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

In many cannibalistic populations, cannibals share resources with their vic tims, 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 i ntercohort competition, habitat heterogeneity, habitat selection, and energ y 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 popu lation 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 su ccessful recruitment of young fish. Habitat use of perch greater than or eq ual to 2 yr old was density dependent, and these fish used only the inshore region at tow densities. The appearance of young fish followed the die-off s of cannibalistic perch and their subsequent absence from the offshore are a, both of which decreased cannibalism on pelagic recruits. Whereas die-off s of larger perch could not be related to competition with young-of-the-yea r (YOY) perch, evidence for a competitive impact of YOY perch on Age-1 perc h was present due to substantial food overlap. A strong depression in pelag ic zooplankton was observed during summer in years with strong recruitment, which resulted in reduced consumption of zooplankton, slow growth, and red uced body condition in Age-1 perch and suggested high mortality of Age-1 pe rch in autumn. Age-1 perch did not appear to profit substantially from cann ibalism on YOY perch because of the short time period that they could effic iently prey on YOY perch. The few larger perch that survived the die-offs g ained 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. Furthermor e, habitat heterogeneity, combined with habitat selection, may limit the ex tent to which cannibals have a stabilizing effect on population dynamics. F inally, the energy gained by cannibals may have important consequences on p opulation dynamics as this energy is allocated into new recruits.