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Anomalous Spawning of Smallmouth Bassin Nebish Lake, Wisconsin: Implicationsfor Early Spawning and Over-WinterSurvivalPeter James Brown a & Michael Anthony Bozek aa Wisconsin Cooperative Fishery Research Unit , U.S. GeologicalSurvey, College of Natural Resoures, University of Wisconsin-StevensPoint , Stevens Point, Wisconsin, 54481, USAPublished online: 11 Jan 2011.

To cite this article: Peter James Brown & Michael Anthony Bozek (2010) Anomalous Spawning ofSmallmouth Bass in Nebish Lake, Wisconsin: Implications for Early Spawning and Over-Winter Survival,Journal of Freshwater Ecology, 25:2, 169-177, DOI: 10.1080/02705060.2010.9665066

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Anomalous Spawning of Smallmouth Bass in Nebish Lake, Wisconsin: Implications for Early

Spawning and Over-Winter Survival P e t e r James B r o w n a and M i c h a e l Anthony Bozek

Wisconsin Cooperative Fishery Research Unit U. S. Geolo~ical Survey, College of Natural Resoures

university of ~isconsi;i-stevens Point Stevens Point, Wisconsin 54481 USA

ABSTRACT We observed that the smallmouth bass (Micropterus dolomieu) population in

Nebish Lake, Wisconsin spawned twice-once during the traditional spring period and then again in summer, well beyond the typical spawning season for north-temperate lakes. We documented this anomalous spawning behavior and compared the characteristics of smallmouth bass nests built during the two distinct spawning seasons. Smallmouth bass built 463 nests (1 10.2 n e s t s h shoreline) as water temperature was rising toward 20°C. During the summer spawning period, 24 nests (5.7 nestslkm shoreline) were constructed between 1 1 August and 29 August as water temperature was falling. Nests built in summer were significantly farther from shore, in deeper water, farther from cover, larger in diameter, and built by larger males than in the spring.

INTRODUCTION The smallmouth bass (Micropterus dolomieu) has adapted to life in north-

temperate lakes by adopting a spring spawning behavior. Rising spring water temperature triggers nest building by males, usually in shallow, gravel areas, and often associated with physical cover to increase survival on nests (Hubbs and Bailey 1938, Scott and Crossman 1973, Baylis et al. 1993). The largest males spawn first (Ridgway et al. 1991), procuring larger broods, which presumably results in better survival and fitness for offspring (Wiegmann et al. 1992). It is also presumed that producing eggs earlier confers greater fitness to offspring because earlier-spawned individuals can attain greater size during the growing season by acquiring more food and can reduce size-dependent mortality (Oliver et al. 1979, Latto 1992, Ludson and DeVries 1997). Because mortality of young-of-year smallmouth bass can exceed 95% by early fall (Frey et al. 2003), increased growth of earlier-spawned young-of-year is believed to translate into increased survival.

Another presumed ecological advantage that the larger, earlier-spawned young-of- year smallmouth bass have is that by attaining a larger size these fish are more prepared for the rigors of over-winter survival, a period of time when little feeding occurs (Oliver et al. 1979, Shuter et al. 1980). Shuter et al. (1 980) demonstrated through modeling and in laboratory studies that smallmouth bass survival to later year classes is contingent on growing large enough by their first fall to sustain them through the winter. They also used this work to explain the northern limit of smallmouth bass distribution. Lakes north of the 16.6"C mean July air temperature isotherm generally had lower over-winter survival because young-of-year were not able to acquire enough lipid stores during the short growing season. They concluded that smallmouth bass cannot become established in lakes with successive winter-induced year-class failures.

It seems logical from a bioenergetics standpoint that spring-spawned fish should survive best and production of these individuals should regulate future year classes and

a Corresponding author; current address; Montana Cooperative Fishery Research Unit, Department of Ecology, Montana State University, Bozeman, Montana 59717 USA; E- mail: pbrown@montana.edu.

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overall population size. However, anomalous spawning by smallmouth bass has been recently observed in several north-temperate lakes, and we studied it in detail in Nebish Lake where the spring spawning occurred as expected for 46" north latitude, between 16 May and 16 June. A second spawning period occurred during summer (I 1-29 August). In this study, we compared differences in parental males, nesting habitat, and fry abundance during both spawning periods.

