Increased intrinsic growth rate is advantageous even under ecologically stressful conditions in coho salmon (<Emphasis Type="Italic">Oncorhynchus kisutch</Emphasis>)

Growth rate is an ecologically important trait, affecting the energy acquisition from, and provisioning to, the surrounding community. One of many costs suggested to counteract the evolution of increased intrinsic growth rate is an associated reduction in tolerance to conditions of nutrient stress. Here we test this concept with individuals possessing experimentally increased intrinsic growth rates (growth hormone transgenic coho salmon, Oncorhynchus kisutch) relative to wild genotypes. Using a series of three experiments, survival and growth of both genotypes were assessed on a physiological and behavioral level while varying food abundance, social interactions, and predation risk. Only in complete absence of exogenous food in newly emerged fry did the high intrinsic growth rate appear costly with a shorter average survival time compared to wild-type (Exp. 1). In experiment 2, genotypes with elevated intrinsic growth showed equal or higher survival and growth than wild-type genotypes In a third experiment, adding very limited amounts of food and allowing for social interactions in a simulated natural environment benefited transgenic individuals relative to wild-types, but at similar magnitudes in both the absence and presence of predators. Populations with transgenic individuals present did not crash under these competitive conditions as previously reported when studied in simple environments where hiding and attack escape were not possible. Our data suggest that transgenic fish have a greater scope for growth under most conditions, but are not obligated to use this capability. Physiological (e.g. appetite and conversion efficiency) and behavioral traits (e.g. competitive ability and risk-taking) found previously to correlate positively with intrinsic growth rate in the transgenic strain likely aided in their survival and growth, even under food limited conditions. Hence, at least in coho salmon, intrinsic growth rate does not appear to strongly affect survival under nutrient stress.     

EVOLUTION OF INTRINSIC GROWTH AND ENERGY ACQUISITION RATES. II. TRADE-OFFS WITH VULNERABILITY TO PREDATION IN MENIDIA MENIDIA

Abstract The Atlantic silverside ( Menidia menidia ) exhibits countergradient latitudinal variation in somatic growth rate along the East Coast of North America. Larvae and juveniles from high-latitude populations display higher intrinsic rates of energy consumption and growth than genotypes from low-latitude populations. The existence of submaximal growth in some environments suggests that trade-offs must counter the oft-cited theoretical benefits of energy and growth maximization (e.g., "bigger is better,""faster is better") in the immature life stages. We hypothesized that energy and growth maximization trades off against investment in defense from predators. We conducted laboratory selection experiments to compare vulnerability to predation of silversides from: (1) fast-growing northern (Nova Scotia, NS) versus slow-growing southern (South Carolina, SC) source populations; (2) phenotypically manipulated fast-growing versus moderately-growing NS fish; and (3) recently fed versus unfed NS and SC fish. Tests involved fish drawn from common-garden environments and were conducted by subjecting mixed-treatment schools of size-matched silversides to natural, common piscine predators. NS silversides suffered significantly higher predation mortality than SC silversides. Parallel results were found in phenotypic manipulation of growth: NS silversides reared on a fast-growth trajectory (∼1.0 mm/day) were significantly more vulnerable to predation than those growing at a moderate rate (∼0.5 mm/day). Food consumption also affected vulnerability to predators: Silversides with large meals in their stomachs suffered significantly higher predation mortality than unfed silversides. Differences in predation vulnerability were likely due to swimming performance, not attractiveness to predators. Our findings demonstrate that maximization of energy intake and growth rate engenders fitness costs in the form of increased vulnerability to predation.     

