Status of Lake Trout (Salvelinus namaycush) in …Status of Lake Trout (Salvelinus namaycush) in Lake Simcoe, Ontario Science and Information Branch Southern Science and Information

Status of Lake Trout (Salvelinus namaycush) in Lake Simcoe, Ontario

Science and Information BranchSouthern Science and Information

By Rebecca Dolson

Aquatic Science Unit Report 2012-1

Ministry of Natural Resources

1 Lake Simcoe Fisheries Assessment Unit Science and Information Branch Aquatic Science UnitOntario Ministry of Natural Resources26465 York Rd. 18Sutton West, ONL0E 1R0

(905) [email protected]

Status of Lake Trout (Salvelinus namaycush) in Lake Simcoe, Ontario

Science and Information BranchSouthern Science and Information

By Rebecca Dolson1

Aquatic Science Unit Report 2012 –1

Ministry of Natural Resources

SIB ASU Report 2012–1ii

Abstract

Lake Simcoe is a large inland lake in central Ontario, of which the coldwater fi sh community is of considerable ecological and economic importance. The coldwater fi sh community of Lake Simcoe has experienced recreational fi shery exploitation, habitat degradation, an infl ux of exotic species, and ultimately population collapse. Lake trout (Salvelinus namaycush) are a highly sought-after sport fi sh and also serve as an indicator of the health of the coldwater fi sh community. The status and temporal trends of the lake trout population were assessed using long-term monitoring data collected by the Lake Simcoe Fisheries Assessment Unit. The monitoring programs utilized in analyses included recreational creel surveys, winter angler sampling, summer gill netting, and fall trap netting. Lake trout relative abundance and biological attributes (growth, condition, survival, maturity, and fecundity) were examined between 2000 and 2010, with comparisons to historical trends. The status and trends of the recreational fi shery (angler catch, effort, and harvest) were also assessed. Lake Simcoe supports both stocked and natural (naturally reproduced) lake trout and the prevalence of each was assessed. The stocking density of yearling lake trout in Lake Simcoe between 2000 and 2010 was low (0.022-0.049 kg/ha; total surface area). A weak positive linear relationship between recruitment to age fi ve and the number of fi sh stocked in that year class was found (F1,16 = 7.4, P = 0.02). An increasing number of natural lake trout have been observed in Lake Simcoe, with six year classes identifi ed. Temporal analysis of summer gill netting data (2003-2010) indicated a signifi cant decline in the relative abundance of stocked lake trout (S = -3.22, P = 0.001). Analysis of winter angler sampling and fall trap netting data also indicated a decline in total catch of stocked lake trout during the same time period. Growth of stocked lake trout decreased between the 1990s and 2000s, and size (mean fork length) at age fi ve and six decreased over the same time period. Lake trout condition did not vary substantially between 2000 and 2010, and in 2010 the mean estimated condition coeffi cient was 2.98 in the summer, and 2.73 in the winter. Annual survival of the stocked lake trout population was estimated as 80.5% between 2005 and 2009, and displayed little variability between the 1990s and 2000s. Age at 50% maturity for stocked lake trout was estimated as three years old for males, and four years old for females. Mean female lake trout fecundity was estimated as 2,145 eggs/kg between 2008 and 2010. Catch-per-unit-effort (CUE) of the winter recreational fi shery was estimated at 0.01 fi sh/angler hour in 2010. Overall, temporal changes in the status of Lake Simcoe lake trout reveal several recent and important changes. Continued long-term monitoring and data collection will provide the information necessary to meet legislative and adaptive management requirements for lake trout in Lake Simcoe.

Keywords:

Lake Simcoe, Salvelinus namaycush, growth, relative abundance, fi shery, natural reproduction

SIB ASU Report 2012–1iii

Résumé

Le lac Simcoe est un vaste lac intérieur situé dans le centre de l'Ontario. Ses populations de poissons d'eau froide sont d'une grande importance écologique et économique. Exploitées par la pêche de loisir, envahies par des espèces exotiques et souffrant de la dégradation de leur habitat, elles sont aujourd'hui beaucoup appauvries. Le touladi (Salvelinus namaycush) est un poisson très prisé des pêcheurs. Il est aussi un indicateur de l'état de santé des populations de poissons d'eau froide. L'état et les tendances temporelles de sa population du lac Simcoe ont été surveillés au fi l des ans grâce aux données recueillies par l'Unité d'évaluation des pêches du lac Simcoe. Sont parmi les données utilisées pour les analyses : questionnaires sur les prises des pêcheurs; prélèvement de poissons en hiver; pêche au fi let maillant en été; pêche au fi let-piège en automne.

L'abondance relative et les attributs biologiques (croissance, état physiologique, taux de survie, développement et fécondité) ont été examinés entre 2000 et 2010, et comparés aux tendances chronologiques. L'état des pêches de loisir et les tendances relatives à celles-ci (prises par les pêcheurs à la ligne, prises par unité d'effort et prélèvements) ont également été examinés. Le lac Simcoe comprend tant des touladis introduits par empoissonnement que des touladis issus d'une population naturelle (reproduits naturellement), et l'abondance relative des deux a été examinée. La densité d'empoissonnement (touladis de l'année) entre 2000 et 2010 a été faible (de 0,022 à 0,049 kg/ha; superfi cie totale). Un faible rapport linéaire positif a été constaté entre le recrutement par poissons jusqu'à cinq ans d'âge et le nombre de poissons de cette classe d'âge introduits par empoissonnement (F1,16 = 7,4, P = 0,02). On a constaté un nombre croissant de touladis issus d'une population naturelle, et six classes d'âge ont été relevées. L'analyse temporelle des données obtenues par la pêche au fi let maillant (2003-2010) révèle un important appauvrissement de l'abondance relative des touladis introduits par empoissonnement (S = -3,22, P = 0,001). L'analyse des données obtenues par la pêche hivernale et la pêche automnale au fi let-piège révèle également une diminution, durant la même période, du nombre total de prises de touladis introduits par empoissonnement.

Le taux de croissance des touladis introduits par empoissonnement a diminué entre les années 1990 et les années 2000, et la taille (longueur moyenne, mesurée du museau à la fourche de la nageoire caudale) à l'âge de cinq et de six ans a également baissé durant cette période. L'état physiologique du touladi du lac Simcoe n'a pas beaucoup changé entre 2000 et 2010. En 2010, le coeffi cient moyen de l'état physiologique du touladi a été estimé à 2,98 en été et à 2,73 en hiver. Le taux de survie annuel du touladi introduit par empoissonnement a été estimé à 80,5 % entre 2005 et 2009, et il a peu varié entre les années 1990 et les années 2000. L'âge à un degré de développement de 50 % pour le touladi introduit a été estimé à trois ans pour les mâles et à quatre ans pour les femelles. Le taux de fécondité moyen des femelles a été estimé à 2 145 œufs/kg entre 2008 et 2010. Pour la pêche de loisir en hiver, les prises par unité d'effort ont été estimées à 0,01 poisson par pêcheur et par heure en 2010. Globalement, plusieurs changements à la fois récents et importants ont été constatés dans l'état du touladi du lac Simcoe.

Les données qui seront obtenues en continuant les activités de surveillance au cours des prochaines années apporteront les renseignements nécessaires pour observer les prescriptions (règlements et gestion adaptative) relatives au touladi du lac Simcoe.

Mots clés : Lac Simcoe, Salvelinus namaycush, croissance, abondance relative, pêches, reproduction naturelle

SIB ASU Report 2012–1iv

This project was conducted by the Lake Simcoe Fisheries Assessment Unit (LSFAU) of the Aquatic Science Unit (ASU) within the Ontario Ministry of Natural Resources (OMNR). I would like to extend my thanks to all of the current and past LSFAU staff who participated in the fi eld data collection and data management. I would like to thank Mike Rawson and Julie Swaine for their assistance with data entry. Thank you to Susan Mann and Nancy Drummond for age interpretation. I greatly appreciate the advice and input from Steve Sandstrom and Nigel Lester while preparing this report. Many thanks to Andy Cook, Steve Chong, Tim Haxton, Todd Langley, Jake La Rose, Gabrielle Liddle, Melissa Robillard, Justin Trumpickas, and Cam Willox, who provided helpful comments on earlier drafts of this report. I would also like to thank Lyn Thompson for her assistance in fi nal formatting and publishing.

Acknowledgements

SIB ASU Report 2012–1v

Contents

Abstract ....................................................................................................................................... ii

Résumé ...................................................................................................................................... iii

Acknowledgements ................................................................................................................... iv

List of Tables ............................................................................................................................... vi

List of Figures ........................................................................................................................... vii

List of Appendicies .................................................................................................................... vii

1.0 Introduction ...........................................................................................................................1

2.0 Methods ................................................................................................................................2

2.1 Study Area ........................................................................................................................22.2 Monitoring Programs .......................................................................................................22.3 Fish Sampling ...................................................................................................................42.4 Data Analysis ....................................................................................................................4

2.4.1 Stocking ...................................................................................................................42.4.2 Relative Abundance and Year Class Success ...........................................................52.4.3 Recreational Fishery Catch, Harvest, and Effort .....................................................62.4.4 Lake Trout Biological Attributes ..............................................................................7

3.0 Results and Discussion ..........................................................................................................8

3.1 A History of Stocking .......................................................................................................83.2 Relative Abundance and Year Class Success .................................................................10

3.2.1 Natural Lake Trout .................................................................................................103.2.2 Stocked Lake Trout ................................................................................................103.2.3 Lake Trout Abundance, Stocking, and Community Interactions ..........................14

3.3 The Recreational Fishery ................................................................................................153.4 Lake Trout Biological Attributes ....................................................................................18

3.4.1 Length and Age Frequency Distributions ..............................................................183.4.2 Growth ...................................................................................................................183.4.3 Condition ...............................................................................................................223.4.4 Survival...................................................................................................................223.4.5 Maturity Schedule..................................................................................................233.4.6 Fecundity ...............................................................................................................25

3.5 Lake Trout In A Changing Lake Simcoe ........................................................................253.5.1 Habitat ...................................................................................................................253.5.2 Exotic Species ........................................................................................................263.5.3 Diet and Food Web ...............................................................................................27

4.0 Summary ..............................................................................................................................27

5.0 Literature Cited ...................................................................................................................29

SIB ASU Report 2012–1vi

List of Tables

Table 1. Lake trout biological attributes sampled during winter angler catch sampling (WACS), summer gill netting (OSBIN), and fall trap netting. ...................................................................................................................................................................... 4

Table 2. Age frequency of stocked and natural lake trout captured in all sampling programs and sampling years combined. .. 8

Table 3. Estimated von Bertalanffy growth function parameters for stocked lake trout captured in winter angler sampling and summer gill netting programs on Lake Simcoe. ....................................................................................................................19

Table 4. Condition of stocked lake trout in 2000, 2005, and 2010 captured during winter angler catch sampling, summer gill netting, and fall trap netting. ...................................................................................................................................................... 22

Table 5: Estimated total annual survival and mortality based on a weighted catch curve regression of stocked lake trout captured in winter angler sampling and summer gill netting. ..................................................................................................... 23

Table 6: Frequency of non-spawning and spawning stocked male and female lake trout captured during summer gillnetting. ........................................................................................................................................................................................ 23

Table 7. Percentage of male and female stocked lake trout captured during fall trap netting. ................................................. 24

