Risk-taking behaviour in foraging young-of-the-year perch varies with population size structure.

I investigated if risk-taking behaviour of young-of-the-year (YOY) perch Perca fluviatilis was connected with population-specific predation patterns in four lakes in northern Sweden. The lakes differ in perch size distribution, according to earlier fishing surveys. Thus, the most intense predation pressure by cannibals is assumed to occur at different prey-size windows in the four lakes. In an aquarium study, I observed groups of perch, and registered time spent foraging in an open habitat and number of prey attacks in the presence of a predator. Perch from Angersjön, with the highest proportion of large fish in the population, spent more time in the open area than those from Fisksjön that has a dense population of mainly small perch. The Angersjön perch also made more prey attacks than did perch from Fisksjön and Bjännsjön. Relative differences in predation risk in the four lakes were estimated as cannibalistic attack rates, on a range of sizes of YOY perch, calculated from population size distributions. Principal component analysis on predation risk patterns resulted in two components, of which PC1 explained 79.1% of the variation. High scores of PC1 indicated low cannibalistic attack rates on smaller perch (10-20 mm) and high rates on larger fish (> or =30 mm), while low scores indicated the opposite. The level of risk-taking behaviour in the aquarium study positively correlated with lake-specific PC1 scores. The perch with the most cautious behaviour in the aquaria originated from the population with the highest predation pressure on early stages. The boldest perch came from the lake with low predation on the smallest, but with higher predation on larger YOY perch. Thus, the influence of predation risk on behaviour patterns in perch may depend on the timing of the highest exposure to predators.