Summary1. In gape-limited predators, body size asymmetries determine the outcome of predator-prey interactions. Due to ontogenetic changes in body size, the intensity of intra-and interspecific interactions may change rapidly between the match situation of a predator-prey system and the mismatch situation in which competition, including competition with the prey, dominates. 2. Based on a physiologically structured population model using the European perch (Perca fluviatilis), analysis was performed on how prey density (bream, Abramis brama), initial size differences in the young-of-the-year (YOY) age cohort of the predator, and phenology (time-gap in hatching of predator and prey) influence the size structure of the predator cohort. 3. In relation to the seasonality of reproduction, the match situation of the predator-prey system occurred when perch hatched earlier than bream and when no gape-size limitations existed, leading to decreased size divergence in the predator age cohort. Decreased size divergence was also found when bream hatched much earlier than perch, preventing perch predation on bream occurring, which, in turn, increased the competitive interaction of the perch with bream for the common prey, zooplankton; i.e. the mismatch situation in which also the mean size of the age cohort of the predator decreased. 4. In between the total match and the mismatch, however, only the largest individuals of the perch age cohort were able to prey on the bream, while smaller conspecifics got trapped in competition with each other and with bream for zooplankton, leading to enlarged differences in growth that increased size divergence. 5. The modelling results were combined with 7 years of field data in a lake, where large differences in the length-frequency distribution of YOY perch were observed after their first summer. These field data corroborate that phenology and prey density per predator are important mechanisms in determining size differences within the YOY age cohort of the predator.6. The results demonstrate that the switch between competitive interactions and a predator-prey relationship depended on phenology. This resulted in pronounced size differences in the YOY age cohort, which had far-reaching consequences for the entire predator population.

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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-ofthe-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.