Table 8. Percentage of male and female stocked lake trout captured during summer gill netting. ........................................... 24

Table 9: Summary of the temporal changes, by program, in key population attributes of stocked lake trout. .......................... 28

SIB ASU Report 2012–1vii

List of Appendicies

Appendix 1: Summary of winter creel program data availability. ..............................................................................................35

List of Figures

Figure 1. Bathymetric map of Lake Simcoe, Ontario. ...................................................................................................................3

Figure 2. Lake trout stocking history of Lake Simcoe. ..................................................................................................................8

Figure 3. Relative rate of return of age fi ve stocked lake trout captured during fall trap netting plotted against the number stocked of that year class. ........................................................................................................................................................... 10

Figure 4. Total catch at age of natural lake trout captured during summer gill netting. ............................................................. 11

Figure 5. Temporal changes in the area weighted relative abundance of stocked and natural lake trout captured during summer gill netting. .................................................................................................................................................................... 11

Figure 6. Temporal changes in the area weighted relative abundance of stocked lake trout, by age, captured during summer gill netting. .................................................................................................................................................................... 12

Figure 7. Year class strength indices (YCI) of stocked lake trout based on summer gill netting. ............................................... 13

Figure 8. Adjusted proportional abundance of stocked lake trout at age for each year class captured during fall trap netting . ....................................................................................................................................................................................... 13

Figure 9. Total annual catch of stocked and natural lake trout during fall trap netting. ............................................................. 14

Figure 10. Temporal changes in the area weighted relative abundance of stocked and natural lake whitefi sh, rainbow smelt, and lake herring, captured during summer gill netting. .............................................................................................................. 15

Figure 11. Estimated winter angler catch and harvest, and total effort of the lake trout fi shery based on several 50 day standardized roving winter creel surveys. ....................................................................................................................... 16

Figure 12. Estimated winter annual harvested weight of lake trout from the recreational winter fi shery. .................................. 17

Figure 13. Estimated summer angler catch and harvest (bars), and total effort (circles) of the lake trout fi shery based onseveral roving summer creel surveys ........................................................................................................................................... 18

Figure 14. Fork length frequency distribution of stocked male and female lake trout captured during summer gill netting. ........................................................................................................................................................................................ 19

Figure 15. Age frequency distribution of stocked male and female lake trout captured during summer gill netting. .............. 19

Figure 16. Observed and predicted fork length at age of stocked lake trout captured during winter angler sampling and summer gill netting. ....................................................................................................................................................................20

Figure 17. Mean fork length at age fi ve of stocked lake trout captured from 1989 to 2009 during winter angler sampling, summer gill netting, and fall trap netting. ................................................................................................................................... 21

Figure 18. Mean fork length at age six of stocked lake trout captured during winter angler sampling, summer gill netting, and fall trap netting. .................................................................................................................................................................... 21

Figure 19. Relative fecundity at age of stocked lake trout captured during modifi ed fall trap netting. .....................................25

SIB ASU Report 2012–11

1.0 IntroductionLake Simcoe supports one of the largest recreational fi sheries in Ontario. Local revenue from the fi shery is estimated at well over $200 million annually (Young et al. 2010). Lake trout (Salvelinus namaycush) are a primary target of the winter fi shery along with lake whitefi sh (Coregonus clupeaformis), yellow perch (Perca fl avescens) and prior to the 2001 fi shery closure, lake herring (C. artedi). Several attributes of Lake Simcoe ensure an intensive year round fi shery, including its size (722 km2), accessibility, and location. Located approximately one hour north of Toronto, Lake Simcoe is an attractive fi shing destination for nearby residents, those in the greater Toronto area, as well as internationally. The shoreline of Lake Simcoe is almost entirely developed and much of the land in the surrounding watershed is agricultural (Winter et al. 2002). Historical and recent land use changes have resulted in the degradation of water quality (changing levels of nutrients, pH, dissolved carbon, water clarity, and dissolved oxygen) in Lake Simcoe (Evans et al. 1996; Winter et al. 2007; Young et al. 2010). In an effort to restore the health of the lake, the Lake Simcoe Protection Act (2008) and implementation plan (Lake Simcoe Protection Plan 2009) were enacted by the provincial government. Previous efforts (1985–2008) to improve the health and quality of Lake Simcoe were recommended by the Lake Simcoe Environmental Management Strategy (LSEMS 2003). Targeted efforts to reduce the amount of phosphorous entering the lake has resulted in recent improvements in water quality (decreased phosphorous and increased late summer dissolved oxygen concentration; Young et al. 2010). Another objective of the Plan is to restore a self-sustaining coldwater fi sh community, specifi cally lake trout (Lake Simcoe Protection Plan 2009). Lake trout are an important sport fi sh species and also serve as an indicator of ecosystem health due to their specifi c habitat requirements (Edwards et al. 1990).

The coldwater fi sh community of Lake Simcoe persists today despite a history of habitat degradation and fi shery collapse. Several coldwater fi sh species of Lake Simcoe (lake trout, lake whitefi sh, lake herring, and rainbow smelt, Osmerus mordax) have experienced drastic population declines and ultimately near collapse over the last four decades (Amtstaetter 2002; Evans et al. 1991; Johanson and Amtstaetter 2004; MacRae 2001). Lake trout experienced a population collapse in the 1960s which resulted in the population being maintained through a rehabilitative stocking program (Borwick et al. 2009) that has continued into 2011. From the late 1960s to the early 2000s limited and subsequently no natural reproduction/recruitment of lake trout was observed. Habitat degradation was identifi ed as a primary factor which led to natural lake trout recruitment failure and ultimately the decline of natural lake trout in Lake Simcoe (Evans et al. 1991).The fi rst evidence of natural lake trout reproduction in several decades was reported in 2001 (Willox 2001). Each year between 2001 and 2005 an increasing number of natural lake trout were observed (La Rose and Willox 2006).

The population dynamics and biological attributes of lake trout are important variables to consider when evaluating the health of Lake Simcoe’s coldwater fi sh community. Many factors exist in the lake which may infl uence the dynamics of the lake trout population, including an extensive recreational fi shery, annual stocking, changes in prey abundance, state of water quality and habitat, climate change, establishment of several exotic species, and the reappearance of natural recruitment (Evans et al. 1991; Evans and Waring 1987; MacRae 2001; Willox 2001). For example, global climate change may impact several life history characteristics of vulnerable lake trout populations (Shuter and Lester 2004). The establishment of several exotic species may negatively impact lake trout either directly or indirectly; potentially harmful species include zebra (Dreissena polymorpha) and quagga mussels (D. bugensis), rusty crayfi sh (Orconectes rusticus), and round goby (Neogobius melanostomus; Jimenez et al. 2011; Jonas et al. 2005; Ozersky 2010; Vander Zanden et al. 2004). In addition, the exotic spiny water fl ea (Bythotrephes longimanus) may also impact the coldwater fi sh community, including lake trout (Madenjian et al. 1998).


MacRae (2001) summarized the historical population dynamics and key attributes of the lake trout population in Lake Simcoe between 1981 and 2000. MacRae (2001) reported a high level of recreational fi shery exploitation and harvest. Additionally, MacRae (2001) estimated an absolute average abundance of mature (>5 years old) lake trout of 1.37 trout/ha, or 99,255 individuals in 2000. Variable total mortality, size (length) at age, and year class strength were observed between 1980 and 1999, but no overall directional trend was detected. Metcalfe (2008) observed decreased growth (size at age) of lake trout for the most commonly caught ages (4–7 years old) beginning in the mid 1990s; however the observed decrease was within the magnitude of past changes.

The objective of this report was to provide an update on the status and trends of the lake trout population in Lake Simcoe. This report provides information useful to fi sheries management, policy implementation, ongoing science, and contributes to an adaptive management approach.

Several attributes of Lake Simcoe’s stocked lake trout population were assessed including the relative success of a stocking event, relative abundance, the recreational fi shery, growth, size and age structure, condition, survival, maturity, and fecundity. The relative abundance of natural lake trout was also assessed. At the time of this report, annual sample sizes of natural lake trout were insuffi cient to perform an assessment of many biological attributes and life history characteristics.

The Ontario Ministry of Natural Resources (OMNR) has conducted long term monitoring of the Lake Simcoe fi sh community since 1961. This report highlights data collected between 2000 and 2010 by the Lake Simcoe Fisheries Assessment Unit (LSFAU) of the OMNR. A historical context has been referred to throughout the report where comparisons to earlier decades can be made. For a thorough description of lake trout dynamics and status between 1980 and1999, see MacRae (2001). Several fi sh community monitoring programs were used to describe the population dynamics of lake trout in the lake.

2.0 Methods2.1 Study AreaLake Simcoe is a deep multi-basin lake with a surface area of 722 km2. Lake Simcoe has a mean depth of 17 m and a maximum depth of 42 m in the western-most basin, Kempenfelt Bay. There are three major basins in Lake Simcoe, including the main basin (mean depth 14 m), Kempenfelt Bay (mean depth 20 m), and Cook’s Bay (mean depth 13 m; Eimers et al. 2005). Cook’s Bay is classifi ed as eutrophic, while Kempenfelt Bay and the main basin are mesotrophic. Figure 1 illustrates the geographic location and bathymetry of Lake Simcoe.

2.2 Monitoring Programs

Winter Creel Survey and Winter Angler Sampling (WACS; 1981–2010)A creel survey is a whole-lake stratifi ed roving angler survey. Creel surveys included counting and interviewing anglers regarding their catch, harvest, and effort. Fish biological attribute data were collected during winter angler catch sampling which ran concurrently with the winter creel survey. The OMNR has performed winter season creel surveys on Lake Simcoe since 1961. Data used in this report span intermittently between 1981 and 2010 (Appendix 1). However, data collected between 1961 and 1980 were discussed to provide a historical context, but were not used in analyses presented herein. The winter fi shery is dependent on weather conditions which determine the timing and duration of ice cover on the lake. Survey results were standardized to a 50 day period between January 25 and March 15. The survey was also stratifi ed by sector (10), mode (2), day-type (2), and day-period (2). Catch, harvest, and effort calculations were based on the software application FISHNET (v.2; Lester et al. 1996). For information on winter creel survey fi eld and analytical methods, see DesJardine and Lawrence (1979a, 1979b), Willox (1985), and McIntyre (1989).


Summer Creel Survey (1981–2010)The summer creel survey is a whole-lake stratifi ed roving angler survey. Summer creel surveys were conducted in the same manner as a winter survey but anglers were approached by boat. Interviews and angler counts were performed to estimate catch, harvest, and effort. Survey results were stratifi ed by season, day-type (weekday or weekend/holiday), period (length of time within the summer season), and space (sector of the lake). A total of 120 strata were designed and defi ned. The summer creel survey typically occurred between the second Saturday in May (start of the lake trout angling season) and Labour Day. Catch, harvest, and effort calculations were based on the software application FISHNET (v.2; Lester et al. 1996). For information on summer creel survey fi eld and analytical methods see DesJardine and Lawrence (1979a, 1979b), Willox (1985), and McIntyre (1989).