METHODS The entire littoral zone of Nebish Lake, Wisconsin, was surveyed for smallmouth

bass nesting activity every other day during the spring of 2001 and again in summer. Three methods were used to search for bass nests. Nests in water <I .0 m deep were observed from a boat with polarized glasses, nests 1.0-2.0 m deep were observed by snorkeling or SCUBA diving, and nests 2.0 - 3.0 m deep were located by towing a SCUBA diver at the deep water edge of the littoral zone. No bass nests were located in water >3.0 m deep. In spring, 463 smallmouth bass nests were located. Since the large number of nests precluded enumeration of eggs, Fry, and habitat variables on all nests, a random sub sample of 49 nests, chosen at intervals throughout the spring spawning period, was examined. These nests were marked with numbered flags and visited every other day. Fry were estimated while they were oriented on the top of the substrate in the black fry stage immediately before they disperse. Fry were visually estimated while SCUBA diving using a 36 x 36 cm grid. At each nest, the number of fry in each grid square (6 cm x 6 cm) was estimated individually, and all grid squares were summed for an estimate of total abundance. The same diver made all estimates, and estimates were corrected for observer bias by verifying estimates on six nests not included in the study. Estimates were verified by first estimating the number of eggs and then counting every egg. A correction factor was developed from the regression of these egg estimates vs. egg counts (number of fry = 0.7499~ + 83.496, ? = 0.98). Estimates of fry abundance for each study nest were then corrected using this regression equation.

During the summer spawning period, fry production was estimated for all nests encountered (24). Because initial egg deposition was not observed during the late summer spawning, the deposition date was back-calculated from the young-of-year black fry stage using the incubation and development times observed at similar water temperatures during spring (8 d). Thus, initial egg deposition was determined to be 20 August. At each nest, the total length of the parental male was visually estimated using the 36 x 36 cm grid. In both spring and summer, immediately after fry dispersed from nests, habitat variables were measured at each nest site. These included nest depth, nest concavity, site slope, distance to shore, distance to nearest active nest, nest diameter, substrate size, substrate embeddedness, cover type (i.e., rock or wood), and distance to cover. Nest depth was measured from the nest rim to the water surface. Concavity was the difference between the water depth at the rim of the nest and the water depth at the center of the nest. Site slope was calculated as the difference between two depth measurements taken 2m from the center of the nest, toward the shore and away from the shore. Distance to shore (the closest landmass) was measured from the center of the nest. Distance to nearest nest was measured from the center of the nest to the center of the nearest nest used during the same spawning season. Nest diameter was measured as the average of two orthogonal measurements made from rim to rim. Substrates located within the nest rim were categorized using particle size classes modified from Wentworth ( 1 922) and Platts et al. (1983)- silt (c0.2 mm diameter), sand (0.2-6.3 mm diameter), gravel (6.4- 76.0 mm diameter), cobble (76.1-149.9 mm diameter), rubble (1 50.0-303.9 mm diameter), small boulder (304.0-609.9 mm diameter), large boulder (2610.0 mm diameter), and bedrock, and the coverage of each size class was visually estimated. Each substrate size larger than sand was assigned an embeddedness rating fiom 0 to 4, with 0

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indicating clean substrate with no fines in the top two layers of larger substrates and 4 being highly embedded substrate (Saunders 2001). Rocks larger than small boulder (>304 mm diameter) and large pieces of woody structure (>50.9 mm dia. and >1.0 m length) within 10 m of the nest were recorded as cover, and the distance to each rock or wood cover item was measured from the nest rim to the closest point on the cover item. Habitat measurements were made by a SCUBA diver and recorded underwater. Differences in nest habitat characteristics were compared between early and late nests using the Mann- Whitney U-test, and alpha was set at 0.05.

Average daily water temperature was calculated for Nebish Lake using linear regression. A subset of temperature measurements recorded for nearby Escanaba Lake was used to develop a temperature model ( R ~ = 0.99). This model was then used to estimate Nebish Lake water temperatures throughout the entire season.

