Proceedings of the Fourth International Workshop on Lobster Biology and Management, 1993

THE INFLUENCE OF TEMPERATURE, SALINITY, AND

POSTLARVAI, TRANSPORT ON THE DISTRIBUTION OF JUVENILE SPINY LOBSTERS, PANULIRUS ARGUS (LATREILLE, 1804), IN

FLORIDA BAY

BY

JENNIFER M. FIELD 1) and MARK J. BUTLER, IV2)

Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529-0266, U.S.A.

ABSTRACT

Florida Bay is the major nursery for the Caribbean spiny lobster, Panulirus argus, population in south Florida. This region is characterized by a series of shallow hardbottom or seagrass-covered basins separated by carbonate mudbanks that serve as barriers to water circulation and presuma- bly to transport of planktonic larvae. Temperatures fluctuate dramatically in the bay and salinities there range from 35 ppt-50 ppt. In this study, we investigated the physiological tolerance of P. argus postlarvae to various combinations of temperature and salinity representative of the condi- tions found in Florida Bay and also determined the extent of postlarval recruitment into the interior of the bay.

We measured postlarval settlement, juvenile abundances, and postsettlement habitat availabil- ity monthly (March 1992-July 1992) along 5 transects extending north from the main Florida Keys into the interior of Florida Bay. Concurrently, P. argus postlarvae were reared in the laboratory, in a completely crossed design, at four temperatures (18°C, 22°C, 29°C and 33°C) and four salinities (25, 35, 45 and 50 ppt). Survival, time:-to-metamorphosis, and growth to the first juvenile stage were measured.

Few postlarvae settled at sites beyond the emergent banks ringing Florida Bay, and lobsters were only found in one basin (Twin Keys Basin) where habitat and environmental conditions were favorable. Laboratory results indicate that at high (33°C) and low temperatures (18°C) survival at salinities other than 35 ppt is greatly reduced. Our results indicate that postlarvae are not regularly transported into the interior of Florida Bay and that the lack of suitable nursery habitat and high salinity further limit recruitment, especially at extreme temperatures. Given the present conditions in Florida Bay, recruitment of P. argus is restricted to the southernmost reaches of Florida Bay nearest the Florida Keys.

RÉSUMÉ

La baie de Floride est la principale nursery pour la population caraïbe de la langouste Panulirus argus en Floride du Sud. Cette région est caractérisée par une série de bassins peu profonds à fond dur ou couvert d'algues, séparés par des bancs de vase carbonatée qui servent de barrière à la circulation de l'eau et, suppose-t-on, au transport des larves planctoniques. Les températures

1) Currcnt address: Oak Ridge National Laboratories, Environmental Sciences Division, P.O. Box 2008, Oak Ridge, TN 37831-6037, U.S.A. 2) Corresponding author.

27

fluctuent de façon spectaculaire dans la baie et les salinités vont de 35 ppt à 50 ppt. Au cours de cette étude, nous avons testé la tolérance physiologique des postlarves de P. argus à différentes combinaisons de température et salinité, représentatives des conditions observées dans la baie de Floride, et aussi déterminé l'importance du recrutement postlarvaire à l'intérieur de la baie.

Nous avons mesuré chaque mois (mars 1992-juillet 1992) la sédentarisation postlarvaire, les abondances de juvéniles et la disponibilité des habitats de post-sédentarisation le long de 5 transects s'étendant du nord des principales Florida Keys jusqu'à l'intérieur de la baie de Floride. Simultanément, des postlarves de P. argus étaient élevées au laboratoire, sous les combinaisons complètes de 4 températures (18°C, 22°C, 29°C et 33°C) et de 4 salinités (25, 35, 45 et 50 ppt). La survie, le temps jusqu'à la métamorphose et la croissance jusqu'au premier stade juvénile ont été mesurés.