Offshore Benthic Index Gill Netting (OSBIN; 2003–2010)The OSBIN program (hereafter referred to as summer gill netting) is a standardized mid-summer benthic (lake bottom) gill netting program. The study design allowed the comparison of relative abundance and biological attributes of fi sh species across years within Lake Simcoe. The summer gill netting program incorporated a random design which controlled for site selection bias and seasonal/spatial variation in species distribution. Spatial and depth strata were delineated within the lake and sites were selected randomly from a list of suitable sites. The strata were defi ned as: Main Basin Deep (>25 m), Main Basin Shallow (20–25 m), and Kempenfelt Bay (>25 m). The sampling allocation to each stratum was proportional to the amount of area the stratum contributes to the whole lake. However, Kempenfelt Bay was often sampled in excess of its proportional area to increase the number of lake trout caught in a given year of the program. Roughly 30 samples (net sets) were completed annually, with the exception of 2008 when a larger project was conducted and 50 samples were completed. Gill nets were 137.2 m long and 2.4 m high with green monofi lament stretched mesh and a hang ratio of 2:1. The nets were comprised of nine panels with

Figure 1. Bathymetric map of Lake Simcoe, Ontario. A star represents the location of the Lake Simcoe Fisheries Assessment Unit1.


mesh sizes (mm) 25, 38, 51, 64, 76, 89, 102, 114, and 127. Net set orientation was dependent on the prominent wind direction for ease of deployment and retrieval. Nets were set overnight, typically fi shing for 18 hours.

Fall Trap Netting (FITN; 1989–2006)Fall trap netting was a monitoring program that collected information about the spawning lake trout and lake whitefi sh populations in the lake, including total catch, size at age, and fecundity. Two sites were sampled annually (Strawberry Island and North Georgina Island) for the entire length of the spawning run (late September–early December). At each site a 10 foot (3.05 m) trap net was deployed and fi shed daily (except during extreme weather). For a detailed description of sample design and sampling methodology, see Willox (1986), McMurtry (1989), and Amtstaetter (1999).

2.3 Fish SamplingTotal catch and/or relative abundance were recorded per-unit-effort for all species captured in each sampling program. Biological attributes of lake trout collected in each sampling program was summarized in Table 1. Lake trout sex and maturity were determined by visual examination of the gonads. Fall trap netting fi sh sampling methodology changed over the course of the program’s existence and incorporated complete sampling or sub-sampling (1989–1991), size-stratifi ed sampling (1992–2000), and random sub-sampling (2001–2006). A review of the sampling methodology can be found in Amtstaetter (1999). Natural lake trout always received complete detailed sampling (of all attributes) when encountered in any programs.

The methodology used for age interpretation of lake trout was fairly consistent between the mid 1980s and 2006. A sub-sample of lake trout were selected for age interpretation from each year and sampling program. Lake trout age was assessed using a combination of completed scale annuli counts, fork length, and clip code (using the Peterson Method; Bagenal and Tesch 1978). An age-length key was developed and an assessed age assigned to all lake trout captured in the specifi c year/sampling program. Confi dence in the assigned age was high for younger (<8 years old) stocked trout where only one clip code (and therefore age) was possible. Scale age assessment typically underestimates the age of older fi sh (Casselman 1987). However, using a combination of the Peterson Method and clip code it is believed that the age of only a few lake trout would have been grossly underestimated. Lake trout captured after 2006 (in all sampling programs) were aged using completed annuli counts of the otolith (embedded in epoxy, sectioned and mounted). Assessed age for natural lake trout was always determined by an examination of aging structures (scales and otoliths post 2006).

2.4 Data Analysis

2.4.1 StockingThe number of stocked lake trout released into Lake Simcoe was recorded annually and maintained in the Fish Stocking Information System database. Prior to release in Lake Simcoe, lake trout had one or more fi ns removed manually and the specifi c

Attribute WACS OSBIN FITN*

Fork Length (mm) X X X

Round Weight (g) X X X

Sex X X

Maturity X X

Otolith (post 2006) X X X

Scales X X X

Clip Code X X X

Table 1. Lake trout biological attributes sampled during winter angler catch sampling (WACS), summer gill netting (OSBIN), and fall trap netting (FITN). * FITN sampling methodology has changed marginally over the years, and not all attributes were collected for all lake trout, in all years.


combination of fi ns that were removed denoted the clip code. The clip codes rotated on a nine year cycle and provided information about individual fi sh year class and origin (naturally reproduced lake trout will have no clips). Clips were applied with 99% accuracy (i.e., 99% have some portion of the correct fi n removed and 96% of all fi sh have full fi n removal), and a negligible number of stocked lake trout could be misinterpreted as being of natural origin (Amtstaetter and Willox 2004).

The annual stocking rate of lake trout in Lake Simcoe (i.e., density stocked annually) was examined between 2000 and 2010. Additionally, a density dependent stocking analysis was completed based on fall trap netting data (1989–2006) using a variation of the method from MacRae (2001) and Amtstaetter (1999). The relative success of stocking at different densities was evaluated by relating the relative catch of age fi ve fi sh to the number of fi sh stocked in that year class (i.e., number stocked fi ve years previously). The relative rate of return of age fi ve fi sh was calculated as the number of age fi ve fi sh divided by the sum of catch for all fi sh ages 3–12 years old caught in an individual sampling year. The relative rate of return of age fi ve lake trout (as opposed to absolute rate of return) was calculated in order to control for interannual variation in catch which may mask changes in the stocking success of specifi c year classes. The method outlined above calculated absolute catch of all age classes as the dependent variable in the stocking analysis, whereas in MacRae (2001) the absolute catch of age fi ve fi sh divided by the relative (to number stocked) catch of ages 3–12 years was calculated. This change was made to remove the number of lake trout stocked per year from the dependant variable of the regression.

2.4.2 Relative Abundance and Year Class SuccessThe relative abundance of lake trout was estimated using data collected in the summer gill netting program and was available from 2003 to 2010. Relative abundance was represented as the area weighted catch-per-unit-effort (CUE; net set) per year (Sandstrom and Lester 2009). CUE was weighted by area using equation (1):

(1)

where W is the proportion of total sampling area, A is the mean CUE of lake trout, and h is the spatial stratum. The variance of the area weighted CUE was calculated using equation (2):

Vcue )1()(

1 h

hhin

t nAAh

(2)

where i is the individual sample catch, and n is the number of net sets (samples) in each stratum (Sandstrom and Lester 2009). Relative abundance was calculated using the arithmetic mean. For stocked lake trout, relative abundance was also calculated for individual age classes. Relative abundance was calculated (as described above) for each age class per stratum between 2003 and 2009. Total catch per age class of natural lake trout was calculated per year (and not relative abundance) due to limited sample size.

The relative abundance of several coldwater species (lake whitefi sh, lake herring, and rainbow smelt) known to interact with lake trout was also calculated as described above.

Lake-wide relative abundance of lake trout was evaluated for the presence of a monotonic temporal trend using the S-statistic of the Mann-Kendall non-parametric trend test. The S-statistic is the difference between the number of pairwise slopes that are positive minus the number that are negative. Large positive numbers indicate an increasing trend, while large negative numbers indicate a decreasing trend (USEPA 2000). Sen’s slope was used as a non-parametric estimate of slope and was calculated as the median rate of change among all years (Yan et al. 2008). The Mann-Kendall test is a modifi ed Kendall’s Rank test (Kendall’s Tau) and evaluates the rank order

hIh

hcue AWA *


correlation between two variables. A signifi cant trend in slope would indicate a signifi cant increase or decrease in the relative abundance of lake trout.

An index of year class strength (YCI) was estimated using the method presented in Marshall (2011). Stocked lake trout year class strength was estimated using summer gill netting data between 2003 and 2009 for those year classes (1991–2001) with at least three years of data for fi sh between the ages of six and 14 (i.e., fully recruited to the gear).

(3)

where YCI is a dimensionless index of year class strength, CUE is the mean area weighted CUE of lake trout, i represents the age-class, and n the year class. YCI describes the strength of a year class relative to the average strength of all year classes during the same sampling period. A YCI value greater than the overall upper third quartile represented an exceptionally strong year class, while a YCI value less than the fi rst quartile represented an exceptionally weak year class.

Adjusted proportional abundance was calculated for each year class captured in the fall trap netting program between 2001 and 2006 and plotted against age (3–12 years old; see MacRae 2001). Adjusted proportional abundance was calculated as the number at age (age 3–12) of lake trout caught divided by the number stocked in that year class and multiplied by 10,000. This produces the relative rate of return of lake trout per 10,000 stocked. The relative rate of return of each age group was then divided by the sum of relative rate of return per 10,000 stocked for all fi sh ages 3–12 years old caught in an individual sampling year. This produces the adjusted proportional abundance at age for all lake trout aged 3–12 years old. This alternative estimate of year class strength (and not YCI) was used for fall trap netting data because lake trout catch was recorded in total number per year (and not CUE).

2.4.3 Recreational Fishery Catch, Harvest, and EffortEstimates of Lake Simcoe’s recreational fi shery catch, harvest and total effort were based on creel survey data (see Section 2.2). FISHNET (v2) performed a two-stage estimation process. First, daily estimates were obtained and averaged across the stratum (i.e., area, day type, period), then multiplied by stratum size (i.e. number of days) to produce the estimated total for each stratum. Estimated survey totals were generated by summing across the strata (Lester and Korver 1996). Herein, total fi shing effort (effort for all species combined) was used in lake trout CUE and harvest-per-unit-effort (HUE) calculations, as opposed to targeted lake trout effort, because total effort was a more robust measure. Additionally, there was good agreement (>85% winter and summer seasons) between total and targeted CUE (LSFAU unpublished data). MacRae (2001) also illustrated a strong relationship between total catch and targeted lake trout CUE. It is acknowledged that using total fi shing effort for all species will result in an underestimate of the annual CUE, specifi cally for lake trout.

Winter annual harvest (kg/ha) was estimated as the annual lake trout harvest (stocked and natural) multiplied by the mean weight (kg; stocked and natural) of angled lake trout from that year (1986–2010), divided by the total area of the lake (72,200 ha). Estimated winter harvest rates did not include a three week period at the beginning of the season where the lake trout fi shery was open but the angler survey had not begun due to potentially unsafe conditions (January 1–25). Summer and fall estimates of harvest were also omitted from the aforementioned analysis. Summer harvest was omitted because the average weight of angled summer lake trout was not collected. Fall harvest information was also not collected for Lake Simcoe. Therefore, estimated winter annual harvest represented the minimum value and underestimated the true harvest of lake trout from Lake Simcoe.

10014

6

14

6

ii

ini

n

CUE

CUEYCI


2.4.4 Lake Trout Biological AttributesLength and age frequency distributions for male and female stocked lake trout were calculated based on summer gill netting data pooled between 2003 and 2009.

Lake trout growth was characterized using the von Bertalanffy growth parameters. The parameters were calculated from the best fi t non-linear regression equation of mean size at age for those age classes with fi ve or more lake trout (ages 3–16 years) using the Fisheries Analysis and Simulation Tool software (FAST; Splike and Maceina 2006). Winter angler sampling (1989–2009) and summer gill netting (2003–2009) data were used to characterize the growth trajectory of the stocked lake trout population on a decadal scale. The decade was chosen as the time interval of interest to account for the high annual variability of size at age. The von Bertalanffy growth parameters are:

Lt = L∞ (1 – e-k(t – to)) (4)

where Lt is size (length) at age t, L∞ is the maximum theoretical length, k is the growth coeffi cient, and to is the year when length is zero.

