Yellow perch (Perca flavescens) phenotypic plasticity: Rapid changes following the arrival of a predator

Predators are known to strongly influence the phenotypic expressions and distributions of prey species within aquatic communities. I examined behavioural, life-historical, physiological and morphological variation among yellow perch (Perca flavescens) populations with respect to predation pressure in eighteen lakes in the Sudbury Region, Ontario, Canada. The fish communities in these study lakes differed in the presence or absence of interspecific predator species and duration of predator establishment. Lakes were classified into one of three different predator regimes; lakes with no predators, lakes with recently introduced predators, and lakes with established predator populations.;Life history analysis showed that male and female perch allocated energy towards reproduction differentially in the face of predation. Male perch had larger gonads when predators were absent and the density of perch was high. Such evidence supports the hypothesis that sperm competition exists, and males invest more in gonads to increase gene transfer in lakes with high abundances of perch. In contrast, reproductive investment (i.e. gonad weight) was higher in females from lakes with established predators, but neither fecundity nor egg size varied significantly with predator regime, contrary to predictions from life history theory. Egg lipid content was found to be lowest in lakes with recently invaded predators, a possible result of the earlier ontogenetic diet shifts to benthic macroinvertebrates which has lower percent lipid compared to zooplankton. Finally, average total length (fall) for young-of-the-year perch was highest in lakes with no predators and lowest in lakes with recently invaded predators. The early diet shifts to benthos or decreased egg lipid content of eggs from the recently invaded lakes are two possible explanations for the low first year growth in these lakes.;Metabolic enzyme activity rates of nucleoside diphosphate kinase (NDPK) were used to estimate the growth potential of perch (age 1 and age 2) from all study lakes. Analysis of NDPK indicated that perch from lakes with established predator populations had elevated growth potentials which were manifested as higher round weight at length. The consequence of lower perch biomass in lakes with established predators may reduce intraspecific competition for resources, allowing elevated growth in this predator regime. Further enzymatic analysis showed no effect of predation on perch swimming behavior inferred by cellular activity rates of lactate dehydrogenase (LDH) and citrate synthase (CS). These results indicate that differences in growth among predator regimes are not explained by differences in anaerobic and aerobic swimming activity, respectively.;Landmark based shape analysis of perch from all lakes indicated that there was a distinct body shape common to perch from each of the various predator groupings. The morphological index identified that perch in lakes without predators had slender upwards bending bodies, which is typical of pelagic forms feeding exclusively on open water food items. Perch from lakes with either recently invaded or established predators were shown to be typical of littoral and/or benthic feeding forms having a thickened body and caudal peduncle ideal for maneuvering in the littoral region of the lake. Also, the more anterior position of the spiny dorsal fin, in perch from lakes with predators, may represent a defense mechanism where a greater body depth was achieved when spines were erected in response to gape-limited predators.;Quantitative assessments of the fish communities, using multimesh gillnets, revealed that yellow perch biomass dramatically decreased as a function of total predator biomass and this pattern was interpreted as a direct effect of predation. Diet analysis indicated that perch from lakes with recently introduced predators shifted at a smaller size onto benthos compared to perch from lakes with no predators and established predators. Early shifts onto benthos were interpreted as an indirect effect of predation, where perch were forced to utilize food from less optimal habitats in order to minimize predator risk.;This study indicates that recent introductions of few piscivorous predators may have profound effects on the overall biology of perch in these study lakes. Early shifts to a diet of benthos by perch may for example facilitate the recovery of zooplankton in a damaged lake. Knowing how perch respond to the invasions of predators will be useful to explain other limiting factors that impact recovery at various trophic levels.