Peu de postlarves se sont sédentarisées à des endroits situés au-delà des bancs émergés entourant la Baie de Floride, et les langoustes ont été trouvées dans un seul bassin (Twin Keys Basin) où l'habitat et les conditions d'environnement étaient favorables. Les résultats en laboratoire indi- quent que, à haute (33°C) et basse (18°C) températures, la survie à des salinités autres que 35 ppt était fortement réduite. Nos résultats indiquent que les postlarves ne sont pas régulièrement transportées vers l'intérieur de la baie de Floride et que le manque d'un habitat de nursery approprié ainsi qu'une salinité élevée limitent davantage le recrutement, en particulier aux températures extrêmes. Etant donné les conditions actuelles dans la baie de Floride, le recrute- ment de P. argus est restreint aux approches les plus méridionales de la baie de Floride, au plus près des Florida keys.

INTRODUCTION

The Caribbean spiny lobster, Panulirus argus (Latreille, 1804), has a complex life cycle, which in south Florida is intricately tied to the Florida Bay nursery. Postlarvae arrive along the coast of south Florida each month during the new

moon and swim (Calinski & Lyons, 1983) or are transported at night on rising tides through the channels between the Florida Keys and into Florida Bay

(Sweat, 1968; Witham et al., 1968; Little, 1977; Little & Milano, 1980; Marx,

1986). In the bay, postlarvae settle primarily into clumps of the red macroalgae Laurencia spp. (Marx & Herrnkind, 1985; Marx, 1986; Herrnkind & Butler, 1986; Butler & Herrnkind, 1991) where they metamorphose into the benthic

juvenile stage. Juvenile lobsters are solitary and remain within the algae until

reaching 20 mm carapace length (CL) when they become gregarious and reside

under sponges, coral heads, solution holes, seagrass undercuts and other struc-

tures (hereafter referred to as "post-algal" stage juveniles; Herrnkind et al., in

press). After 2-3 years, the lobsters migrate, as adults, to the reef (Davis 1978; Davis & Dodrill, 1980, 1989; Kanciruk, 1980).

Processes limiting recruitment of P. argu.s in Florida Bay may operate at both

local and regional scales. Locally, the availability of postlarvae arriving at an

area establish a minimum level of recruitment (Herrnkind & Butler, in press), while the availability of appropriate macroalgae settlement substrate and post- settlement shelter (e.g., sponges, octocorals, etc.) may limit post-settlement survival of juveniles (Marx & Herrnkind, 1985; Herrnkind & Butler, 1986; Smith & Herrnkind, 1992). On a large scale, the physical structure and condi-

tion of Florida Bay may impose regional limits to recruitment. Florida Bay is

characterized by a series of shallow (<3 m) seagrass-covered or hardbottom

28

(carbonate bedrock with patches of sponges, octocorals and small coral heads) basins separated by seagrass-covered carbonate mudbanks <1 1 m in depth

(Zieman et al., 1989). These mudbanks restrict water circulation in the bay and

Holmquist et al. ( 1989a) have suggested that transport of planktonic larvae into

these basins may be inhibited. Most basins are hypersaline (35-50 ppt) due to

reduced freshwater flow from the Everglades and high rates of evaporation

(Enos & Perkins, 1979; Fourqurean et al., 1992; Walters et al., 1992), and

seasonal fluctuations in temperature can be dramatic (Holmquist et al., 1989b). Therefore, physiological tolerance to temperature and salinity may influence

survival of postlarvae gaining access to these basins. Recently, hypersaline conditions within Florida Bay have sparked a cascade of disturbances, includ-

ing the loss of seagrass meadows (Robblee et al., 1992) and blooms of cya- nobacteria that led to the mass destruction of sponges in the bay (Butler et al.,

unpubl. ms.). Thus, the declining availability of sponges, which serve as shelter

for post-algal stage juvenile lobsters, may also create a recruitment bottleneck

on a regional scale (Wahle & Steneck, 1991; Butler & Herrnkind, in press). There appears to be a northern boundary within Florida Bay beyond which

P. argus is not found (Davis & Dodrill, 1989). Is this boundary a result of

physical barriers to transport, limited settlement or post-settlement habitat, or

constraints set by the physiological tolerance of postlarvae to the temperature and salinity conditions present in the region? Only onc study of postlarval

transport and recruitment has been conducted in Florida Bay (Butler & Herrn-

kind, 1992; Herrnkind & Butler, in press). That study did not, however, include

the interior regions of the bay beyond the fringing mudbanks where the spiny lobster distributional boundary lies. Moreover, there is no detailed information

on the distribution and structure of the hardbottom habitats in these areas, and

thus their suitability as lobster nurseries. Similarly, the physiological tolerances

of P. argus postlarvae and juveniles to the temperatures and salinities typical of

Florida Bay are not known. In this study, we evaluated three mechanisms that

might determine the northern limit to the distribution of P. argus in Florida Bay;

(i) postlarval transport, (ii) settlement and postsettlement habitat suitability, and (iii) physiological tolerance of postlarvae to spccific temperature and

salinity regimes.

