Cannibalism and competition in eurasian perch: population dynamics of an ontogenetic omnivore

In many cannibalistic populations, cannibals share resources with their victims, leading to a size‐dependent mixture of cannibalistic and competitive interactions. We analyze the impacts of such interactions on the population dynamics of Eurasian perch (Perca fluviatilis) by considering effects of intercohort competition, habitat heterogeneity, habitat selection, and energy gain made by cannibals. Over a six‐year period, we measured mortality and recruitment patterns, individual growth, body condition, resource levels, diets, and habitat use as functions of density for an allopatric perch population in a low‐productivity lake. During the course of the study, two major die‐offs took place, selectively affecting larger cannibalistic individuals, followed by several years of successful recruitment of young fish. Habitat use of perch ≥2 yr old was density dependent, and these fish used only the inshore region at low densities. The appearance of young fish followed the die‐offs of cannibalistic perch and their subsequent absence from the offshore area, both of which decreased cannibalism on pelagic recruits. Whereas die‐offs of larger perch could not be related to competition with young‐of‐the‐year (YOY) perch, evidence for a competitive impact of YOY perch on Age‐1 perch was present due to substantial food overlap. A strong depression in pelagic zooplankton was observed during summer in years with strong recruitment, which resulted in reduced consumption of zooplankton, slow growth, and reduced body condition in Age‐1 perch and suggested high mortality of Age‐1 perch in autumn. Age‐1 perch did not appear to profit substantially from cannibalism on YOY perch because of the short time period that they could efficiently prey on YOY perch. The few larger perch that survived the die‐offs gained substantial energy from cannibalism in years with strong recruitments, which increased both growth rates and per capita fecundity. Size‐dependent intercohort competition may have strong impacts on cannibal–victim interactions when victims share resources with cannibals. Furthermore, habitat heterogeneity, combined with habitat selection, may limit the extent to which cannibals have a stabilizing effect on population dynamics. Finally, the energy gained by cannibals may have important consequences on population dynamics as this energy is allocated into new recruits.