Foraging and breeding performance of Japanese cormorants in relation to prey type

Seabirds are high trophic predators in marine ecosystems and are sensitive to change in food supply and thus seabirds can be used as monitors of the marine environment. In order to study the foraging responses of Japanese cormorants Phalacrocorax filamentosus breeding at Teuri Island, Hokkaido to changes in fish availability, the diet was assessed from the regurgitations of parents and chicks, and diving behavior was measured by using time-depth recorders. Breeding performance (brood size, chick growth, breeding success) was monitored using conventional methods to study their breeding responses. Japanese cormorants changed the diet and foraging behavior over four summers. The birds fed mainly on epipelagic schooling fish when they were available and on demersal fish when pelagic fish availability was low. They tended to dive deeper and longer in a year when they fed mainly on demersal fish than the other years, reflecting the change in the depth distribution of prey fish. Chick growth rate did not differ among years, but fledging success was lower in the years of demersal fish as their meal delivery rate was low. When epipelagic schooling fish were considered scare, parents maintained chick growth by reducing brood size. High variability and unpredictability in pelagic fish abundance are key factors affecting the foraging and breeding performance of Japanese cormorants, which could potentially be used to monitor fish resources.     

How does the presence of a conspecific individual change the behavioral game that a predator plays with its prey?

Behavioral games predators play among themselves may have profound effects on behavioral games predators play with their prey. We studied the behavioral game between predators and prey within the framework of social foraging among predators. We tested how conspecific interactions among predators (little egret) change the predator–prey behavioral game and foraging success. To do so, we examined foraging behavior of egrets alone and in pairs (male and female) in a specially designed aviary consisting of three equally spaced pools with identical initial prey (comet goldfish) densities. Each pool was comprised of a risky microhabitat, rich with food, and a safe microhabitat with no food, forcing the fish to trade off food and safety. When faced with two versus one egret, we found that fish significantly reduced activity in the risky habitat. Egrets in pairs suffered reduced foraging success (negative intraspecific density dependence) and responded to fish behavior and to their conspecific by changing their visiting regime at the different pools—having shorter, more frequent visits. The time egret spent on each visit allowed them to match their long-term capture success rate across the environment to their capture success rate in the pool, which satisfies one aspect of optimality. Overall, egrets in pairs allocated more time for foraging and changed their foraging tactics to focus more on fish under cover and fish ‘peeping’ out from their shelter. These results suggest that both prey and predator show behavioral flexibility and can adjust to changing conditions as needed in this foraging game.     

Adaptive variation in energy acquisition and allocation among latitudinal populations of the Atlantic silverside

Understanding the evolution of growth rate requires knowledge of the physiology of growth. This study explored the physiological basis of countergradient variation (CnGV) in somatic growth across latitudinal populations of the Atlantic silverside, Menidia menidia. Energetics of northern (Nova Scotia, Canada) and southern (South Carolina, USA) genotypes were compared across resource levels, temperatures, and fish sizes to identify trade-offs to rapid growth. Offered unlimited resources, genotypes differed in both energy acquisition and allocation. Food consumption, growth, and efficiency of northern genotypes were consistently higher than in southern genotypes, across temperatures and body sizes. Feeding metabolism (specific dynamic action; SDA) was proportional to meal size, differing between genotypes to the extent that food consumption differed. Given limited resources, northern and southern genotypes displayed similar growth, efficiency, routine activity, and SDA across temperatures and fish sizes. Routine metabolism was equal at 17°C and 22°C, yet was significantly higher in northern fish at 28°C. Growth rates in M. menidia do not appear to trade off across environments or body sizes, i.e., at no temperature, ration, or size do southern fish outgrow northern conspecifics. Nor does submaximal growth result from increased costs of maintenance, tissue synthesis, or routine activity. Based on our findings, we propose that CnGV consumption and growth in M. menidia likely result from trade-offs with other energetic components, namely sustained and burst swimming. Received: 26 January 1999 / Accepted: 14 September 1999     

Prey or predator: 0+ perch (Perca fluviatilis) in the trade-off between food and shelter

The 0+ cohort of perch can split into a slow-growing planktivorous and a fast-growing piscivorous cohort during their first months of life. Both cohorts are, however, vulnerable to predation by piscivorous fish. Laboratory experiments were performed to test the behavior of 0+ perch as a predator of cyprinids, and in the trade-off between food and shelter from the threat of predators. In the foraging trials, 0+ perch attacked bream faster than they did carp, and vegetation hampered the aggression against bream. In the second experiment, the habitat selection of two size classes of 0+ perch under the threat of predation was monitored. Overall, vegetation structures were preferred by both size classes of 0+ perch. When small fish were offered to the 0+ perch as food, the open water becomes more attractive. The results of the habitat use trials further show that the two size cohorts of 0+ perch may also differ in their behavior, in that the availability of fish as food becomes more important than the shelter of vegetation structures for the larger perch.     

