Size‐Specific Interactions in Lake Systems: Predator Gape Limitation and prey Growth Rate and Mortality

To study the effects of different predators on the behavior and dynamics of their prey, we compared the performance of perch (Perca fluviatilis) and the abundance of their prey resource in four lakes with two different types of piscivory. In two of the lakes, perch were the only fish species present and piscivory was restricted to cannibalism. In the other two lakes, perch co—occurred with piscivorous pike (Esox lucius). Pike grow to larger sizes and can capture larger prey than piscivorous perch. Therefore, perch reach a size refuge earlier in the lakes without pike. Perch in the lakes lacking pike used both the epilimnion and hypolimnion habitats of the lakes, and a high proportion of the perch occurred below the thermocline. In contrast, perch in the lakes with pike were mainly captured along the littoral zone and were never captured below the thermocline. This difference in habitat use was reflected in different diets of perch in the different lakes. Perch in the lakes with pike were also less abundant than those in the lakes lacking pike and the growth rates of individual perch and the densities of predator—sensitive prey (pelagic and littoral) were higher, indicating that the presence of pike resulted in decreased intraspecific competition in perch. Evidence for a behavioral response to predation risk was found in 1 + (1 yr old) perch, which was suggested to be due to predation risk from both larger perch and pike. It is hypothesized that the use of the cold—water hypolimnion habitat by perch in the lakes lacking pike was a response of perch to reduce metabolic demands under conditions of severe resource limitation. Differences in gape limitation between predators species are suggested to have major effects on size—dependent mortalities and growth rates in prey and thereby on overall community dynamics. Evidence for this latter effect was observed in differences in zooplankton size structure between the lakes with and without pike.