MATERIALS AND METHODS

Laboratory methods

We examined the effects of temperature and salinity on Panulirus argus post- larvae in a laboratory study conducted from February 1992 to August 1992 at

the Keys Marine Laboratory, Long Key, Florida. Postlarvae were collected at

night on rising new moon tides using stationary plankton nets (1 1 m2 diameter,

29

750 pm mesh) deployed from the Tom's Harbor Channel Bridge, Florida, and

were retained overnight in the laboratory in water of the same temperature and

salinity from which they were collected. The following morning, postlarvae were placed into 400 ml experimental beakers ( postlarva/beaker). Postlarvae

were randomly assigned to treatments and were gradually exposed over 8-12 hrs to the treatment conditions by slowly changing the salinity and temperature within each experimental beaker. Postlarvae were exposed, in a completely crossed design, to four temperatures (18° and 22°C representative of winter

temperatures; and 29° and 33°C representative of summer temperatures) and

four salinities (25, 35, 45, 50 ppt). Postlarvae were exposed to the 18° and 22°C treatments during winter and early spring, and the 29" and 33"C treatments

during summer. These treatments span a range of environmental conditions

present within Florida Bay (Fourquerean et al., 1992). The laboratory experiments were conducted in water tables, each with a

separate sea water system that recirculated sea water between a 40 1 reservoir

(equipped with a carbon and subgravel filter) and up to 20 experimental beakers at a rate of 0.3 1/hr. A 2 cm2 piece of air conditioning filter (which mimics macroalgal structure) was placed in each beaker as a shelter. The

number of replicates per treatment varied from 13 to 52, depending on the

number of postlarvae captured from the plankton. Experimental temperatures were maintained using heaters or by chilling the laboratory room. Salinities

were maintained by adding sea salt (Instant Ocean Brand) or freshwater to

natural seawater. Temperature and salinity were monitored twice daily; tem-

peratures were maintained to within ± 2°C, and salinities were maintained to

within ± 1 ppt. Dissolved oxygen, pH, nitrate nitrogen and ammonia were

monitored weekly. We observed postlarvae twice daily and recorded survival,

time-to-metamorphosis (TTM) to the first benthic juvenile stage, and juvenile

length following metamorphosis (mm carapace length; CL). Survival of postlarvae was analyzed using a three-way log-linear categorical

analysis to test for an interaction between temperature, salinity, and postlarval survival. If the three-way interaction was not significant, we then used two-way

contigency tables analyses to examine the relationship between various tem-

perature or salinity treatments and postlarval survival. The effect of tempera- ture and salinity on postlarval time-to-metamorphosis was analyzed using a

two-factor model I ANOVA on In transformed data. Similarly, we used a two-

factor model I ANOVA on square root transformed data to test the effect of

temperature and salinity on the carapace length of first stagc juveniles at

metamorphosis. The Ryan-Einot-Gabriel-Welsch Multiple F test was employed to determine the significance of the levels of temperature and salinity for both

analyses. The alpha level for significance was set at 0.025 to insure an experi- mentwise error rate of 0.05 for the two separate ANOVAs.