Predators select against high growth rates and risk-taking behaviour in domestic trout populations

Domesticated (farm) salmonid fishes display an increased willingness to accept risk while foraging, and achieve high growth rates not observed in nature. Theory predicts that elevated growth rates in domestic salmonids will result in greater risk-taking to access abundant food, but low survival in the presence of predators. In replicated whole-lake experiments, we observed that domestic trout (selected for high growth rates) took greater risks while foraging and grew faster than a wild strain. However, survival consequences for greater growth rates depended upon the predation environment. Domestic trout experienced greater survival when risk was low, but lower survival when risk was high. This suggests that animals with high intrinsic growth rates are selected against in populations with abundant predators, explaining the absence of such phenotypes in nature. This is, to our knowledge, the first large-scale field experiment to directly test this theory and simultaneously quantify the initial invasibility of domestic salmonid strains that escape into the wild from aquaculture operations, and the ecological conditions affecting their survival.     

Countergradient variation in growth rate: compensation for length of the growing season among Atlantic silversides from different latitudes

Summary How do organisms adapt to the differences in temperature and length of the growing season that occur with latitude? Among Atlantic silversides (Menidia menidia) along the east coast of North America, the length of the first growing season declines by a factor of about 2.5 with increasing latitude. Yet body size at the end of the first growing season does not decline. High-latitude fish must, therefore, grow faster within the growing season than do low-latitude fish. This geographical pattern has a genetic basis. Laboratory experiments on fish from six different locations revealed a latitudinal gradient in the capacity for growth (i.e., maximum growth potential). In two subsequent experiments using fish from Nova Scotia (NS), New York (NY) and South Carolina (SC) that had been separately reared in a common environment for several generations, differences in growth rate among populations were highly significant. The rank order was NS>NY>SC, but the difference among populations depended on temperature. High-latitude fish outperformed those from low latitudes primarily at the high temperatures that low-latitude fish would be expected to experience most often in nature. These results suggest that instead of being adapted for growth at low temperatures, fish from high latitudes are adapted for rapid elevation of growth rate during the brief interval of the year when high temperatures occur. Selection on growth rate results from sizedependent winter mortality: the importance to winter survival of being large increases with latitude but the length of the growing season simultaneously decreases. The end result is countergradient variation in growth rate, a phenomenon that may be much more widespread than currently recognized.     

Shifting the balance between foraging and predator avoidance: the importance of food distribution for a schooling pelagic forager

It is widely held that when predator avoidance conflicts with other activities, such as feeding, avoidance of predators often takes precedence. In this study, we examine how predation risk and food distribution interact to influence the schooling behavior and swimming speed of foraging juvenile walleye pollock, Theragra chalcogramma. Fish were acclimated to either spatially and temporally clumped, or spatially and temporally dispersed food for 3 weeks. Fish were then monitored while feeding in the absence and presence of predatory sablefish, Anoplopoma fimbria. Fish foraging for clumped food swam rapidly in a loose school when predators were absent, but swam more slowly and adopted more cohesive schooling in the presence of predators, trading-off foraging opportunity for decreased vulnerability to predators. Fish foraging for dispersed food swam about slowly and did not engage in cohesive schooling in either the absence or presence of predators. These fish accepted greater predation risk in order to continue foraging, suggesting that the cost of schooling, in terms of decreased foraging opportunity, was greater when food was dispersed than when it was clumped. This lower responsiveness to predators among fish receiving dispersed food demonstrates that predator avoidance does not always take precedence over other activities, but rather, that a balance is maintained between predator avoidance and feeding, which shifts as food distribution changes.     

Why do juvenile fish utilise mangrove habitats?

