James E. Garvey

Overwinter Growth and Survival of Largemouth Bass: Interactions among Size, Food, Origin, and Winter Severity

Abstract Winter severity (temperature, duration, and photocycle), geographic origin, food availability, and initial body size likely influence growth, survival, and, therefore, recruitment of age-0 largemouth bass Micropterus salmoides. We collected age-0 largemouth bass (70–160 mm total length) from low (33°N), intermediate (40°N), and high (45°N) latitudes throughout their natural range (origin), and we subjected all three groups of fish to three experimental winters that mimicked these latitudes (N = 9 largemouth bass per treatment). Within each winter and origin, one-half of the largemouth bass were fed fish prey, whereas the remaining one-half were starved. Winter strongly influenced survival; overall survival rates in the high-, intermediate-, and low-latitude winters were 34.9, 59.4, and 61.1%, respectively (χ2 test, P < 0.05). Largemouth bass from 33°N suffered high mortality in the high-latitude winter. Across all winters, more fed fish (64.5%) survived than did starved fish (38.1%) (χ2 test, P <...

Evaluating How Chela Size Influences the Invasion Potential of an Introduced Crayfish (Orconectes rusticus)

-In northern Wisconsin lakes, the crayfish Orconectes rusticus is replacing 0. propinquus, a previous invader, and 0. virilis, a native crayfish. Because chela size dictates success in aggressive encounters, predatory defense and reproduction, interspecific size differences could lend insight to this ongoing replacement. To quantify chela size differences among these three species, we measured chela lengths and widths of Form II (FII, nonreproductive) males, Form I (Fl, reproductive) males and females. For Fl and FII males, both 0. rusticus and 0. propinquus had larger chelae than similar-sized 0. virilis. Females differed only slightly in chela size. Females had smaller chelae than their male conspecifics. Chela size differences, coupled with differential body size, likely contributes, via differential predatory susceptibility and reproductive success, to the replacement of 0. propinquus and 0. virilis by 0. rusticus. INTRODUCTION The crayfishes, Orconectes rusticus, 0. propinquus and 0. virilis occur within littoral zones of N temperate lakes, often in sympatry (Crocker and Barr, 1968; Berrill, 1978; Capelli and Capelli, 1980; Corey, 1988). In northern Wisconsin lakes, 0. virilis is endemic; both 0. propinquus and 0. rusticus are exotics (Capelli, 1982). First to invade these lakes, 0. propinquus appears to displace 0. virilis, whereas 0. rusticus, the most recent invader, displaces both 0. virilis and 0. propinquus (Capelli, 1982; Lodge et al., 1986). Mechanisms explaining this replacement remain elusive, though predation, competition and reproductive interference all have been implicated (Capelli and Capelli, 1980; Capelli, 1982; Capelli and Munjal, 1982; Lodge et al., 1986). Herein, we quantify interspecific morphological differences that potentially could contribute to this replacement. Orconectes rusticus, 0. propinquus, and 0. virilis share similar gross external morphological characteristics (Capelli and Capelli, 1980; Hobb and Jass, 1988). However, chela size differs among these crayfishes. Chelae comprise a substantial portion of crayfish dry weight (Stein, 1976) and though energetically costly to grow, large chelae benefit crayfish in a variety of ways. Crayfish with large chelae are less susceptible to fish predation (Stein, 1976) and able to dominate similar-sized crayfish having small chelae (Bovbjerg, 1956; Stein, 1976; Snedden, 1990; Garvey, 1992). Furthermore, males with large chelae are better able to secure and hold females during copulation than similar-sized males with small chelae (Stein, 1976; Snedden, 1990). Though chela size is important, 0. rusticus and 0. virilis grow larger than 0. propinquus (Corey, 1988; Olsen et al., 1991). Because large crayfish are less susceptible to predation and able to dominate small crayfish (Stein and Magnuson, 1976; Stein, 1977), chela plus body size differences likely combine to influence interspecific interactions among these crayfishes. By quantifying chela length and width for these crayfishes, we can assess chela size differences. We then can evaluate the effect of chela size coupled with body size differences on the replacement of 0. virilis and 0. propinquus by 0. rusticus.