RESULTS Smallmouth bass in Nebish Lake spawned twice during 2001- once in spring from

16 May to 16 June (i.e., initial egg deposition through last young-of-year swim-up) and again in summer fiom 11 August to 29 August (Fig. 1). No nesting activity occurred between 17 June and 1 1 August. Nest construction in spring began on 5 May as water temperature reached 15°C. Larger males were first to move into the littoral zone to excavate new nests and clean old nests. Total length of males guarding nests in spring was 235 f 13.16 mm (mean f 95% confidence interval). Females were first observed in the littoral zone on 2 May, and eggs were laid from 15 May to 4 June. Incubation (i.e., egg to swim-up) lasted an average of 9.6 d and was temperature-dependent (minimum 4 d at 20°C to maximum 16 d at 15 OC), and fry swam fiom nests between 2 1 May and 12 June. A total of 463 nests (1 10.2 nestslkm shoreline) was counted in the spring. Thirty- four percent of spring nests failed (i.e., 0% survival), with average survival to black fry stage of 59% + 9.0 on successhl nests. The highest nest densities occurred on mixed sand, gravel, and cobble shorelines, although some nests (n=15) were found in

Figure 1. Water temperature of Nebish Lake, Wisconsin between 1 May and 1 November 200 1. Shaded areas indicate periods when eggs or fry occupied nest.

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macrophyte (Myriophyllum tenellum) beds with eggs adhered to vegetation. Average nest depth was 0.95 m f 0.2 1 and average nest distance to shore was 8.37 m f 1.65. Nests were excavated in coarse sand and gravel substrates and had an average diameter of 0.50 m rt 0.05 and at an average concavity of 0.1 1 m f 0.0 1. Seventy-four percent of nests had cover present-rock (22%), wood (48%), or both (4%). Of nests with cover present, the average distance to cover was 0.24 m f 0.1 1, and all cover items were within 1.25 m of the nest rim.

Upon emergence, spring-spawned young-of-year smallmouth bass moved into areas immediately adjacent to nest sties (rock and wood habitats within the littoral zone), and parental male guarding continued. Young-of-year were never observed in silt habitats from swim-up to late August. After the parental male stopped guarding offspring, fewer young-of-year were observed in the littoral zone. Smallmouth bass that hatched in spring were on average 14.4 mm f 0.6 on 22 June, and 48.5 mm f 0.7 when nesting activity resumed in late August.

The second spawning period began in summer as lake temperature started to drop (Fig. 1). New nests were excavated and some nests from the spring spawning period were cleaned beginning on I I August. A total of 24 nests was observed during the summer spawning period. Nests were less dense than in spring (5.7 nestslkm shoreline) and were not evenly distributed along the shoreline; they were grouped. Average length of males guarding nests was significantly larger than in spring; (34 1 mm f 18.39). Average nest depth was 2.14 m f 0.1 1, significantly deeper than spring nests (Fig. 2). Late summer nests were also significantly farther from shore at 27.6 m f 5.78. Summer nests were excavated in similar substrates and to a similar concavity, compared to spring nests, but were significantly larger in diameter (mean 0.90 m, f0.06). Summer-hatched young-of year swam from their nests on 29 August when they were 7.4 mm f 0.1 ; they moved into habitats adjacent to nest sites.

Following swim-up in late August, the length-distribution of young-of-year smallmouth bass was bimodal, with two cohorts (i.e., spring-spawned and summer-

Spring Summer Spring Summer

Figure 2. Significant differences in Nebish Lake spring and summer nests. Horizontal bar represents the mean, box represents *l standard error, vertical bar represents represent a 95% confidence interval, and dots represent outliers.

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spawned). Through September and October, summer young-of-year started using habitat similar to that used by spring-spawned fish. As water temperature reached 8°C in late October, spring and summer young-of-year took rehge in cavities found immediately under pieces of cobble. Young-of-year that hatched in the spring had 146 d to develop and store energy before water temperature fell below 8"C, while summer-hatched fish only had 7 1 d to develop before taking refhge. In the following spring, routine electrofishing surveys captured 24 young-of-year smallmouth bass. Length-frequency distribution of fish was again bimodal, with fish varying from 52 mm to 79 mm in the first mode and two fish measured as 12 1 and 122 mm in the second mode (Fig. 3).