Three hypotheses to discern the strong positive association between juvenile fish and mangrove habitat were tested with field and laboratory experiments. Artificial mangrove structure in the field attracted slightly more juvenile fish than areas without structure. Artificial structure left to accumulate fouling algae attracted four-times the total number of juvenile fish than areas without structure or areas with clean structure. Community composition of fish attracted to structure with fouling algae was different when compared with areas with no structure or clean structure; five species were attracted by structure with fouling algae whilst two species were associated with structure regardless of fouling algae. Algae were linked to increased food availability and it is suggested that this is an important selection criteria for some species. Other species were apparently attracted to structure for different reasons, and provision of shelter appears to be important. Predation pressure influenced habitat choice in small juvenile fish in laboratory experiments. In the absence of predators, small juveniles of four out of five species avoided shelter but when predators were introduced all species actively sought shelter. Large fish were apparently less vulnerable to predators and did not seek shelter when predators were added to their tank. Feeding rate was increased in the mangrove habitat for small and medium-sized fish compared with seagrass beds and mudflats indicating increased food availability or foraging efficiency within this habitat. Larger fish fed more effectively on the mudflats with an increased feeding rate in this habitat compared with adjacent habitats. The most important aspect of the mangrove habitat for small juvenile fish is the complex structure that provides maximum food availability and minimises the incidence of predation. As fish grow a shift in habitat from mangroves to mudflat is a response to changes in diet, foraging efficiency and vulnerability to predators.     

Effects of food availability on temporal activity patterns and growth of Atlantic salmon

1. Patterns of sheltering and activity are of fundamental importance in the ecology of animals and in determining interactions among predators and prey. Balancing decreased mortality risk when sheltering with increased feeding rate when exposed is believed to be a key determinant of diel patterns of sheltering in many animals. 2. Despite lower foraging efficiency at night than during the day, Atlantic salmon Salmo salar parr are nocturnal during winter and at low summer temperatures. Nocturnal activity also occurs at warm water temperatures during summer, but little is known about the functional significance of this behaviour. 3. This study aimed to determine: (1) the preferred activity and shelter pattern of Atlantic salmon parr during warm summer months, and (2) their response to variations in food availability when balancing growth rate (G) and mortality risk (M), as expressed through time out of shelter. We differentiated among four potential responses to reduced food availability: (1) no response; (2) G decreases but M remains constant; (3) G remains constant but M increases; and (4) G decreases and M increases. 4. Time exposed from shelter was inversely related to food availability. Fish subject to high food availability were significantly less active during the day than those with restricted rations. However, food availability had no significant effect on the extent to which fish were active at night. There was no evidence of variation in growth rate with food availability. 5. Salmon were predominantly nocturnal at high ration levels, consistent with their previously reported behaviour during winter. Rather than switching to diurnal behaviour at high temperatures per se, as previously was supposed, it appears that the fish are diurnal only to the extent needed to sustain a growth rate, and this extent depends on food availability. 6. Atlantic salmon parr modulate the amount of time they are active rather than growth when responding to variations in food availability over an order of magnitude.     

Social behavior of juvenile chum salmon, Oncorhynchus keta, under risk of predation: the influence of food distribution

Synopsis Social interactions can influence both foraging reward and vulnerability to predators. We examined social interactions in groups of juvenile chum salmon, Oncorhynchus keta, receiving food that was either spatially dispersed, with many food items appearing synchronously, or spatially clumped, with individual food items appearing asynchronously. These experiments were conducted both in the presence and absence of predators. when food was dispersed and predators were absent, juvenile chum formed schools and all individuals had access to food, despite frequent agonistic interactions. When predators were present, schooling and feeding continued, but agonistic interactions ceased. In contrast, when food was clumped, dominant fish utilized aggression to monopolize food regardless of whether predators were present or absent, resulting in decreased group cohesion. These results illustrate that food distribution and social interaction may play a role in determining how fish balance predation risk against foraging reward.     

