Does the use of alternative food resources induce cannibalism in a size-structured fish population?

Cannibalism in fish has various effects at population and individual level. Cannibals derive energetic benefit by feeding on their smaller conspecifics, while at the same time reducing competition for the shared resource. Avoiding intracohort competition can force fish to use alternative feeding strategies, which may subsequently stimulate cannibalism. This study examined diet variation and intracohort cannibalism by young-of-the-year (YOY) European perch (Perca fluviatilis) after hatching in three stocking experiments in ponds based on stomach content and stable isotope analyses. Because zooplankton biomass decreased, YOY perch started to diverge in diet use, suggesting that increased intraspecific competition forced fish to try alternative feeding strategies. Larger YOY perch then fed on energetically richer resources (bream larvae or odonates) and hence outgrew their smaller conspecifics, becoming large enough to enable cannibalism. Thus, the widening of initial size differences by feeding on high-energy resources served as a stepping stone towards cannibalism. This differentiation into two size classes and two different feeding strategies was shown to be a stable pattern within the YOY perch population, as indicated by stable isotope analysis.