From Star Charts to Stoneflies: Detecting Relationships in Continuous Bivariate Data

Within many ecological systems, relationships between controlling factors and associated response variables are complex. In many cases, the response should vary little when the controlling factor exerts strong effects. Conversely, when the effect of the controlling factor is weak or absent, the response may vary greatly with effects of other factors. Correlation or regression analyses often may not be appropriate for testing these relationships, because variance of the response changes with values of the controlling factor. We suggest using a technique from the astronomy literature, a two-dimensional Kolmogorov-Smirnov (2DKS) test, to detect relationships in bivariate data with these patterns of variance. This technique successfully identified simulated bivariate data composed of paired independent values as having nonsignificant relationships and simulated bivariate data in which mean and variance of y was constrained at high levels of x as having significant relationships. Using these simulations and examples from aquatic and terrestrial systems, we demonstrate that the 2DKS is a robust test for detecting nonrandom patterns in bivariate distributions that commonly arise in many ecological systems.

Protracted Reproduction in Sunfish: the Temporal Dimension in Fish Recruitment Revisited

Understanding how life histories influence reproductive success under un- certain conditions is necessary to predict population dynamics. For many organisms, pro- tracted reproduction may increase expected offspring recruitment in variable environments, requiring that temporal patterns of reproduction be considered when developing manage- ment or conservation strategies. We explored the interrelationships among birth date, pro- duction of embryos on nests, survival of larvae to the open-water stage, and survival of juveniles through the first fall and winter of life for bluegill and pumpkinseed sunfish (Lepomis macrochirus and L. gibbosus) in Lake Opinicon, Ontario, during May 1998 through May 1999. Age-0 sunfish were sampled with nesting surveys for embryos, surface tows for free-swimming larvae, and seining/trawling for juveniles. Age of juveniles (days post swim-up) was quantified using otoliths. The abundance of embryos on nests and the density of open-water larvae were unrelated across all dates and sites. Although 40% of larvae had appeared in the open water by 12 June, most juveniles sampled in the fall were produced after that time, suggesting that high mortality of early produced larvae occurred. Larval survival to the juvenile stage was generally unrelated to the abundance of edible zooplankton taxa during swim-up from nests. Larval survival was often highest at tem- peratures .23.58C. Fall length of age-0 sunfish increased with increasing age. Both age- specific length and mean lengths shifted positively between October 1998 and May 1999, suggesting that growth of all individuals and perhaps selective mortality of small juveniles occurred. Although early reproduction may increase sizes reached by fall and thereby improve overwinter survival, early hatched larvae are subject to variable environmental factors that may reduce survival. Late-hatched larvae may reach relatively smaller sizes by fall, but have higher survival probabilities during this life stage. Protracted reproduction appears to be a response to variable environmental factors influencing growth and survival across multiple life stages. As such, all reproducing adults, rather than those perceived to produce offspring during typically favorable times, must be protected from exploitation or other human-induced perturbations.

Linking Landscapes and Food Webs: Effects of Omnivorous Fish and Watersheds on Reservoir Ecosystems

Abstract Ecologists increasingly recognize the need to understand how landscapes and food webs interact. Reservoir ecosystems are heavily subsidized by nutrients and detritus from surrounding watersheds, and often contain abundant populations of gizzard shad, an omnivorous fish that consumes plankton and detritus. Gizzard shad link terrestrial landscapes and pelagic reservoir food webs by consuming detritus, translocating nutrients from sediment detritus to the water column, and consuming zooplankton. The abundance of gizzard shad increases with watershed agriculturalization, most likely through a variety of mechanisms operating on larval and adult life stages. Gizzard shad have myriad effects on reservoirs, including impacts on nutrients, phytoplankton, zooplankton, and fish, and many of their effects vary with ecosystem productivity (i.e., watershed land use). Interactive feedbacks among watersheds, gizzard shad populations, and reservoir food webs operate to maintain dominance of gizzard shad in highly productive systems. Thus, effective stewardship of reservoir ecosystems must incorporate both watershed and food-web perspectives.

Movement and Habitat Selection by Invasive Asian Carps in a Large River

Abstract We evaluated the habitat use and movements of 50 adult bighead carp Hypophthalmichthys nobilis and 50 silver carp H. molitrix by means of ultrasonic telemetry during spring-summer 2004 and 2005 to gain insight into the conditions that facilitate their establishment, persistence, and dispersal in the lower Illinois River (river kilometer 0-130). Movement and habitat use were monitored with stationary receivers and boat-mounted tracking. The relative availability of four macrohabitat categories (main channel, island side channel, channel border, and connected backwater) was quantified to determine selection; discriminant function analysis was used to evaluate changes in physical characteristics within each category. A flood pulse occurred in spring through early summer of 2004 but not 2005. Movement rates (km/week) of both species were positively correlated with flow but not with temperature. Including data from stationary receivers greatly increased estimates of daily movement. During low summer f...