Size at age (a cohort specifi c measure of growth) was examined for stocked lake trout caught in the winter angler sampling, summer gill netting, and fall trap netting programs over the entire range of data available. To establish a point of reference against which annual measures of size at age could be compared and to describe variability in the data set, an overall mean (all programs and years combined) and 95% confi dence intervals (CI) was calculated. Lake trout aged fi ve and six were the most frequently caught age classes in LSFAU sampling programs. An analysis of stocked lake trout size at age fi ve and six allowed a specifi c examination of the temporal patterns in growth of the most abundant portion of the population.

Lake trout condition was defi ned as the relationship between fi sh length and weight and interpreted as the robustness or well-being of an individual (Schneider et al. 2000).The condition of stocked lake trout was calculated using the weight-length relationship described in Anderson and Neumann (1996) based on the ordinary least-squares regression model:

W = aLb (5)Log10(W) = a’ + b (log10L) (6)

where W and L are weight and length respectively, a’ is the y-intercept, and b is the slope of the line. Condition was compared among each sampling program (winter angler sampling, summer gill netting, and fall trap netting) for the years 2000, 2005, and 2010 where data were available. The 95% confi dence intervals of the slope, b, were evaluated. Overlapping confi dence intervals would suggest no signifi cant difference between time periods (within one sampling program). An estimate of slope, b, greater than three would imply that fi sh were robust and heavy for their length (i.e., good condition; Schneider et al. 2000).

Catch curve analysis (Ricker 1975) was used to estimate the annual total mortality (fi shing plus natural mortality) for stocked lake trout between the ages of fi ve (winter angler sampling) and six (summer gill netting) through 15 years old (i.e., those fi sh fully recruited to the gear). Fish older than 15 years were not included in the analysis due to low sample size per age class (N < 5). Sampling years were combined on fi ve year intervals due to limited catch at age data in recent years. The software FAST of Splike and Maceina (2006) was used to perform the analysis. In FAST a weighted regression technique was followed (Steel and Torrie 1980) to reduce the infl uence of older age classes.

Age of maturity of stocked lake trout was estimated by relating visual inspection of captured fi sh gonads to its estimated age. To increase the sample size, summer gill netting data from 2003 to 2009 were pooled to estimate age at 50 and 100% maturity.

Reproductive effort for stocked lake trout was approximated based on total and relative fecundity of adult female fi sh captured during a modifi ed (reduced effort and


spatial sampling) fall trap netting program in 2008, 2009, and 2010. Fecundity was estimated as the total number of eggs produced by a female by applying the average of three replicate sub-sample egg counts to the whole ovary weight. Relative fecundity was estimated as the number of eggs produced per kilogram of total mass (total fecundity divided by weight).

3.0 Results and Discussion3.1 A History of StockingLake trout were fi rst stocked in Lake Simcoe in 1880 (MacCrimmon and Skobe 1970) and have been stocked annually since 1914 (McMurtry et al. 1997). Stocking was fi rst initiated to enhance the commercial and recreational fi sheries but became a rehabilitative effort after the collapse of the lake trout population in the 1960s. Figure 2 illustrates the stocking history from 1980 onward, which involved stocking yearling Lake Simcoe strain lake trout in the spring. Prior to 1978 fry and two year old lake trout were also stocked on a trial basis to determine if survival varied based on age of planting (MacLean et al. 1981).

Fingerlings were not stocked after 1978 due to low survivorship (Evans and Willox 1991). During 1974–1977 limited capture of adult lake trout prevented the collection of eggs and milt, necessitating stocking of non-native strains (Lake Superior, Lake Manitou, and Clearwater Lake; MacRae 2001). MacLean et al. (1981) found that Lake Simcoe and Clearwater strains stocked into Lake Simcoe had similar survival. Based on the life expectancy of stocked lake trout in Lake Simcoe (93% of aged fi sh did not survive past 11 years old; Table 2) and the complete recruitment failure during the 1980s, it is unlikely that lake trout of non-native strains continue to persist in

Age Stocked Natural

1-5 13403 250

6-10 14363 214

11-20 2161 136

>21 20 0

Total 29947 600

Table 2. Age frequency of stocked and natural lake trout captured in all sampling programs (winter angler sampling, summer gill netting, and fall trap netting) and sampling years (1989-2009) combined.

1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010

Num

ber o

f Lak

e Tr

out S

tock

ed (*

1000

)

0

20

40

60

80

100

120

140

160

Figure 2. Lake trout stocking history of Lake Simcoe from 1980 to 2010. Lake trout stocking has occurred annually and consistently in the early spring when lake trout were roughly 16 months old.


appreciable numbers or in terms of genetic contribution to the naturally reproduced population. Prompted by the reappearance of natural trout in 2001, the annual stocking rate of lake trout was reduced in 2010 to 50,000, or roughly 0.02 kg/ha (total area; Borwick et al. 2009). In order to promote natural reproduction the stocking rate will remain at 50,000 for fi ve years after which the stocking success and objectives will be re-evaluated (Borwick et al. 2009).

The annual stocking rate of lake trout has generally approached the target (prior to 2010) of 100,000 yearlings over the last three decades with variation between a minimum of 21,091 (1988) and a maximum of 146,126 (2002; Figure 2). The low stocking rate in 1988 was due to reduced survival of yearling fi sh in the hatchery (MacRae 2001). The annual stocking rate of lake trout between 2000 and 2010 varied between the maximum in 2002 and a minimum of 51,360 in 2010. Stocking at this level translated into relatively low density of lake trout released (0.022–0.049 kg/ha, total surface area; Figure 2) in reference to the provincial guideline of 0.1 kg/ha/yr (OMNR 2000).

The relationship between annual stocking rate of lake trout and relative rate of return of age fi ve fi sh was analyzed. The relative rate of return of age fi ve fi sh captured in the fall trap netting program between 1989 and 2006 suggested that recruitment was weakly, yet signifi cantly, proportional to the number of fi sh stocked (R2 = 0.32, F1,16 = 7.4, P = 0.02; Figure 3). Only 32% of the variation in the relative rate of return of age fi ve fi sh was explained by the number of fi sh stocked per year class, suggesting other variables may be infl uencing whether a particular stocked year class will be successful. If the analysis is repeated after removing the 1987 year class (only 21,091 stocked and known to have poor survival prior to release) the relationship between annual stocking rate of lake trout and relative rate of return of age fi ve became non-signifi cant and not linearly related (R2 = 0.09, F1,15 = 1.5, P = 0.23). MacRae (2001) found a positive linear relationship between the annual stocking rate of lake trout and relative rate of return of age fi ve lake trout in Lake Simcoe. Evans and Waring (1987) found that lake trout in Lake Simcoe exhibited a density dependent stocking relationship when more than 80,000 yearling lake trout were stocked per year. Powell and Carl (2004) discuss the infl uence of habitat, prey abundance, and intraspecifi c competition on the success of a stocking event. It is not clear what promotes a successful stocking event in Lake Simcoe. For example, a large stocking event in 2002 (2001 year class) of 146,000 yearlings did not result in an exceptionally high return of age fi ve fi sh. The relative rate of return of the 2001 year class was estimated as 0.32, compared to the 1992 year class where 95,376 fi sh were stocked and relative rate of return was estimated as 0.49 (Figure 3).

The stocking analysis presented herein contrasts that of MacRae (2001; see section 2.4.1) who found a strong positive relationship between the annual stocking rate and the relative rate of return of age fi ve stocked lake trout in Lake Simcoe. If the annual stocking rate is not linearly related to the success of a year class (measured as relative rate of return of age fi ve fi sh) further manipulation of the annual stocking rate may be required to determine the optimum stocking rate required to meet management objectives.

Stocking methodology (i.e., timing, location, existing species in the receiving lake, etc.) can greatly infl uence the success of a stocking event. The availability of suitable habitat for the stocked fi sh is an important variable to consider to ensure a successful stocking event (Kerr and Lasenby 2001). Spring yearling lake trout (approximately 16 months old) were stocked annually in the early spring (mid-late April) when water temperatures do not restrict movement (prior to lake stratifi cation). The site of release for stocked trout has been variable. Since 1988 the total number of fi sh stocked has been divided among at least two release sites, typically at an eastern and western lake location. In the last decade (1999–2010 year classes) the mean annual lake trout size at stocking was 140 mm (fork length), and 27 g (weight).


3.2 Relative Abundance and Year Class Success

3.2.1 Natural Lake TroutIn 2001, two young-of-the-year lake trout were captured in LSFAU monitoring programs providing the fi rst evidence of lake trout natural reproduction and recruitment in Lake Simcoe since the 1960s (Willox 2001). Natural lake trout were captured in several LSFAU index netting programs between 2001 and 2006 (La Rose and Willox 2006), and the proportion of natural lake trout captured increased on an annual basis until 2010. Winter anglers caught an increased proportion of natural lake trout in 2009 and 2010. For example, in 2009 and 2010 over 40% of the estimated winter angler catch was natural lake trout; a prevalence not observed since the late 1970s (MacRae 2001).

The reappearance of natural lake trout was an encouraging step toward population rehabilitation in Lake Simcoe. Also encouraging was the appearance of several year classes of natural lake trout within the population. Figure 4 depicts the total catch at age of natural lake trout from the summer gill netting program between 2003 and 2009, where six year classes were represented. Four year classes were represented between the ages of 2–6 years in 2009. The 2004 year class comprised the largest fraction of natural lake trout caught between 2005 and 2009. Natural lake trout contributed between 17 and 32% of the total lake trout catch during summer gill netting from 2007–2010 (Figure 5).

The specifi c conditions that contributed to the re-establishment of successful natural reproduction after roughly 30 years of recruitment failure are not known. Potential explanatory variables may include changes in the relative abundance of stocked trout (discussed below), water quality, the amount of optimal habitat (mean volume weighted hypolimnetic dissolved oxygen; Young et al. 2010), forage density, as well as intra and interspecifi c competition.

3.2.2 Stocked Lake TroutThe relative abundance (represented by the mean area weighted CUE) of stocked lake trout decreased signifi cantly between 2003 and 2010 (S = -3.22; P = 0.001; Sen’s slope = -0.34; Figure 5). This result was based on all of the age classes captured in the summer gill netting program.

Number of Lake Trout Stocked (*1000)

0 20 40 60 80 100 120 140 160

Rel

ativ

e R

etur

n of

Age

Fiv

e S

tock

ed L

ake

Trou

t

0.0

0.1

0.2

0.3

0.4

0.5

0.6 R2 = 0.32

Figure 3. Relative rate of return of age fi ve stocked lake trout captured during fall trap netting from 1989 to 2006 plotted against the number stocked of that year class.


2003 2004 2005 2006 2007 2008 2009

Tota

l Cat

ch o

f Nat

ural

Lak

e Tr

out

0

10

20

30

40

50age 1 age 2 age 3 age 4 age 5 age 6 age 8

Figure 4. Total catch at age of natural lake trout captured during summer gill netting from 2003 to 2009.

Figure 5. Temporal changes in the area weighted relative abundance (catch-per-unit-effort) of stocked and natural lake trout captured during summer gill netting. Relative abundance is expressed as the mean number of trout caught per net set annually, weighted by the stratum’s proportion of total surface area. There is a signifi cant decline in the relative abundance of stocked lake trout from 2003 to 2010 based on a non-parametric test of slope (S = -3.22; p = 0.001). Error bars represent the standard error of the mean.