Ontogenetic changes in foraging tactics of the piscivorous cornetfish <Emphasis Type="Italic">Fistularia commersonii</Emphasis>

Foraging behaviors of the piscivorous cornetfish Fistularia commersonii were observed at shallow reefs in Kuchierabu-jima Island, southern Japan. This fish foraged on two types of prey fishes: one was reef fish that typically dwell on or near substrata (e.g., Tripterygiidae and Labridae), and the other was pelagic fish that shoal in the water column (e.g., Clupeidae and Carangidae). The prey sizes, prey types and foraging behaviors changed as the predator size increased. Prey sizes were largely limited by gape size of the cornetfish, and small predators consumed small prey. The small cornetfish (10–30 cm in total length) fed only on reef fish captured after stalking (where the fish slowly approaches the prey and then suddenly attacks). The stalking was done either solitarily or in foraging association with conspecifics. Large fish (30–120 cm) fed on both types of fishes by stalking and/or chasing (where the fish chases the prey using its high mobility and attacks), either solitarily or in foraging association with con- or heterospecifics. Thus, chasing was only performed by the large cornetfish against pelagic prey fish in associative foraging with other con- and heterospecific predators. As their body sizes increased, F. commersonii began to show a diversification of foraging behaviors, which was strongly related not only to the habitat types and anti-predatory behaviors of the prey fishes but also to associative foraging with con- or heterospecifics, which improves their foraging success.     

Selection on increased intrinsic growth rates in coho salmon, Oncorhynchus kisutch

Substantial evidence from the animal kingdom shows that there is a trade-off between benefits and costs associated with rapid somatic growth. One would therefore expect growth rates under natural conditions to be close to an evolutionary optimum. Nevertheless, natural selection in many salmonid species appears to be toward larger size and earlier emergence from spawning redds, indicating a potential for increased growth rate to evolve. We tested how selection for genetic variants (growth hormone transgenic coho salmon, Oncorhynchus kisutch, with more than doubled daily growth rate potential relative to wild genotypes) depended on predator timing and food abundance during the early period of life (fry stage). In artificial redds, fry of the fast-growing genotypes showed a highly significant developmental shift, emerging from gravel nests approximately two weeks sooner, but with an 18.6% reduced survival, relative to wild-genotype fry. In seminatural streams, fry of the fast-growing genotypes suffered higher predation than those of wild genotypes when predators were present at the time of fry emergence, but this difference was less pronounced when food was scarce. In streams where predators were introduced after emergence, fry survived equally well regardless of food availability. Surviving fry grew faster in habitats provided with more food, and fast-growing genotypes also grew faster than wild genotypes when predators arrived late and food was abundant. Fewer fish migrated downstream past a waterfall when food availability was high and in the presence of predators, and wild-genotype fry were more likely to migrate than fry of the fast-growing genotypes. After being returned to the experimental streams after migration, fast-growing genotypes survived equally well as those of the same genotypes that did not migrate, whereas migrating wild genotypes experienced higher mortality relative to those of the same genotypes that did not migrate. Comparisons of growth rates between siblings retained under hatchery conditions and those from habitats with the fastest growth in the experimental stream revealed that growth rates were similar for wild genotypes in both environments, whereas the fast-growing genotypes in the streams only realized 90% of their growth potential. The present study has shown that a major shift in developmental timing can alter critical early stages affecting survival and can have a significant effect on fitness. Furthermore, ecological conditions such as food abundance and predation pressure can strongly influence the potential for fast-growing variants to survive under natural conditions. The large-scale removal of many predatory species around the world may augment the evolution of increased intrinsic growth rates in some taxa.     

VARIATION IN ENVIRONMENTAL AND GENOTYPIC SEX-DETERMINING MECHANISMS ACROSS A LATITUDINAL GRADIENT IN THE FISH,MENIDIA MENIDIA