31

at a particular site for a few months may not accurately reflect the annual

potential for natural settlement in the area. Therefore, we censused the post-

algal stage juvenile lobster population near the benthic collector sites as an

independent estimate of recent recruitment. Lobsters < 35 mm CL are not

nomadic (Kanciruk, 1980; Herrnkind & Butler, 1986) and arc likely to have

settled in the vicinity near their capture within the last few months (Yoshimura & Yamakawa, 1988). Post-algal stage juveniles are also diurnal crevice dwellers

and can be visually censused, whereas algal-dwelling juveniles cannot. Surveys of post-algal stage juveniles are, thus, useful measures of recent recruitment

because the number of juveniles ? 35 mm CL at a site represents an integration of settlement and post-settlement survival over the previous 3-5 months (For- cucci et al., in press). Surveys were conducted by two divers who searched and

collected lobsters from natural shelters (e.g., sponges, octocorals, solution

holes, etc.) in an area of hardbottom for 30 minutes (1 diver hour/site), yielding a catch-per-unit-effort index of post-algal juvenile abundance. All lobsters

found were measured (nearest 0.1 mm) and released. Diver surveys were not

conducted in Johnson Key Basin (site 21), Ninemile Bank (site 22), Panhandle

Key Basin (site 23) or the Crab Keys Basin (site 24) due to the lack of suitable hardbottom habitat or juvenile shelter in those areas.

At the same sites where juvenile lobster surveys were conducted, we also

quantified habitat structure. Four haphazardly-positioned 25 m line-transects

were established at each site and the proportion of bottom covered by macro-

algae (primarly Laurencia spp.), seagrass (Thalassia testudinum Koenig), sponge, octocoral, and other potential post-algal stage lobster shelters (e.g., coral heads

and solution holes) recorded. At a subset of sites (10 total), four 25 m X 2 m belt-

transects were established over the original line transects to estimate the density of sponges, octocorals, coral heads, solution holes and other potential lobster shelters. The shortest possible distance (km) to each site from the inlets between

the main islands of the Florida Keys (i.e., the source of postlarvae for the bay), and the number of mudbanks that postlarvae would pass over enroute to a

specific site were also determined, as was the temperature and salinity at each

site.

We used a multiple regression analysis (best-fit approach) to determine

whether the number of settling postlarvae, recently-recruited juveniles 35

mm CL) or larger resident juveniles (>35 mm CL) could be predicted using the

following factors as potential regressor variables: distance to channel, number of banks crossed, salinity, number of postlarvae/benthic collector/month, the

proportion of bottom covered by Laurencia, seagrass, sponge, octocoral or other

habitats, and the density of sponges, octocorals, corals, and solution holes.

Because only a subset of the sites included shelter densities estimated from belt-

transects, as opposed to the line-transect data (proportion of bottom cover) collected at all sites, separate sets of multiple regression analyses were per- formed on these two data sets. An important assumption of multiple regression

32

analysis is that the regressor variables not be correlated. We used Spearmans rank correlation analysis to test for significant correlations among the non-

normally distributed regressor variables collected from the line-transect sam-

pling method, and Pearsons correlation analysis to test for correlations among the normally distributed regressor variables collected using the belt-transect

sampling method. Several regressor variables collected from the line-transect

sampling method could not be transformed to meet the assumption of nor-

mality for multiple regression, therefore, these data were ranked prior to

analysis (Iman & Conover, 1979; Conover & Iman, 1981). Separate analyses were run on the line- or belt-transect habitat data to predict: (i) number of

juvenile lobsters 35 mm CL/site, (ii) number of juvenile lobsters >35 mm CL/

site, and (iii) number of settling postlarvae/site. The dependent variables, the

habitat sampling method employed, and the uncorrelated regressor variables

used for each analysis are listed in table I.

TABLE I

Summary of the multiple regression analyses listing the predicted variables, the

types of habitat data (i.e., data obtained from line- or belt-transects), and the

uncorrelated regressor variables used in each analysis

Predicted variable Habitat data used Uncorrelated regressor variables

Number of juvenile Line-transect Number of mudbanks Panulirus argus <_35 mm CI. Proportion of algae (square root transformed) Proportion of spongcs

Proportion of octocorals

Belt-transect Density of sponges Density of octocorals (In transformed) Density of corals

Number of juvenile Line-transect Proportion of algae Panulirus argus >35 mm CL Proportion of octocorals (square root transformed) Proportion of other shelters

Proportion of sand

Belt-transect Salinity Density of octocorals (In transformed)

. Density of corals Density of solution holcs

Number of postlarvae Line-transect Number of mudbanks settling (In transformed) '

Salinity Proportion of sand

Belt-transect Salinity Density of spongcs Density of octocorals (In transformed) Density of solution holes