2003 2004 2005 2006 2007 2008 2009 2010

Mea

n La

ke T

rout

Rel

ativ

e A

bund

ance

0

1

2

3

4

5

Stocked Lake TroutNatural Lake Trout


Lake trout ages six years and older were considered fully recruited (i.e., vulnerable) to the summer gill netting gear. An assessment of the relative abundance of age six and older fi sh also revealed a signifi cant decline between 2003 and 2009 (S = -2.85; P = 0.004; Sen’s slope = -0.21). Figure 6 depicts the relative abundance of stocked lake trout by age class. Lake trout between the ages of one and fi ve years old (i.e., not fully recruited to the gear) may exhibit variability in catch unrelated to their relative abundance, but possibly due to the infl uence of gear selectivity of smaller/younger fi sh (Hansen et al. 1997). Gear selectivity of smaller/younger fi sh may be attributed to the gear confi guration and/or changes in growth of young lake trout. Between 2003 and 2005 older stocked lake trout (> eight years old) contributed substantially to the annual estimate of relative abundance (Figure 6). However, after 2005 the contribution of older stocked lake trout decreased. Figures 7 and 8 illustrate the relative success of each stocked year class captured in the summer gill netting (2003–2009) and fall trap netting programs (2000–2006), respectively. Data from both sampling programs suggest that the 1992 and1995 year classes were substantially more successful than other year classes during the sampling period. The 1996 YCI falls below the fi rst quartile implying very low success. All year classes since 1995 (except 2000) fall below the median quartile (105.4) implying moderate to low success. Due to the requirements of the YCI analysis, the successes of recent year classes (2002 to 2006) could not be calculated.

The factors contributing to the observed decrease in stocked lake trout relative abundance between 2003 and 2010, as well as the limited year class success between 1996 and 2001 are not known. Potential explanatory variables may include habitat quality, exotic species, inter and intraspecifi c competition, and angling pressure. The potential infl uences of these variables are discussed in sections 3.2.3 and 3.5.

High interannual variation in total catch was observed throughout the fall trap netting program (Figure 9). However, a consistent decline in total catch was observed between 2000 and 2006, and this pattern was congruous with summer gill netting and winter angler sampling trends. Total catch is a coarse index of the lake trout spawning population observed at two spatially explicit spawning shoals. In the last three years of the fall trap netting program (2004 to 2006) total catch of lake trout was less than 800, which was lower than any catch recorded since 1989.

2003 2004 2005 2006 2007 2008 2009

Mea

n S

tock

ed L

ake

Trou

t Rel

ativ

e A

bund

ance

0

1

2

3

4age - 1age - 2 age - 3 age - 4 age - 5 age - 6 age - 7 age - 8 age - 9 age - 10 age - 11 age - 12 age -13 age-14 age 15+

Figure 6. Temporal changes in the area weighted relative abundance (catch-per-unit-effort) of stocked lake trout, by age, captured during summer gill netting. Relative abundance is expressed as the mean number of trout caught per net set annually, weighted by the stratum’s proportion of total surface area.


Age

2 4 6 8 10 12 14Adj

uste

d P

ropo

rtion

al A

bund

ance

of S

tock

ed L

ake

Trou

t

0.0

0.1

0.2

0.3

0.4

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Figure 7. Year class strength indices (YCI) of stocked lake trout based on summer gill netting data from 2003 to 2009. Year classes included in the analysis (1991-2001) are fully recruited to the gear (i.e., from 6 to 14 years old) with at least three years of data. Dotted lines represent the overall fi rst and third quartiles. A YCI value greater than the third quartile represents an exceptionally strong year class, whereas below the fi rst quartile is exceptionally weak year class. Number stocked (*1000) per year is shown.

Figure 8. Adjusted proportional abundance of stocked lake trout at age for each year class captured during fall trap netting from 2000 to 2006.


The analyses presented above strongly suggest a directional change in stocked lake trout relative abundance, especially as the pattern was seen across several monitoring programs. An approximation of the magnitude of change in relative abundance was not available (Hubert and Fabrizio 2007).

3.2.3 Lake Trout Abundance, Stocking, and Community Interactions Two scenarios are often described in fi sheries science to generalize the response of natural fi sh populations to stocking; stocked over natural (Scenario 1; Evans and Willox 1991; Evans 1992; and Evans et al. 1991) and natural over stocked (Scenario 2; Hoyle 1990; Loftus 1985). Scenario 1 involves planting stocked fi sh into a lake with an existing natural population (of the same specie). Several studies have suggested that this may result in the extirpation of the native population, under conditions of high fi shing pressure (Evans and Willox 1991; Evans 1992; and Evans et al. 1991). Genetic homogenization, limited recruitment, competition, and increased angler harvest (due to increased anticipation of greater catch rate when a lake is stocked) are cited as factors that promote the success of stocked fi sh over a natural population (Dunlop and Brady 1995, 1997; Evans 1992; Evans et al. 1991; Evans and Willox 1991). Scenario 2 involves planting stocked fi sh into a lake with an existing natural population, but in the absence of high fi shing pressure, the natural population suppresses the success of the stocking event (i.e., establishment of the stocked population; Hoyle 1990; Loftus 1985). For example, Powell et al. (1986) found that the survival of stocked lake trout was negatively infl uenced by the abundance of natural lake trout, lake whitefi sh, and lake herring due to density dependant interactions. Lake Simcoe has traditionally been considered an example of Scenario 1 given the conditions of high fi shing pressure and low natural lake trout abundance. Stocked lake trout relative abundance declined between 2003 and 2010; whether or not the observed decline will result in Lake Simcoe entering into a Scenario 2 stocking situation is unknown.

Predator-prey and interspecifi c interactions are strong drivers of community dynamics in freshwater lakes (Vander Zanden and Vadeboncoeur 2002). Lake Simcoe has a diverse coldwater fi sh community and many species may interact with lake trout. Angler catch and harvest data from the 1980s–1990s suggests that lake whitefi sh, lake herring, and rainbow smelt all experienced signifi cant population declines (Amtstaetter 2002; Evans and Waring 1987; Johanson and Amtstaetter 2004; McMurtry 1999). Summer gill netting data illustrated increased relative abundance

1975 1980 1985 1990 1995 2000 2005

Tota

l Lak

e Tr

out C

atch

(Nor

th G

eorg

ina

Is a

nd S

traw

berry

Is)

0

1000

2000

3000

4000

5000

Stocked Natural

Figure 9. Total annual catch of stocked and natural lake trout during fall trap netting.


of lake whitefi sh (stocked and natural), lake herring, and rainbow smelt between 2003 and 2009 (Figure 10). The observed increase in the relative abundance of other coldwater species is in contrast to the recent downward trend in stocked lake trout relative abundance. Lake whitefi sh, lake herring, and rainbow smelt may negatively interact with one another, interact with juvenile lake trout, and potentially predate on lake trout eggs (Evans and Waring 1987; Jonas et al. 2005; Vander Zanden et al. 2004). Evans and Olver (1995) suggest the growth and survival of stocked lake trout can be expected to decrease in proportion to an increase in the density of other interacting coldwater species.

An analysis of the causal factors infl uencing the magnitude and direction of predator-prey and interspecifi c interactions among coldwater species in Lake Simcoe is not available. Both intra and interspecifi c competition, as well as predator-prey dynamics may provide insight into the changing relative abundances of both stocked and natural lake trout.

The relative abundance of natural lake whitefi sh also increased between 2005 and 2010 (Figure 10). The stocked lake whitefi sh population however, has neither increased nor decreased appreciably since 2003. Determining the causal factors leading to the revival of the natural lake trout and lake whitefi sh populations in Lake Simcoe will assist in the management of the entire coldwater fi sh community.

3.3 The Recreational FisheryLake Simcoe supports one of the largest recreational fi sheries in Ontario. A substantial portion of the overall lake trout angling effort was expended during the winter season. Total winter angling effort (for all species) has ranged within 6.8 and 9.7 angling hrs/ha between 2000 and 2010. MacRae (2001) illustrated the estimated recreational catch, harvest, and effort of lake trout between 1961 and 2001. Winter recreational angling effort increased continually since the 1960s and has been consistently high since 2000. Figure 11 depicts the temporal variability of estimated lake trout catch, harvest, and effort of the winter fi shery on Lake Simcoe between 1981 and 2010. Effort was tightly linked to weather conditions and the safety of the ice. In 2010, lake trout winter angling CUE was 0.011 fi sh/angler hr and HUE

2003 2004 2005 2006 2007 2008 2009 2010

Mea

n R

elat

ive

Abu

ndan

ce

0

5

10

15

20

25Stocked Lake Whitefish Natural Lake Whitefish Lake Herring Rainbow Smelt

Figure 10. Temporal changes in the area weighted relative abundance (catch-per-unit-effort) of stocked and natural lake whitefi sh, rainbow smelt, and lake herring, captured during summer gill netting. Relative abundance is expressed as the mean number of fi sh caught per net set annually, weighted by the stratum’s proportion of total surface area. Error bars represent the standard error of the mean.


was 0.009 fi sh/angler hr (for both stocked and natural populations). Winter annual harvested weight (kg/ha) of lake trout has been variable since 1986, with the highest harvest of lake trout occurring in the winter of 2003 with 0.76 kg/ha harvested (Figure 12). Notably, in 2010 harvested weight of lake trout was the lowest observed in the time series (0.16 kg/ha). Winter mean annual weight (kg) of angled lake trout was higher in the 1990s than in the 2000s. However, interannual variability of mean angled lake trout weight was high. Figure 11 suggests that winter angler catch decreased between 2003 and 2010. Together these results suggest that a decline in winter harvested weight (kg/ha) may have been driven by a decline in the number of lake trout caught as well as a decline in the weight of angled lake trout (kg). The winter annual harvested weight of lake trout presented in Figure 12 represents the absolute minimum total annual harvest because data for summer and early fall/winter was not collected (see methods). However, the data represent the period of highest angling effort (i.e., winter).

In most years, winter angling effort was positively related to lake trout catch. However, in 1993 and 2005 an increase in effort was not accompanied by an increase in catch. Low angling success in 1993 was observed for both lake trout and yellow perch and the cause remains unknown (MacRae 2001). Reduced angling success for lake trout in 2005 may have been infl uenced by the reduced contribution of the successful 1995 year class to the fi shery. Estimated lake trout winter angler catch and winter annual harvested weight illustrate a decline between 2003 and 2010, which is congruent with summer gill netting and fall trap netting catch trends. Together these results strengthen the argument that a decrease in the relative abundance of stocked lake trout occurred between 2003 and 2010.

Natural lake trout did not contribute meaningfully to the winter fi shery between the early 1980s and 2000s. In 2003, winter anglers once again captured natural lake trout. The proportion of natural lake trout caught by winter anglers increased to over 40% of the total lake trout catch in 2009 and 2010. For the same years, the proportion of natural lake trout captured in the winter fi shery was greater than the proportion captured in summer gill netting. This may suggest a particularly strong year class had entered the fi shery (and was not yet recruited to summer gill netting gear), or natural lake trout were more vulnerable to winter anglers than their stocked counterparts.

1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010Est

imat

ed W

inte

r Ang

ler L

ake

Trou

t Cat

ch a

nd H

arve

st

0

5000

10000

15000

20000

25000

Est

imat

ed W

inte

r Ang

ler E

ffort

(Ang

ler h

rs *

1000

)

0

200

400

600

800Catch Harvest Effort

Figure 11. Estimated winter angler catch and harvest (bars), and total effort (circles) of the lake trout fi shery based on several 50 day standardized roving winter creel surveys (January to March) from 1981 to 2010. Error bars represent the standard error of the mean. Standard error estimates are not available from 1981 to 1983.