DISCUSSION Previous research has described the spawning behavior, chronology, and habitat

used by smallmouth bass in rivers (Reynolds 1965, Pflieger 1966, Lukas and Orth 1995) and lakes (Beeman 1924, Ridgway et. al. 1991, Bozek et al. 2002). Typically, smallmouth bass spawning occurs as water temperature is rising in the spring. Early studies by Beeman (1924) on lakes in New York and Connecticut described mid-May spawning season temperature of 15- 17°C and Tester (1930) described the spawning

250

0 0 16 32 48 64 80 96 112 128

Fish length (mm) Figure 3. Length-frequency histogram of young-of-year smallmouth bass cohorts

collected in Nebish Lake during August 200 1 (age 0) after both spawning periods and June 2002 (age 1).

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season of smallmouth bass in Lake Nipissing, Ontario, as occurring between 8 June and 24 June with water temperature of 12-16°C. More recent studies by Baylis et al. (1993) and Wiegrnann et al. (1 992) of smallmouth bass in Nebish Lake described the spawning season as occurring at water temperature of 15-20°C and between 18 May and 8 June. The timing of the spring spawning is consistent with observations made during other smallmouth bass spawning studies in Nebish Lake (Baylis et al. 1993) and in other north- temperate lakes such as Pallette Lake, Wisconsin (Gillooly and Baylis 1999, Saunders et al. 2002), Lake Opeongo, Ontario (Ridgway et al. 1991), and Two Island Lake, Ontario (Picard et al. 1993) as well as in laboratory studies (Cantin and Bromage 1991).

The observations made during this study clearly deviate from the observations of previous researchers and call into question our understanding of the ecology, energetics, and population dynamics of smallmouth bass spawning in north-temperate lakes. In this study, the population of smallmouth bass in Nebish Lake spawned twice and produced viable (i.e., swim-up) offspring from both spawning periods. The summer spawning period was unexpected and deviates substantially from previous observations and ecological theory on spawning and survival strategies of smallmouth bass. The summer spawning period lasted for 14 d and took place as water temperature was falling in the lake. Fry production of individual summer nests was not significantly different from fry production of individual spring nests.

Summer spawning by smallmouth bass may not be anomalous. Other observations of late spawning activity by smallmouth bass have been made, following these initial observations, in other proximal northern Wisconsin lakes. Smallmouth bass were observed building nests and laying eggs on Little Muskellunge Lake and Katherine Lake during summer of the same year as this study, although detailed observations of survival were not made. Interestingly, no summer nesting activity was observed during 2002 in Nebish Lake. During 2003, nests were observed with eggs in Plum Lake (pen. comm., K. Gauthiera), an ovulating 408 mm smallmouth bass was collected from nearby Pallette Lake, Wisconsin during late July. Further, dissected smallmouth bass from Nebish Lake commonly have eggs and sperm present in the gonads throughout the summer (pen. comm., S. Newmrina).

The habitat used for spawning during both spring and late summer spawning periods in this study was similar to that observed by Tester (1930), Neves (1975) and Saunders et al. (2002) for north-temperate lakes; however, few similarities existed between spawning seasons. Spawning substrates were not significantly different between seasons in this study and were similar to reports from nearby lakes. Saunders et al. (2002) reported 88% and 94% of nests were found in sand or gravel substrates in nearby Pallette Lake and Sanford Lake, respectively. The density of nests during the spring spawning season was clearly much higher than the summer nest density and that reported by Saunders et al. (2002) in nearby lakes. This extremely high density of nests suggests that spawning habitat may be saturated in spring. Moreover, in Nebish Lake smallmouth bass used aquatic macrophytes as substrate in nest construction and egg deposition during the spring spawning but did not do so in summer nests, further suggesting habitat saturation.

Summer nests were significantly farther from shore, deeper, larger in diameter, and farther from cover. In summer, adult smallmouth bass may be using deeper nest sites to take advantage of cooler water in the hypolimnion, which may be similar to spring temperatures. Maximum average daily temperature during nest building was near the lethal limit for smallmouth bass (28"C), and smallmouth bass were only observed building nests after dark. Positioning a nest in deeper water would provide lower temperature at the nest site and shorter distances to deeper, cooler water. After nests were constructed, water temperature dropped and smallmouth bass were observed guarding the

'Wisconsin Department of Natural Resources

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nests throughout the day during young-of-year development. Environmental cues were also different between spawning seasons. In spring, smallmouth bass initiated nesting activity as water temperature was rising to 20°C during a photoperiod of 15.0 h (sunrise to sunset). In the summer spawning, nesting activity began as water temperature was falling toward 25°C during a photoperiod of 13.5 hours.