ENERGETICS, PREDATION, AND RATION AFFECT SIZE‐DEPENDENT GROWTH AND MORTALITY OF FISH DURING WINTER

Winter temperatures may reduce energy costs for ectotherms. However, variable mid-temperate and low-latitude winters may interact with scaling of size, metabolism, and energy reserves to cause energy deficits and require trade-offs between foraging and predation. A dynamic optimization model explored how ration, fall fat, and both non- and size-selective predation influenced foraging (i.e., fast or forage) and energy allocation (i.e., length or fat) decisions that maximize winter survival of age-0 largemouth bass (Micropterus salmoides). During a mid-latitude (38° N) winter, a pond experiment in which age-0 fish occurred with or without adult conspecific predators tested a subset of the model. In the model without predators, winter foraging occurred, with small size only reducing survival when low ration and low fall fat caused small fish to exhaust reserves. With predation, all sizes foraged to maintain mass and fat reserves when ration was sufficiently high, with small fish also growing in length. When modeled predation was nonselective, size-dependent mortality varied in complex ways. In contrast, size-selective predators consistently reduced survival of small fish. Generally consistent with the model, fish in ponds without predators gained mass and energy content, while those with predators only maintained these parameters. All small individuals grew more than large counterparts in length. Mortality in ponds never depended on size but was ∼20% higher with predators. Energy deficits often demand active foraging during mid-temperate winters, with predation rather than energy depletion influencing size-dependent survival.

EXPLORING SPATIAL AND TEMPORAL VARIATION WITHIN RESERVOIR FOOD WEBS: PREDICTIONS FOR FISH ASSEMBLAGES

In the limnetic zones of small, highly productive reservoirs, young-of-year (YOY) gizzard shad (Dorosoma cepedianum ) or threadfin shad (D. petenense) (henceforth, shad) often attain high densities during spring. Environmental factors facilitating early growth and survival of shad plus potential interspecific competition for zooplankton may reduce growth and survival of YOY bluegill (Lepomis macrochirus), another common species in reservoirs. We hypothesized that fewer YOY bluegill moving from the limnetic zone to the littoral zone in late spring probably slows or prevents the ontogenetic switch to piscivory by YOY largemouth bass (Micropterus salmoides), reducing their oversummer growth, overwinter survival, and hence recruitment to their second year. To determine whether shad and bluegill abundances indeed vary inversely in reservoirs, we quantified densities of YOY shad and bluegill in four reservoirs across several years (1987-1994; N 5 22 reservoir-years). To assess how YOY bluegill abundance influenced the ontogenetic dietary switch to fish by largemouth bass, we conducted experiments with differing levels of realism and control (4-m 2 littoral cages, 4.5-m 2 outdoor pools, and 0.33-m 2 laboratory aquaria). In reservoirs, peak YOY bluegill density declined weakly in the limnetic zone but strongly in the littoral zone when peak limnetic YOY Dorosoma spp. exceeded 10 indi- viduals/m 3 . In pools and aquaria, largemouth bass grew more rapidly at

Linking Bluegill and Gizzard Shad Prey Assemblages to Growth of Age-0 Largemouth Bass in Reservoirs

3 than at zero bluegill per largemouth bass. Using known temperatures and largemouth bass growth in a bioenergetics model, we discovered that YOY largemouth bass in pools and aquaria ate #65% of their maximum daily consumption potential (in grams of wet mass) at

SCALE-DEPENDENT ASSOCIATIONS AMONG FISH PREDATION, LITTORAL HABITAT, AND DISTRIBUTIONS OF CRAYFISH SPECIES

6 bluegill per largemouth bass. In cages, largemouth bass consumed only 40% of their maximum and grew less at bluegill abundances similar to those in pools and aquaria, probably because dense vegetation and depletion of bluegill inhibited predatory success. In reservoirs with abundant shad, reduced littoral bluegill density likely compromises first-year growth and recruitment of largemouth bass. However, variable abiotic and biotic factors may modify YOY bluegill abundance and hence invalidate our predictions for largemouth bass recruit- ment success. To better predict fish community structure and develop management actions for reservoir ecosystems, multi-scale experimentation should be combined with whole- system manipulations (e.g., via adaptive management) to bound these variable interactions.