In Lake Superior the catchability of stocked and natural lake trout differed and it was suggested that differences in bathymetric distribution may be an important factor (Nieland 2006).

Anglers sought primarily yellow perch, smallmouth bass (Micropterus dolomieu), and lake trout in the summer recreational fi shery on Lake Simcoe. Lake trout targeted effort in 2010 was estimated as 15.1% of total targeted fi shing effort for all species. In 2010, lake trout angling CUE was 0.03 fi sh/angler hour, and HUE was 0.02 fi sh/angler hour (for both stocked and natural populations). Summer total angling effort fl uctuated between 1.68 angler hrs/ha in 2005, 0.88 angler hrs/ha in 2009, and 1.63 angler hrs/ha in 2010. Long-term trends in the summer recreational fi shery reveal that targeted fi shing effort for lake trout in the summer had declined between the early 1980s and 1998 (Allen et al. 1998). Figure 13 illustrates a decreasing rate of lake trout catch and harvest between 1981 and 2005, with an increase in 2009 and 2010. Total effort also decreased between 1981 and 2009, with an increase in 2010.

1986 1989 1992 1995 1998 2001 2004 2007 2010

Lake

Tro

ut H

arve

st (k

g/ha

)

0.0

0.2

0.4

0.6

0.8

1.0

Figure 12. Estimated winter annual harvested weight (kg/ha; total lake surface area) of lake trout from the recreational winter fi shery from 1986 to 2010. Mean annual weight of angled lake trout is estimated from winter angler sampling data for those years with estimated angler catch and harvest data. The overall mean (all sampling years) of harvested weight is demarcated by the solid line.


1980 1985 1990 1995 2000 2005 2010

Est

imat

ed S

umm

er L

ake

Trou

t Cat

ch a

nd H

arve

st

0

2000

4000

6000

8000

10000

Est

imat

ed S

umm

er A

ngle

r Effo

rt (A

ngle

r hrs

*10

00)

0

50

100

150

200

250

300

350CatchHarvest Effort

Figure 13. Estimated summer angler catch and harvest (bars), and total effort (circles) of the lake trout fi shery based on several roving summer creel surveys (May to September) from 1980 to 2010. Error bars represent the standard error about the mean.

3.4 Lake Trout Biological Attributes

3.4.1 Length and Age Frequency DistributionsThe length frequency distribution of captured male and female stocked lake trout suggested that the summer gill netting program more frequently caught smaller males than females (Figure 14). However, there were few fi sh in each of the 10 mm size bins. The age distribution of captured male and female lake trout is shown in Figure 15. Females were most frequently caught at age six, while aged four and six year old males were caught at the same frequency (Figure 15).

3.4.2 GrowthEstimated von Bertalanffy growth (VBG) parameters for stocked lake trout captured in the winter angler sampling and summer gill netting programs are shown in Table 3. The predicted (VBG) and observed growth trajectories are shown in Figure 16. Annual data were pooled within a program to highlight the difference, if any, between the last two decades (winter angler sampling was not conducted in 2002 and 2007). Between the 1990s and 2000s there was a decrease in the estimated maximum theoretical length (L∞) as well as growth per year (ω) of stocked lake trout observed in the winter angler sampling program (Table 3). Lake trout captured in the summer gill netting program exhibit a lower growth trajectory than those captured in the winter angler sampling program (Figure 16). This observation was most likely related to the timing of sampling. For example, summer gill netting was conducted in the middle of the growing season while winter angler sampling was conducted during a period when individuals had completed most of their growth for the year. The growth parameters suggested that stocked lake trout in Lake Simcoe grew faster (K > 0.19) than the provincial average (K = 0.16; Shuter et al. 1998). Lake Simcoe’s stocked lake trout population displayed a growth trajectory and life history that was similar to lake trout populations in other exploited (Healey 1978a), large lakes (i.e., Lake Erie; Coldwater Task Group 2010).

Mean size at age of stocked lake trout was examined for the most frequently caught ages in all three sampling programs (age four, fi ve, six, and seven years old). The time series of stocked lake trout mean size at age fi ve and six (relative to an overall mean)


0

5

10

15

20

25

30

35

40

45

1 3 5 7 9 11 13 15 17 19 21 23 25 27 32Age

Num

ber o

f Lak

e Tr

out

MaleFemale

Figure 15. Age frequency distribution of stocked male and female lake trout captured during summer gill netting from 2003 to 2009.

Table 3. Estimated von Bertalanffy growth function parameters for stocked lake trout captured in winter angler sampling (1989–1999; 2000–2009; WACS) and summer gill netting (2003–2009; OSBIN) programs on Lake Simcoe. Data were pooled within a sampling program to illustrate temporal changes between decades for stocked lake trout aged 3 to 16 years old (i.e., those ages for which there was greater than fi ve individuals). L∞ - maximum theoretical length, k - growth coeffi cient, To - year when length is zero, and is growth per year (mm).

Parameters

Program Sampling Yrs Ages N L ∞ (mm) K To (mm/yr)

OSBIN 2003–2009 3-16 477 724 0.22 -0.6 161

WACS 2000–2009 3-16 1076 790 0.19 -1.3 150

WACS 1989–1999 3-16 1690 834 0.20 -0.6 163

0

2

4

6

8

10

12

14

16

18

20

<400

410-

419

430-

439

450-

459

470-

479

490-

499

510-

519

530-

539

550-

559

570-

579

590-

599

610-

619

630-

639

650-

659

670-

679

690-

699

710-

719

730-

739

750-

759

770-

779

790-

800

Fork Length (mm)

Num

ber o

f Lak

e Tr

out

MaleFemale

Figure 14. Fork length (mm) frequency distribution of stocked male and female lake trout captured during summer gill netting from 2003 to 2009.


Figure 16. Observed (circle) and predicted (line) fork length (mm) at age of stocked lake trout captured during winter angler sampling (WACS; 1989-2009) and summer gill netting (OSBIN; 2003-2009) for both sexes combined. The von Bertalanffy predicted growth trajectory are demarcated by solid lines (1989-1999 WACS – red; 2000-2009 WACS – blue; 2003-2009 OSBIN - green).

Age

2 4 6 8 10 12 14 16 18

Fork

Len

gth

(mm

)

300

400

500

600

700

800

900

WACS 1989-1999WACS - 2000-2009 OSBIN 2003-2009

are illustrated in Figure 17 and 18, respectively. Years prior to 2006 had a strong infl uence on the overall mean size at age due to a markedly higher fall trap netting sample size in those years. Fall trap netting data illustrated a decline in size at age fi ve between 2002 and 2006 (Figure 17). However, fall trap netting samples represent only a portion of the spawning population. Variable size at age fi ve with an overall declining trend between 2003 and 2010 was observed for stocked lake trout captured by winter anglers. Similarly, lake trout captured in the summer gill netting program displayed variable mean size at age fi ve between 2003 and 2010. However, the small contemporary sample size of winter angler sampling and summer gill netting data (2007–2009, N < 10 per age class) did not allow an assessment of the magnitude of change in size at age fi ve for stocked lake trout. Stocked lake trout mean size at age six decreased between the 1990s and 2000s and variability about the mean increased post 2000 (Figure 18). Metcalfe (2008) also documented decreased size at age for stocked lake trout in Lake Simcoe beginning in the 1990s. The VBG parameters, growth trajectory, and size at age data all suggest that there has been a decrease in growth of stocked lake trout between the 1990s and 2009. However, this decrease may be within the magnitude of past changes (Metcalfe 2008). Fabrizio et al. (2001) suggested that a decrease in growth may have been associated with a reduction in food availability for a recovering population of lake trout in Lake Superior. Similarly, Madenjian et al. (1998) found that offshore lake trout which endured low prey food availability (of appropriate size) experienced slower growth rates than nearshore lake trout with ample prey food availability (of appropriate size) in Lake Michigan.

Fish growth can be infl uenced by several factors, including forage availability at various life stages, life history decisions, habitat, and intra and interspecifi c interactions (Martin 1970, Madenjian et al. 1998; Matuszek et al. 1990; Mebane and Arthaud 2010). A decrease in growth may suggest that any one of the above factors is no longer favourable for the lake trout population. Furthermore, Evans and Waring (1987) stated that changes in growth rate may lead to a change in recruitment. Regardless of the cause, decreased growth is a cause for concern because it may be a signal of a stressed population (Mebane and Arthaud 2010). It is unlikely that the decrease in growth of stocked lake trout was caused by changes during hatchery development or the actual stocking event. Stocking practices have been consistent between the 1990s and 2009, and lake trout size at the time of release has increased


1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Mea

n La

ke T

rout

For

k Le

ngth

(mm

) at A

ge S

ix

400

450

500

550

600

650

700

FITNWACSOSBIN

Figure 18. Mean fork length (mm) at age six of stocked lake trout captured from 1989 to 2009 during winter angler sampling (WACS), summer gill netting (OSBIN), and fall trap netting (FITN). The overall mean (all years and programs pooled) is indicated by the horizontal solid line with 95% confi dence limits about the mean demarcated by the dotted lines. Error bars represent the standard deviation of the annual mean.

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Mea

n La

ke T

rout

For

k Le

ngth

(mm

) at A

ge F

ive

400

450

500

550

600

650

FITNWACSOSBIN

Figure 17. Mean fork length (mm) at age fi ve of stocked lake trout captured from 1989 to 2009 during winter angler sampling (WACS), summer gill netting (OSBIN), and fall trap netting (FITN). The overall mean (all years and programs pooled) is indicated by the horizontal solid line with 95% confi dence limits about the mean demarcated by the dotted lines. Error bars represent the standard deviation of the annual mean.


between 2000 and 2010. Determining the causal factors leading to a decrease in stocked lake trout growth is needed in order to support the management of stocked and natural lake trout populations in Lake Simcoe. A discussion of an observed change in lake trout forage base and potential implications for growth are discussed in section 3.5.3.

3.4.3 ConditionThe condition of stocked lake trout based on all sampling program data in 2000, 2005, and 2010 were provided in Table 4. The 95% confi dence intervals of the condition coeffi cient ‘b’ from the 2010 winter angler sampling data overlaps nearly all estimates of ‘b’ among programs and years (Table 4). The data suggests that condition has been consistent over the last decade for stocked lake trout, despite a decrease in growth.

The difference in estimated condition of stocked lake trout among sampling programs likely refl ects the infl uence of seasonality. The results presented in Table 4 are also similar to those presented in MacRae (2001) suggesting condition has been consistent over a sustained time period, despite changes in forage abundance and other variables. A decrease in growth (size at age) not accompanied by a decrease in condition suggests that stocked lake trout weight (g) has also decreased. Lake trout populations from small southern inland lakes typically exhibit a mean condition coeffi cient of 3.18 (McDermid et al. 2010; Payne et al. 1990), while populations from the Laurentian Great Lakes have been found to vary between 3.11–3.19 (excluding humper and siscowet morphotypes, Carlander 1969). Table 4 suggests that mean condition of stocked lake trout in Lake Simcoe in the last decade (2000–2010) often fell below these typical values.