A drop in water temperature was a prominent characteristic of the summer spawning. While average daily water temperatures were higher during the summer spawning season, they were within the 12-25°C range reported suitable for smallmouth bass eggs by Webster (1945). Many studies have described a rise in temperature as a spawning trigger (Scott and Crossmann 1973, Coble 1975, Baylis et al. 1993); however, the summer spawning in Nebish Lake came as the temperature was falling.

It is unclear whether smallmouth bass attending nests in fall were multiple spawning fish (i.e., fish that had spawned in spring) or if the summer spawning was their first attempt. It is clear that on average males were significantly larger than spring spawning counterparts. Larger centrarchids are able to devote more energetic resources to gonad development and might be able to redevelop or cany viable gametes through the summer (Danylchuck and Fox 1994). Retention or redevelopment of gametes triggered by suitable water temperature may be possible for smallmouth bass and would make the Nebish Lake smallmouth bass population a multiple spawning population. Fox and Crivelli (1 998) suggest multiple spawning in large bodied centrarchids may be an adaptation that allows offspring to take advantage of seasonal variation in resources rather than to maximize yearly gamete output.

While it is not clear why smallmouth bass spawned in the summer, several studies are worth revisiting. In Nebish Lake, larger males spawn first, attract larger females procuring larger broods and have higher survival of offspring than smaller males (Wiegmann et al. 1992). Also in Nebish Lake, there are two sizes of the same age fish, and a large number of these fish fail to breed during any given spring (Baylis et al. 1993). Baylis et al. (1993) proposed an alternating life history pattern. Fish that are hatched later in the spawning season are not able grow large enough by their third year to spawn and therefore spawn early in their fourth year. Wiegmann et al. (1997) M e r studied the Nebish Lake population, synthesized the two previous studies (Wiegmann et al. 1992, Baylis et al. 1993), and proposed that parental size at first reproduction determines the life history pathway for offspring. Smallmouth bass that hatch early in the spawning season can develop and spawn late in their third year; however, smallmouth bass that spawn late in the season cannot develop until their fourth year. Our research suggests that the fish spawning in their fourth year may be fall-spawned.

Shuter et al. (1980) developed models for over-winter survival of young-of-year smallmouth bass. They theorized that fish hatched near the end of the spring spawning period are not able to grow and develop sufficient lipid stores before the onset of winter and therefore have poor survival. They also found higher mortality rates for smaller young-of-year smallmouth bass when winter conditions lasted longer than 200 days. In Nebish Lake, we found two distinctly different ages of fish entering winter. Spring- hatched young-of-year had at least 2,800 growing degree-days before winter, while late summer-hatched fish had less than 1,035 degree-days to build energy stores before winter.

The specific reasons for a multiple spawning strategy in smallmouth bass remain unclear, and these observations raise additional questions. Why would fish expend energy on gametes that will not contribute to the population? In this study, 48 smallmouth bass spawned in late August when their offspring would not, according to current theory, recruit to the population. Do these two separate groups of fish have a genetic basis for differentiated spawning strategies? Steelhead have evolved multiple strains to take advantage of varying resource availability (Withler 1966, Scott and Crossman 1973), and

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other centrarchids have evolved multiple spawning strategies to provide better environmental conditions for offspring (Fox and Crivelli 1998). Is the littoral zone habitat in certain lakes limited such that some fish are forced to wait until summer in order to spawn? Given the sub-optimal habitat used during spring (i.e., macrophytes), this is plausible; however, it is unlikely because small fish would be forced to wait until summer to spawn. In fact larger fish were observed spawning in summer which indicates that other mechanisms are in place. Can a smallmouth bass that is hatched in late August survive the winter? Using the descriptions of summer young-of-year and the criteria of Shuter et al. (1980) it is plausible that summer young-of-year could survive winter, and our 2002 spring electrofishing survey of Nebish Lake confirms this.

ACKNOWLEDGEMENTS M. Brown, B. Achuff, B. Tomnson, M. Catalano, M. Goerlitz helped with underwater surveys. This research was carried out under Wisconsin Department of Natural Resources collectors' permit number: scpnor-174-0606.

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Received: 8 September 2009 k a p t e d : 2 December 2009

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at 0

2:07

09

Dec

embe

r 20

14