33

RESULTS

Laboratory study

The three-way interaction between temperature, salinity, and survival of P.

argus postlarvae to the first benthic juvenile stage approached significance (x2 =

15.57; df = 9; p - 0.08). However, the two-way interactions between the

various levels of temperature or salinity and postlarval survival were all highly

significant in all cases). Survival of postlarvae was highest at 22°C

(55%) and 35 ppt (76%; fig. 2). At temperatures above and below 22°C and

salinities other than 35 ppt, survival of postlarvae declined. When postlarvae were reared at the highest temperature and salinities (i.e., 33°C and 45 and 50

ppt), none survived and < 10% survived extreme salinities (i.e., 25 and 50 ppt) when reared at 18°C. Temperature and salinity interact to influence TTM of

postlarvae (F = 3.15; df = 7; p = 0.004). Postlarval TTM increased with

decreasing temperatures and was greater for postlarvae reared at salinities >45

ppt (fig. 2). However, TTM of postlarvae reared in 35 and 45 ppt was not

significantly different. The interaction of temperature and salinity also signifi-

cantly influenced the size of first stage juveniles at metamorphosis (F = 3.67; df = 7; p = 0.001). Size at metamorphosis was greatest for postlarvae raised at

salinities of 35 ppt and temperatures of 22°C, but the maximum difference

among treatment means (0.31 mm CL) was within measurement error and is

probably not of ecological significance.

Field study

Postlarvae settled in benthic collectors in areas south of the first set of

mudbanks, including: Channel 5 Basin (sites 1-7), several sites within the

Lignum Vitae Basin (sites 10, 17 and 18), and at Ninemile Bank (site 22). Most of these sites (sites 2, 4-10, 13, 14 and 18) also contained juvenile P. argus <35

mm CL (size range 14-<45 mm CL; fig. 3a) indicating that postlarval settle-

ment and recruitment to the postalgal juvenile stage occurred in regions south of the banks. No postlarvae were found within benthic collectors in Johnson

Key Basin (site 21), the Twin Keys Basin (sites 11, 12 and 19), in Panhandle

Key Basin (site 23) or the Crab Keys Basin (site 24) indicating that P. argus

postlarvae are not typically transported across mudbanks and into the interior

basins of Florida Bay. Similarly, no juvenile lobsters <_35 mm CL were found in

Johnson Key Basin, Panhandle Key Basin or the Crab Keys Basin. In Twin Key Basin, however, juveniles were collected from hardbottom habitat in the cen-

tral and western sections of the basin (fig. 3b). At the Twin Key Basin site

nearest Twin Key Bank (site 11), 22 juvenile lobsters (size range: 28.7->60 mm

CL) were collected; 41 % of which were <_35 mm CL. At sites 15 and 16, 5 km

and 6.7 km northeast of the bank (respectively), the abundance of juvenile lobsters decreased; 14 juveniles (size range 28->60 mm CL) at site 15, and 2

37

Bay. However, the number of large (>35 mm CL) juveniles/site and the

number of postlarvae settling/site could be predicted using regressor variables

collected from both line-transect and belt-transect sampling methods (table II). The habitat density data from the belt transects yielded the best models, with

multiple R2 values >0.90, as compared to R2 values of approximately 0.65 for

models incorporating proportional bottom coverage data. However, the den-

sity-based models included fewer sites and, therefore, contain fewer degrees of

freedom than the models based on bottom coverage. Examination of the standardized coefficients for the density-based habitat

data indicates that salinity, and the density of octocorals and hard corals are,

respectively, the most influential variables in the model predicting the number

of juveniles >35 mm CL (R`' = 0.92; table II). The number of postlarvae settling at a site was best predicted by the density of octocorals, salinity, and the density of solution holes at a site, respectively, (R2 = 0.98; table II). For the models

incorporating bottom coverage of various habitats (i.e., line-transect data), the

number of juvenile P. argus >35 mm CL was most influenced by the proportion of bottom covered by octocorals (R2 = 0.66; table II). The number ofpostlarvae settling at a site was negatively related to salinity and the minimum number of

banks over which postlarvae pass enroute to a site (R2 = 0.65; table II).

