How initial predator-prey ratio affects intra-cohort cannibalism and growth in Eurasian perch Perca fluviatilis L larvae and juveniles under controlled conditions

Two experiments were conducted in order to study whether initial density of potential cannibals in a stock or their early life history inducing high initial size heterogeneity affects further quantitative characteristics in Eurasian perch larvae or young juveniles restocked under intensive culture. In experiment 1, populations of half-sibling larvae produced from three females were pooled and sorted out as small and large fish on Day 30 post-hatching. Then, five experimental variants were set with 0, 1, 2, 4 and 8% of the largest fish (considered as potential predators) in each triplicate group of 200-medium fish per 20 l. In experiment 2, half-sibling larvae from stock A were produced 8 days earlier than larvae from stock B. On Day 45 post-hatching, basic stocks were composed of small and/or medium fingerlings of stock B; and 8 variants were set with 0, 2 and 4% of the largest fish of stock B or medium fish of stock A, considered as potential cannibals. Survival and growth related parameters were measured after 39 (experiment 1) or 32 (experiment 2) days. Results from both experiments show that initial weights and their variation increased significantly with the initial density of potential cannibals in the stocks but this did not affect significantly further cannibalism rates whatever the size-class differences or the early life history of cannibals. Nevertheless, type II cannibalism tended to decrease at low initial density of predators and to increase at high proportion in experiment 1. At the end of each experiment, large-growing fish appeared in all groups and their density did not differ. Survival and growth parameters were not affected by the initial intensity of predators or their early life history, except for a slight decrease in growth rate at higher initial density of cannibals in experiment 1 in relation to high initial size heterogeneity. Data from the present study indicate that cannibalism emergence is not consistently size dependent in Eurasian perch larvae or young juveniles, and that the re-establishment of this phenomenon at restocking is independent of the initial predator-prey relationship because size heterogeneity is negatively related to growth rate.