Models of environmental sex determination (ESD) usually assume that genetic influences on sex are polygenic, but the validity of this (or any other) form of genotype-environment interaction is virtually unknown. In the Atlantic silverside, Menidia menidia, sex is determined by an interaction between temperature and genotype and the response of sex ratio to temperature differs among populations from different latitudes. We examined the genetic basis of this pattern by measuring among family variation in the proportion of females, F/(F + M), within and among high (21°C) and low (15°C) temperatures for two populations: one from Nova Scotia (NS) where the level of ESD is low, and another from South Carolina (SC) where the level of ESD is high. In NS fish, temperature had a significant influence on sex ratio in only 1 of 23 families. The distribution of the fraction of females within temperatures for families from NS was highly heterogeneous and tended to fall into distinct classes (0.0, 0.25, 0.5, 1.0) like that expected from Mendelian segregation of a major sex factor(s). In contrast, temperature had a highly significant influence on sex ratio in all SC families examined (N = 24). Family sex ratios within temperatures were highly heterogeneous and, at least at 15°C, did not conform to simple Mendelian ratios. At 21°C, the proportion of females in most SC families was near zero and so the underlying sex tendencies of different families could not be discerned. Based on a previous study, mid-latitude fish appear to have an intermediate form of sex determination: simple Mendelian sex-ratio patterns exist and there is a moderate thermal influence on sex ratio in most but not all families. We suggest that sex determination in M. menidia is controlled by an interaction between major genetic factors, polygenic factors, and temperature and that the relative importance of each component differs with latitude. High latitude populations appear to have evolved a major sex-determining factor(s) that overrides the effect of temperature, and this factor(s) is lacking in low latitude populations.     

Complex predator-prey interactions and predator intimidation among crayfish,piscivorous fish,and small benthic fish

Summary Predator-prey interactions were studied among a small prey fish (the johnny darter Etheostoma nigrum) and two predators (crayfish Orconectes rusticus and smallmouth bass Micropterus dolomieui) with complementary foraging behaviors. When only smallmouth bass were present, darters reduced activity to 6% of control rates and spent most of the time hiding under tile shelters. When only crayfish were present, darter activity and shelter-use were similar to controls. When both crayfish and bass were present, an interaction occurred. Darters, normally inactive in the presence of bass, were often forced to move by approaching crayfish and thus activity increased to 19% of control rates. Also, darters were often evicted from shelters by intruding crayfish. Thus, crayfish increased the vulnerability of small fish to bass by evicting them from shelters and causing increased activity. Conversely, bass increased the vulnerability of small fish to crayfish by forcing these fish to seek cover under shelters occupied by crayfish. Intimidation effects of bass on darters last for some time. After a 30-min exposure to bass, darters showed reduced activity and increased shelter use lasting at least 24 h after the bass was removed. Thus predators, throught intimidation, can influence prey behavior even though the predators are no longer present.The Unit is sponsored jointly by the United States Fish and Wildlife Service, the Ohio Department of Natural Resources, and The Ohio State University     

Habitat heterogeneity and fish behavior: Units of heterogeneity as a resource and as a source of information

A review of studies, mainly experimental, on modifications of fish behavior caused by microscale habitat heterogeneity. Elements or units of heterogeneity influence on decision making in fish either as contestable physical resources, or as information cues or signals. Habitat heterogeneity arises from abiotic physical objects, aggregations of prey, and grouping fish. Feeding behavior of fish including food search, choice, and consumption are significantly dependent on the structure of heterogeneity of the habitat, where fish are foraging. Depending on the parameters of heterogeneity, prey characteristics and a predator foraging mode, heterogeneous habitats can either facilitate feeding behavior, or makes it more difficult. Habitat heterogeneity plays significant and, as a rule, positive role providing various refuges for fish hiding from predators. Landmarks help fish to find the shortest route to shelters. If a habitat is rather homogeneous or in a novel habitat, which appears to be homogeneous, shoaling of fish makes surroundings of each individual in the school structured providing fish with a substitute of shelters and landmarks. Recent experimental and field results convincingly demonstrate that the effects of main biotic and abiotic factors can be significantly modified by the structure (level of spatial heterogeneity) of habitats. When a habitat is physically structured, tendencies to disperse and establish individual territories prevail. In uniform, poorly structured habitats, fish tend to gather in schools or shoals and maintain larger aggregations. Food is considered the major contestable resource, but fish often demonstrate interference competition not for food, but for heterogeneous sites in the habitat, where they vigorously fight either for a shelter or just for visually non-uniform area. Visually heterogeneous sites can be used by fish as a template of a future individual territory, where fish can find not only food but also a refuge from predators. Fish use individual territories for much longer period than food patches. Just the presence of either physical refuge or “social refuge” neutralized the inhibiting effect of kairomons and allowed fish to feed more intensively despite the potential danger. We suggest that the decision-making was influenced only by available information of possibility to use a refuge. Habitat complexity is almost always accompanied by visual and other types of heterogeneity. Adaptive significance of fish attraction to the units of heterogeneity is probably related to the fact that under natural situations vital for fish objects are often tightly coupled with heterogeneous sites. Thus, units of habitat heterogeneity can be reliable signals or information cues in uncertain, i.e. changeable and poorly predictable, habitats.     