DISCUSSION

Implications of postlarval physiological tolerances for recruitment in

Florida Bay

At typical south Florida winter water temperatures and salinities (22°C and

35 ppt), TTM for P. argus postlarvae is 15 days; in the summer TTM decreases

to 9 days at 29°C and 35 ppt. Examination of water temperature records for

Florida Bay for the past 5 years indicate that winter water temperatures

occasionally dip below 20°C (i.e., 13-18°C) and summer temperatures can

exceed 32°C. The consequences of such a thermal regime for recruitment of

postlarvae into Florida Bay may be manifested in effects on both TTM and

survival. Time-to-metamorphosis for P. argus postlarvae raised at 18°C and 35

ppt in our study was approximately 27 days, as compared to 8 days at 33°C. Because postlarvae recruit throughout the year in south Florida, those entering Florida Bay during cold winter months could potentially be transported further

into the bay due to their prolonged postlarval period. However, physical conditions in the bay may nullify this potential. North winds are common in the

winter and may set up south or southeast flowing surface currents that might limit postlarval movement into the bay. During the summer, postlarval recruit-

ment is probably restricted to areas close to the southern chain of keys due to

the rapid TTM at warm temperatures. The laboratory results reported here also suggest that recruitment of postlar-

vae in Florida Bay is likely to vary geographically and seasonally due to the

38

TABLE II

Results of the multiple regression analyses using physical factors (distance-KM and number of banks-BANK), environmental factors (salinity-PPT), and habitat characteristics (proportion of bottom covered by algae-A; seagrass-SG; sand-

SAND, sponges-SPO; octocorals-OCT; and other-O; density of sponges-NSPO; octocorals-NOCT; corals-NC; and solution holes-NSOL) to predict the number

of juvenile Panulirus argus >35 mm CL (JUV), and the number of postlarvae (PL)

settling at a particular site. The number of juveniles <_35 mm CL could not be

predicted. Only the significant models and their corresponding ANOVA tables

and standardized variable contribution estimates are listed

40

temperatures (i.e., 29°C), up to 61 % of recruiting postlarvae could tolerate a

salinity of 45 ppt, whereas only 4% could withstand a salinity of 50 ppt (fig. 5b). Under these conditions, we would expect low postlarval survival and poor recruitment to the early benthic juvenile stage in the interior regions (i.e., at the

northern P. argus boundary) of the bay. No postlarvae could tolerate extreme

summer temperatures of 33°C and salinities >45 ppt; therefore, recruitment to

the bays interior would be unlikely (fig. 5c). Cooler winter temperatures would

also reduce postlarval survival. Approximately 73% of the recruiting postlarvae could withstand winter Florida Bay water temperatures of 22°C at a salinity of

45 ppt, and 26% could withstand a salinity of 50 ppt (fig. 5d). However,

periodic cold snaps, combined with high salinities in the interior of the bay would be lethal to P. argus postlarvae; postlarval survival is only 4% at 18°C and

50 ppt (fig. 5e). Thus, the typical salinity and temperature regimes within Florida Bay proba-

bly limit recruitment of P. argus in the interior portions of the bay. In fact, the

distributional boundary for P. argus in Florida Bay first reported by Davis &

Dodrill (1989) is well described by the expected postlarval survival probabilities associated with salinity-temperature tolerance isopleths (fig. 5). Salinities

between 45-50 ppt and extreme seasonal temperatures (<20°C or >30°C) may be the best descriptor of this geographic boundary.

Plans are underway to restore much of the natural freshwater flow and

hydroperiod within the Everglades. Florida Bay would be the ultimate recipient of this freshwater, so these measures are likely to lower salinities within the bay. A moderate reduction in salinities to near 35 ppt would extend the potential

nursery region northward, assuming adequate postlarval transport and habitat

availability. However, a large decline in salinity (<35 ppt) in the bay would be

as detrimental to P. argus recruitment as high salinities. Postlarval survival at 25

ppt is about 60% at 22-29°C, but at extreme temperatures (18° and 33°C)

postlarval survival decreases to 6% and 13%, respectively. A laboratory study

by Witham et al. (1968) also indicated that low salinities found in the St. Lucie

(Florida ) estuary limit P. argus recruitment. Thus, the geographic limits of P.

argus recruitment in Florida Bay are not only determined by the present

hypersaline conditions, but would also be affected by the hyposalinc conditions

resulting from restoration of the Everglades freshwater flow.