3.4.4 SurvivalCatch curve analysis (Ricker 1975) was used to estimate the annual survival of stocked lake trout among fi ve year intervals. Total mortality (A) between 2005 and 2009 was estimated as 19.5 %, or 80.5% annual survival (S), based on summer gill netting data (Table 5). Total mortality and annual survival between 1990 and 2005 were fairly consistent based on winter angler sampling data. The annual survival estimate derived from winter angler sampling data between 2005 and 2009 was within 6 % of the estimate derived from summer gill netting data of the same time period. The annual survival estimates presented in Table 5 are higher than those presented in MacRae (2001). Higher survival rates post 2000 was unexpected given the recent decline in the relative abundance of stocked lake trout, but is at least partially explained by the exclusion of lake trout greater than 15 years old in the current analysis (where data were insuffi cient). Healey (1978a) also argued that annual survival can be over-estimated during periods of poor year class success; stocked lake trout in Lake Simcoe displayed poor year class success between 1996 and 2001 (Figure 8 and 9). Additionally, Evans and Olver (1995) argued that an increase in lake herring abundance can restrict the growth and survival of stocked

Table 4. Condition of stocked lake trout in 2000, 2005, and 2010 captured during winter angler catch sampling (WACS), summer gill netting (OSBIN), and fall trap netting (FITN). Condition was estimated using the weight-length relationship outlined in Anderson and Neumann (1996); where the condition coeffi cient, b, is the slope, and the length coeffi cient, a, is the intercept. The 95% upper (CI U of b) and lower (CI L of b) confi dence intervals of the slope (b) are shown. The r2 value represents the degree of relationship between mean weight and length.

Program Year N b CI U of b CI L of b a r2 (%)

FITN 2000 105 3.18 2.99 3.38 3.97x10-6 91

2005 82 2.99 2.86 3.13 1.23x10-5 96

WACS 2000 207 2.90 2.81 2.98 2.23x10-5 95

2005 196 2.92 2.82 3.01 1.77x10-5 95

2010 21 2.70 2.73 3.34 9.93x10-6 95

OSBIN 2005 86 3.10 2.97 3.23 7.89x10-6 98

2010 27 2.98 2.77 3.18 1.55x10-5 97


lake trout. Lake herring relative abundance increased in Lake Simcoe between 2005 and 2009 (Figure 10); however the infl uence, if any, on stocked lake trout survival remains to be seen. Annual survival of stocked lake trout in Lake Simcoe (80–86%) was greater than other inland exploited lake trout lakes, such as Lake Joseph (68%, Sandstrom 2010) and Lake Opeongo (65%, Shuter et al. 1998).

Annual estimates of lake trout survival for both the stocked and natural populations will continue to be an essential criterion to evaluate population health. Future estimates will require enhanced data collection, relating survival to decreasing relative abundance, and the delineation of fi shing and natural mortality.

3.4.5 Maturity ScheduleThe age at which 50% of stocked male and female lake trout reached sexual maturity was estimated as three and four years old, respectively (Table 6). Table 6 illustrates that greater than 90% of male stocked lake trout were fully mature at age four, and females at age fi ve. These results are consistent with historic fall trap netting data from

Table 5: Estimated total annual survival and mortality based on a weighted catch curve regression of stocked lake trout captured in winter angler sampling (WACS; 1990–2009) and summer gill netting (OSBIN; 2005–2009). Data were pooled on fi ve year intervals to illustrate temporal changes in total annual mortality for those fi sh fully recruited to the gear. Slope of the regression equation is the estimate of instantaneous annual rate of total mortality (Z). The inverse of this natural logarithm value is the annual survival (S) rate, and annual mortality (A) is annual survival minus one (Ricker 1975).

Period Program Years of Age N Z S (%) A (%)

2005-2009 OSBIN 6-15 161 0.22 80.5 19.5

2005-2009 WACS 5-15 239 0.15 86.3 13.7

2000-2004 WACS 5-15 684 0.18 83.4 16.6

1995-1999 WACS 5-15 536 0.30 74.4 25.6

1990-1994 WACS 5-15 549 0.19 83.0 17.0

*Missing years WACS: 2002,2007

Table 6: Frequency of non-spawning (Non-sp) and spawning (Sp) stocked A) male and B) female lake trout captured during summer gill netting. All years were pooled from 2003 to 2009. Mean fork length (mm) at age is also presented for the same time period.

A) Male

Maturity

Age Non-Sp Sp % Mature Mean FL

1 1 0 0 188

2 6 0 0 299

3 9 10 53 411

4 4 37 90 486

5 2 33 94 517

6 2 39 95 538

7 0 15 100 572

B) Female

Maturity

Age Non-Sp Sp % Mature Mean FL

2 5 0 0 275

3 24 1 4 398

4 9 11 55 482

5 3 31 91 524

6 2 37 95 573

7 1 14 93 612

8 0 22 100 612


the 1990s where full (100%) maturity was estimated as four years old for males and fi ve years old for females (Amtstaetter 1999). The maturity schedule of stocked lake trout in Lake Simcoe is similar to other large lake populations (i.e., Lake Erie) which is also sustained by stocking and endures high fi shing pressure (Coldwater Task Group 2010).

The proportion of male and female lake trout (sex ratio) captured in the fall trap netting program between 1989 and 2006 suggested a skewed sex ratio in favour of male fi sh (Table 7). However, over 34% of the catch was consistently recorded as unknown sex but these fi sh were suspected to be almost exclusively female (R. Allen pers.comm. 2011). Thus fall trap netting data would present a more balanced sex ratio (closer to 1:1) if the unknown lake trout were considered in the proportion of females caught. Male lake trout spend a greater amount of time on the spawning shoals and may be more vulnerable to the trap netting gear. Further support of a balanced sex ratio in the stocked lake trout population was provided by summer gill netting data which suggests the sexes have contributed almost equally to the total catch each year between 2003 and 2009 (Table 8). The proportion of males and females stocked annually is unknown as fi sh were released prior to sexual maturity.

Year Male (%)

Female (%)

Unknown (%)

1989 37.2 28.6 34.2

1990 47.1 13.6 39.3

1991 43.2 12.2 44.6

1992 46.4 8.8 44.8

1993 46.9 8.1 45.0

1994 48.6 9.2 42.2

1995 47.4 5.0 47.6

1996 58.2 4.5 37.3

1997 36.2 15.8 48.1

1998 42.2 13.4 44.4

1999 54.7 11.5 33.8

2000 45.5 14.8 39.7

2001 42.6 9.6 47.8

2002 46.8 9.3 44.0

2003 37.2 18.8 44.0

2004 43.2 2.7 54.1

2005 41.8 2.8 55.4

2006 48.5 17.9 33.6

Average 46.8 18.8 34.4

Table 7. Percentage of male and female stocked lake trout captured during fall trap netting from 1989 to 2006.

Male (%) Female (%)

Unknown (%)

N

2003 42.9 34.5 22.7 119

2004 49.5 38.4 12.1 99

2005 60.9 37.9 1.1 87

2006 50.0 37.5 12.5 72

2007 55.6 27.8 16.7 54

2008 45.7 51.1 3.2 94

2009 45.2 51.6 3.2 31

Average 50.0 39.8 10.2 556 (total)

Table 8. Percentage of male and female stocked lake trout captured during summer gill netting from 2003 to 2009.


3.4.6 FecundityFemale stocked lake trout relative fecundity (number of eggs/kg total weight) was maximized between seven and eight years old (Figure 19). Mean relative fecundity was estimated as 2,145 eggs/kg (+/- 518 eggs/kg) between 2008 and 2010. Female stocked lake trout in Lake Simcoe appeared to be very fecund compared to other inland populations. Healey (1987b) reported that fecundity of northern lake trout populations ranged between 840–1,700 eggs/kg. Similarly, Shuter et al. (1998) reported mean fecundity of mature female lake trout from 13 Ontario lakes as 1,506 eggs/kg. In 2008, 2009, and 2010 mean Lake Simcoe female stocked lake trout fecundity was estimated at 2,354, 2,126, and 1,917 eggs/kg, respectively. Mean fork length (F 2,58 = 0.25, P > 0.05) and round weight (F 2,58 = 0.56, P > 0.05) of female lake trout sampled among 2008, 2009, and 2010 did not differ signifi cantly, suggesting that the observed decline in fecundity was not due to sampling size bias.

3.5 Lake Trout In A Changing Lake Simcoe

3.5.1 HabitatLake trout require well oxygenated coldwater habitat in order to thrive. Land-use changes and urbanization in the Lake Simcoe watershed led to decreased suitable habitat for lake trout in the 1950s and1960s which ultimately led to their recruitment failure and population collapse (Evans et al. 1991). Until 2001, no natural recruitment was observed and this was attributed to the unsuitable water quality (Willox 2001). Young et al. (2010) summarized several decades of water quality information including mean annual late summer volume weighted hypolimnetic dissolved oxygen (MVWHDO). MVWHDO is frequently used to estimate the quantity of optimal (cold and well oxygenated) lake trout habitat within a lake (Evans 2005). Beginning in 1996 and continuing until 2008, an increase in MVWHDO concentration was observed in Kempenfelt Bay (Young et al. 2010). Increased MVWHDO implies more favourable conditions for juvenile lake trout survival and may have contributed to the observed increase in natural lake trout recruitment. Conversely, habitat improvement may be inconsistent with the recent decline in the relative abundance of stocked lake trout.

Age

4 6 8 10 12 14 16 18 20

Lake

Tro

ut R

elat

ive

Fecu

ndity

(egg

s/kg

)

1000

1500

2000

2500

3000

3500

4000

4500

Figure 19. Relative fecundity (eggs/kg) at age of stocked lake trout captured during modifi ed fall trap netting in 2008 and 2009. Relative fecundity is expressed as the number of eggs produced per gram of total mass (round weight in grams).


Spawning shoal evaluations to determine the condition of spawning habitat were last undertaken in the 1970s. By 1979 over 95% of known spawning shoals were buried beneath silt and other material (Thorn et al. 1979). Nutrient loading and eutrophication were identifi ed as the main cause of spawning shoal habitat degradation (MacRae 2001). Young et al. (2010) reported recent improvements in water quality parameters (nutrient loading and phosphorous concentration) in Lake Simcoe. It is not known if the recent improvement in water quality has translated into improved spawning shoal condition or the reappearance of successful natural reproduction.

The infl uence of dreissenid mussel invasion and colonization of potential lake trout spawning shoal habitat is of concern. Marsden and Chotkowski (2001) report that the presence of zebra mussels on spawning shoals may reduce lake trout egg deposition and increase the damage to eggs. The high density of zebra mussels on spawning shoals may also occlude interstitial spaces necessary for lake trout egg protection from several nearshore predators (Marsden and Chotkowski 2001). In 1999, the LSFAU undertook an evaluation of unionid abundance on the Georgina Island Shoal and surrounding Georgina Island shoreline (including locations near the historical fall trap netting site). The survey results showed that zebra mussels covered 50–99% of the available substrate at 24% of all sampling locations; all sampling sites were located in depths less than 2.5 m (LSFAU unpublished data). The contemporary density of zebra and quagga mussels on potential spawning shoals in Lake Simcoe is not known. The presence of quagga mussels, which prefer deeper habitat than zebra mussels, may negatively infl uence natural lake trout reproduction on deep water shoals (Marsden and Chotkowski 2001). Determining the density of dreissenid mussels on lake trout spawning shoals will improve our understanding of the factors infl uencing lake trout recruitment in Lake Simcoe.