Interactions of multiple predators with different foraging modes in an aquatic food web

Top predators can have different foraging modes that may alter their interactions and effects on food webs. Interactions between predators may be non-additive resulting from facilitation or interference, whereas their combined effects on a shared prey may result in emergent effects that are risk enhanced or risk reduced. To test the importance of multiple predators with different foraging modes, we examined the interaction between a cruising predator (largemouth bass, Micropterus salmoides) and an ambush predator (muskellunge, Esox masquinongy) foraging on a shared prey (bluegill sunfish, Lepomis macrochirus) with strong anti-predator defense behaviors. Additive and substitution designs were used to compare individual to combined predator treatments in experimental ponds. The multiple predator interaction facilitated growth of the cruising predator in the combined predator treatments, whereas predator species had substitutable effects on the growth of the ambush predator. The combined predator treatments created an emergent effect on the prey; however, the direction was dependent on the experimental design. The additive design found a risk-reducing effect, whereas the substitution design found a risk-enhancing effect for prey fish. Indirect effects from the predators weakly extended to lower trophic levels (i.e., zooplankton community). Our results highlight the need to consider differences in foraging mode of top predators, interactions between predators, and emergent effects on prey to understand food webs.     

EVOLUTION OF INTRINSIC GROWTH AND ENERGY ACQUISITION RATES. I. TRADE-OFFS WITH SWIMMING PERFORMANCE IN MENIDIA MENIDIA

Abstract Latitudinal populations of the Atlantic silverside, Menidia menidia , show substantial genetic variation in rates of energy acquistion and allocation. Reared in common environments, silversides from northern latitudes consume more food, grow faster and more efficiently, store more energy, and produce greater quantities of eggs than their southern conspecifics. The persistence of seemingly inferior southern genotypes in the face of ostensibly superior northern genotypes suggest that there are hidden evolutionary trade-offs associated with these elevated acquisition and allocation rates. We tested the hypothesis that rapid growth and high levels of food consumption trade-off against locomotory performance in M. menidia . We compared both aerobic (prolonged and endurance) and anaerobic (burst) swimming capacities between intrinsically fast-growing fish from the north (Nova Scotia, NS) and intrinsically slow-growing fish from the south (South Carolina, SC) and between growth-manipulated phenotypes within each population. We also compared swimming speeds and endurance between fasted and recently fed fish within populations. Maximum prolonged and burst swimming speeds of NS fish were significantly lower than those of SC fish, and swimming speeds of fast-growing phenotypes were lower than those of slow-growing phenotypes within populations. Fed fish had lower burst speeds and less endurance than fasted fish from the same population. Thus, high rates of growth and the consumption of large meals clearly diminish swimming performance, which likely increases vulnerability to predation and decreases survival and relative fitness. The submaximal growth rate of southern M. menidia appears to be adaptive, resulting from balancing selection on rates of somatic growth.     

Diel observation on the distribution of the sea urchin Strongylocentrotus intermedius under different food availability and shelter conditions in the laboratory

The present study investigated the diel distribution of Strongylocentrotus intermedius in different laboratory food availability and shelter conditions. The diel rhythm of sheltering behaviour was similar between sea urchins in groups 1 (food inside shelter) and 3 (no food available), but significantly different to group 2 (food outside shelter). Among the three treatments, we found significantly more urchins outside shelter in group 2 than in groups 1 and 3 (p?<?0.05), while no significant difference was found between sea urchins in groups 1 and 3 (p?>?0.05). Kruskal–Wallis test confirmed this conclusion. When food was inside shelter, we found significantly more sea urchins staying on food than at other places inside the shelter (p?<?0.001). When food was outside shelter, significantly more individuals chose to stay in shelter rather than on food outside shelter (p?=?0.001). The present study provides new insights into the trade-off of sea urchins between foraging and sheltering behaviours.