Implications of geography and habitat structure for postlarval recruitment

in Florida Bay

Geographic features in the nursery, such as the number and distribution of

mudbanks, not only restrict water circulation within Florida Bay, but may also

inhibit the transport of postlarvae and, thus, the recruitment of juvenile P.

argus. No postlarvae were found in benthic collectors beyond the first series of

mudbanks suggesting that the banks may indeed be barriers to postlarval

41

transport. Furthermore, field surveys show that fewer than 12% of the juvenile P. argus collected at sites near the northern boundary of the P. argus distribution

in Florida Bay were small individuals ?35 mm CL as compared to 37%

elsewhere (fig. 3). Although large individuals arc nomadic and can immigrate into the bay's interior, juveniles <_35 mm CL are not thought to move far from

where they initially settle as postlarvae (Herrnkind et al., in press). Thus, the

paucity of small lobsters in northern Florida Bay reinforces the benthic collec-

tor data and collectively suggests that postlarval settlement is limited in the

interior regions of Florida Bay. Twin Keys Basin is the only northern basin we surveyed that contained

juvenile P. argus and it is unique among the basins that lie to the north of the

mudbank barrier in terms of physical structure and habitat diversity. The

mudbank to the south of Twin Keys Basin is dissected by several wide, rela-

tively deep channels, so water circulation may not be as restricted in this basin

as it is in others. Perhaps most important is the presence of large hardbottom

areas in Twin Keys Basin that contain abundant postlarval settlement habitat

(e.g., Laurencia) and postsettlement shelter (e.g., sponges, octocorals, etc.). This

type of habitat does not occur in most of the other basins, which contain either

seagrass (Johnson Key Basin) or silted, open hardbottom (e.g., Panhandle Key Basin and the Crab Keys Basin; fig. 4). Apparently few postlarvae reach Twin

Keys Basin, but those that do are more likely to find suitable settlement and

post-settlement habitat there than in adjacent basins behind mudbanks.

Nursery habitat structure plays an important role in determining the distri-

bution of P. argus elsewhere in Florida Bay (Herrnkind & Butler, in press). The

availability of suitable macroalgal settlement habitat influences postlarval sur-

vival to the first benthic juvenile stage (Herrnkind & Butlcr, 1986; Herrnkind et

al., 1988; Smith & Herrnkind, 1992), and the presence of sponges, coral heads, octocorals and other structures is critical to the survival of post-algal stage

juveniles (Smith & Herrnkind, 1992). Thus, by investigating habitat charac-

teristics at a particular site, and combining these data with geographic and

water quality information, we may be able to predict postlarval settlement and

the number of juvenile P. argus found at a site.

The multiple regression model predicting the magnitude of postlarval settle-

ment in an area was significant (R2 = 0.65; table II) and best predicted by the minimum number of banks that postlarvae pass over enroute to a site, salinity, and the proportion of bottom covered by sand. Banks are barriers to physical

transport, high salinities decrease postlarval survival, and both of these predic- tor variables were negatively related to postlarval settlement. The proportion of

bottom covered by sand explained the least variance in the model but it was

positively associated with settlement, which is difficult to explain. Postlarvae

that settle on open sand arc quickly consumed (Herrnkind & Butler, 1986; Smith & Herrnkind, 1992), but hardbottom nursery areas often contain large

expanses of open sand bottom. The model incorporating post-algal shelter

42

densities, rather than percent cover to predict the settlement of postlarvae,

yielded even better results (R2 = 0.98; table II). Again, salinity was important in

predicting postlarval settlement, and we know that abnormal salinites nega-

tively influence lobster survival, but the negative relationship between postlar- val settlement and the most influential variable in the model (the density of

octocorals at a site) is not easily interpreted. The density of solution holes in an

area was also positively associated with the magnitude of postlarval settlement, and was as influential in the model as salinity. Solution holes are indicative of

hardbottom habitat, which may again explain this association.