In the Great Lakes it has been suggested that an increase in water clarity may lead to decreased lake trout movement and reduced capture of lake trout in netting programs (Markham et al. 2008). Young et al. (2010) reported a consistent increase in water clarity in Lake Simcoe beginning in 1996 that has been attributed to the invasion of zebra mussels and phosphorous reduction. The infl uence of increased water clarity on the catchability of lake trout in the LSFAU’s summer gill netting program is not known.

Climate change is predicted to alter the depth, strength, and timing of the thermal stratifi cation process in lakes (King et al. 1999). The infl uence of climate change on biotic and abiotic lake processes may lead to changes in several lake trout life history characteristics (Shuter and Lester 2004); however, the large size of Lake Simcoe may mediate the immediate effects of warming surface waters on vulnerable fi sh populations (King et al. 1999). Kopf and Evans (2011) suggest that the timing of lake trout spawning activity may also be infl uenced by climate change.

3.5.2 Exotic SpeciesSeveral exotic species have recently become established in Lake Simcoe, including zebra and quagga mussels, rusty crayfi sh, spiny water fl ea, and the round goby (Dietrich et al. 2006; McNeice and Johanson 2004; Moles 2010). The infl uence of the above species on lake trout and the entire fi sh community are not known for Lake Simcoe but may be extensive. For example, zebra mussels invaded Lake Simcoe in 1996 and have since established in vast numbers (McNeice and Johanson 2004). Mussels may divert the energy fl ow and availability of planktonic resources from the pelagic food web to the littoral zone which may impact planktivorous juvenile and adult life stages of several coldwater species (Johannsson et al. 2000; MacIsaac 1996; Ozerksy 2010). For example, Mohr and Ebener (2005) associated a negative change in lake whitefi sh body condition with the invasion of dreissenids and loss of the deepwater amphipod, Diporeia. An increase in the water clarity of Lake St. Clair has been linked to a decrease in phytoplankton abundance caused by zebra mussel fi ltering (Vanderploeg et al. 2002). Young et al. (2010) reported an increase in secchi disk depth (surrogate for water clarity) at several sampling locations in Lake Simcoe beginning in 1996 and has linked the increase to the proliferation of zebra mussels.


The spiny water fl ea has become a major dietary item of the lake whitefi sh in Lake Simcoe (Amtstaetter 2002) but not for lake trout (Johanson 2001; LSFAU unpublished data). Madenjian et al. (1998) identifi ed that the spiny water fl ea contributed substantially to the diet of small offshore lake trout (between 200–399 mm) in Lake Michigan. Madenjian et al. (1998) also noted that the same lake trout grew slowly and experience low prey fi sh abundance.

Egg predation by exotic species is also a cause for concern in Lake Simcoe. Jonas et al. (2005) documented that round gobies, mottled sculpin (Cottus bairdii), and the rusty crayfi sh are all predators of lake trout eggs. Rusty crayfi sh (established in 2006) have become very abundant in Lake Simcoe. Egg predation was not likely responsible for the decreased relative abundance of stocked lake trout given that the stocked population was planted as yearlings. However, it is not known if the success of lake trout natural reproduction in Lake Simcoe has been impacted by the range expansion of rusty crayfi sh and round goby.

3.5.3 Diet and Food WebJohanson (2001) demonstrated that rainbow smelt were the primary prey item of adult lake trout and had contributed heavily to lake trout diet since rainbow smelt introduction in the 1960s. This suggests that rainbow smelt were the primary prey item even before the decline of lake herring. Rainbow smelt also replaced lake herring as the preferred prey of lake trout after their introduction into Lake Erie (Coldwater Task Group 2010). The decline of lake trout growth and relative abundance in the 2000s lagged only a few years behind the observed decline in rainbow smelt angler catch in the 1990s. However, Evans and Waring (1987) suggest that rainbow smelt abundance may be up to seven times greater than represented by angler catch. LSFAU unpublished data showed that rainbow smelt made up the largest proportion (by weight and number relative to other species in the diet) of adult lake trout diet in 2008. Lake trout were observed to forage substantially on round gobies after their introduction into Lake Erie and Lake Ontario (Coldwater Task Group 2010; Dietrich et al. 2006) but this has not been observed in Lake Simcoe. Dietary information for lake trout was available for only a snapshot of the summer season and may not represent annual prey item contribution. Madenjian et al. (1998) observed slower growth rates for lake trout with limited access (i.e., low abundance) to appropriately sized prey in Lake Michigan. It may be hypothesized that decreased rainbow smelt abundance in Lake Simcoe in the late 1990s contributed to the observed decline in stocked lake trout growth, based anecdotally on the timing of each event. The infl uence of an increase in relative abundance of rainbow smelt (2005–2009; Figure 10) on lake trout growth and survival remains to be seen. Additionally, the potential infl uence of lake herring as a food source and competitor for lake trout, given the increased relative abundance of lake herring (2006–2009; Figure 10) is not known. Evans and Olver (1995) suggest lake trout—lake herring interactions may negatively impact lake trout under certain conditions.

Early life stage diet can infl uence the life history course of lake trout (Martin 1952). If exotic species and intraspecifi c competition limit juvenile lake trout zooplankton foraging (i.e., zebra mussel induced change in plankton community/abundance, foraging on spiny water fl ea) a change in growth may be expected. If the factors outlined above were negatively impacting the stocked lake trout population, the increased presence of natural lake trout is unexpected. Alternately, these same factors may have masked the natural lake trout resurgence compared to its potential magnitude.

4.0 SummaryThe data presented in this report were collected through a suite of long-term monitoring programs with the intent of providing a contemporary status assessment and detecting temporal changes in the lake trout population of Lake Simcoe. The results presented suggest that several important changes have occurred in the lake trout population over the last two decades, including


• The reappearance of natural lake trout recruitment

• A decrease in the relative abundance of stocked lake trout between 2003 and 2010

• A decrease in the annual winter recreational angler catch of lake trout between 2003 and 2010

• An increase in the prevalence of natural lake trout in winter angler catch in 2009 and 2010

• A decline in the life time growth trajectory and size at age fi ve and six year old stocked lake trout between the 1990s and 2009

Given the observed declines in the relative abundance and growth of the stocked lake trout population, there is strong evidence to suggest that the population has become stressed in the last decade (2000s).

Table 9 summarizes the observed changes in stocked lake trout relative abundance, survival, growth, and condition discussed in this report.

For several variables (total estimated annual winter angler catch, growth, and size at age) high interannual variability was observed. In some cases, the observed variability was within the magnitude of past changes in the time series. High variability may be attributed to several factors, including changes in the spatial distribution of lake trout, sampling variability, or actual changes in the relative abundance and biological attributes of the lake trout population. In this report results were interpreted with a precautionary approach, emphasizing uncertainties in the data where appropriate.

Continued long-term monitoring of recreational fi sheries, index netting and spawning surveys are necessary to maintain an accurate assessment of the lake trout population and increase our understanding of the dynamics of the entire coldwater fi sh community in response to unanticipated stressors.

Several information gaps that, if addressed, would improve our understanding of the trout dynamics in Lake Simcoe include

1) Effectiveness of long-term lake trout stocking efforts

2) Impact of the recreational fi shery on the lake trout population

3) Biological attributes of the natural lake trout population

The Lake Simcoe Protection Plan identifi es the presence and abundance of lake trout as a key indicator of the health of Lake Simcoe (Chapter 3; Lake Simcoe Protection Plan). Continued monitoring of lake trout will allow fi sheries managers to make informed decisions regarding the remediation goals for the lake and the lake trout population.

Table 9: Summary of the temporal changes, by program, in key population attributes of stocked lake trout. Years included in analysis are shown in brackets. ↑ = increased, ↓ = decreased, ↔ = variable or no change, n/a = only one data point available, blank = analysis not conducted.

Program

Attribute

Index of

Abundance

Survival Growth Condition

Winter Angler Sampling ↓ (2003-2010) ↔ (1990-2009) ↓ (1989-2009) ↔ (2000-2010)

Fall Trap Netting ↓ (2000-2006) ↓ (1989-2006) ↔ (2000-2005)

Summer Gill Netting ↓ (2003-2010) n/a ↔ (2003-2009) ↔ (2005-2010)


5.0 Literature CitedAllen, R., Amtstaetter, F., and McMurtry, M. 1998. 1998 Lake Simcoe Summer Creel Survey. Lake Simcoe Fisheries Assessment Unit. Ontario Ministry of Natural Resources. Sutton West, ON.

Amtstaetter, F. 1999. Fall Trapnetting on the spawning grounds of lake trout (Salvelinus namaycush), lake whitefi sh (Coregonus clupeaformis) and Cisco (C. artedi) in Lake Simcoe, 1989–1997. Lake Simcoe Fisheries Assessment Unit. Ontario Ministry of Natural Resources. Sutton West, ON.

Amtstaetter, F. 2002. The status of lake whitefi sh (Coregonus clupeaformis) in Lake Simcoe. Lake Simcoe Fisheries Assessment Unit. Ontario Ministry of Natural Resources. Sutton West, ON.

Amtstaetter, F., and Willox, C. C. 2004. Survival and growth of lake whitefi sh from two stocking strategies in Lake Simcoe, Ontario. N. Am. J. Fish. Manag. 24:1214–1220.

Anderson, R. and Neumann, R. 1996. Length, weight, and associated structural indices. In: Fisheries Techniques (2nd). Ed. Murphy, B.,R., and Willis, D., W. American Fisheries Society, Bethesda, Maryland. pp 447–481.

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YearWinter Creel

Winter Angler Catch Sampling Year

Winter Creel

Winter Angler Catch Sampling

1961 19861962 19871963 19881964 19891965 19901966 19911967 19921968 19931969 19941970 19951971 19961972 19971973 19981974 19991975 20001976 20011977 20021978 20031979 20041980 20051981 20061982 20071983 20081984 20091985 2010

Appendix 1: Summary of winter creel program data availability.

©2012, Queen’s Printer for OntarioPrinted in Ontario, Canada

This publication should be cited as:

Dolson, Rebecca. 2012. Status of Lake Trout (Salvelinus namaycush) in Lake Simcoe. Ont. Min. Natur. Resour., Science and Information Branch, Southern Sci. and Info., SIB ASU Report 2012–1. 35 p.

Cette publication hautement specialisée La situation du touladi (Salvelinus namaycush) dans le lac Simcoe, Ontario n’est disponible qu’en anglais en vertu du Règlement 411/97 qui en exempte l’application de la Loi sur les services en français. Pour obtenir de l’aide en français, veuillez communiquer avec Southern Science and Information Section au ministère des Richesses naturelles au 1-800-667-1840 ou [email protected].

This report was prepared by the Aquatic Science Unit.For more information please contact

Science and Information Branch Aquatic Science UnitLake Simcoe Fisheries Assessment UnitOntario Ministry of Natural Resources

26465 York Rd. 18Sutton West, ON

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1map data source: Ontario Ministry of Natural Resources. (2009). NRVIS (Natural Resource Values Information System) database. Ontario Ministry of Natural Resources, Land Information Ontario, Peterborough, ON. This map should not be relied on as a precise indicator of routes or locations, nor as a guide to navigation. The Ontario Ministry of Natural Resources shall not be liable in any way for the use of, or reliance upon, this map or any information on this map.

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