Both models predicting the number of large juvenile P. argus (i.e., those >35

mm CL) found at a site were also significant. The model based on habitat

coverage (Rz = 0.66; table II) indicated that the number of large juveniles at a

site was positively and strongly associated with octocoral cover. Octocorals are

characteristic of hardbottom areas, their abundance is significantly correlated

with sponge abundance (r = 0.66; p<0.001 ), and juvenile lobsters are often

found under these two shelter types. The association between large juveniles and algae, sand, and the "other" habitat categories (e.g., small coral heads and

solution holes) was negative, but was much weaker than that with octocorals. The size class designation ">35 mm CL" includes a broad range of sizes some of which use these types of shelters (e.g., sub-adults that are 50-75 mm CL often

reside under small coral heads or in solution holes). Among-site differences in

the distribution of individuals of different sizes within this broad size category is

likely to produce very different habitat-specific associations. The model incor-

porating shelter densities (table II) again yielded better predictions (R2 = 0.92) for the number of large juveniles (>35 mm CL) found at a site than the model

incorporating percent cover data. Octocorals were again positively associated

with the number of juveniles whereas coral and solution hole densities were

negatively associated with juvenile abundance. Salinity also had a strong influ-

ence on the number of juveniles >35 mm CL found in region. No one has

exposed juvenile P. argus to the temperatures and salinity conditions present in

Florida Bay, so we cannot be certain that hypersaline conditions are as detri-

mental to juvenile P. argus as they are to postlarvae. No combination of

regressor variables proved useful in predicting the number of small juvenile lobsters mm CL) at a site. Small juveniles <35 mm CL are the most

difficult size class to accurately census, and their abundance at a site may be a

poor reflection of present habitat (e.g., algal abundance) and environmental

conditions (e.g., salinity).

Conclusions

The number of P. argus postlarvae entering the Florida Bay nursery each

month is highly variable (Little, 1977; Forcucci et al., in press) and their

transport into the bay's interior basins appears limited, presumably due to the

43

extensive mudbank system that restricts water circulation in the bay. The

survival of postlarvae that arrive in these basins depends on the salinity and

temperature regimes there and the availability of settlement and post-settle- ment habitat. Salinities in central Florida Bay typically exceed 45 ppt and when

water temperatures are extreme (<20°C or >30°C), postlarval survival, hence

recruitment to later juvenile stages, will be poor. Given the high salinities in the

central bay, postlarval survival is unlikely to exceed 25% at normal

temperatures. The presence of macroalgal settlement habitat for postlarvae and shelter for

postalgal juveniles may also limit recruitment in portions of Florida Bay. With

the exception of Twin Keys Basin, the interior basins studied here lacked

suitable spiny lobster nursery habitat. Thus, the intricate mudbank system,

high salinities, occasionally extreme temperatures, and distribution of nursery habitat in Florida Bay all conspire to limit recruitment of P. argus in the interior

portions of the bay. If so, P. argus recruitment in south Florida may be limited to

a much smaller region, adjacent to the Florida Keys, than was previously

thought. Projected changes in the delivery of freshwater from the Everglades to

Florida Bay are likely to alter the present distribution of nursery areas within

the bay and could potentially enlarge the region suitable for recruitment, given sufficient postlarval abundance and nursery habitat.

ACKNOWLEDGMENTS

We thank W. Herrnkind, J. Hunt, M. Childress, M. Kuhlmann, C. Bowling, T. Matthews, R. Bertlesen, B. Sharp, C. Acosta, G. Hyde, S. Gornak, and J.

Kipp for their assistance and suggestions throughout the study. Many others

also assisted us in collecting postlarvae and we owe them our thanks. For the

use of the Keys Marine Laboratory, we thank the Florida Institute of Ocean-

ography, the Florida Department of Environmental Protection - Florida

Marine Research Institute, and the staff of the Keys Marine Laboratory. The

Everglades National Park permitted us access to many sites and provided historic temperature and salinity data for portions of Florida Bay. This reserach

was supported by a Lerner-Gray Marine Research (American Museum of

Natural History) Grant to J. Field, and by a Florida Sea Grant award (R/LR-

B-30) to M. Butler and W. Herrnkind.

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