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Food-dependent foraging activity by prey fish mediates population-level mortality in whole-system experiments.
Biro, Peter1, Post, John1, 1
ABSTRACT- Behavioural ecologists have long promised that an understanding of behavioural trade-offs would lead to a better understanding of population dynamics. Recently, theoretical and empirical studies aimed at linking individual behaviour with populations have shown that adaptive trade-offs between prey foraging activity and risk of predation can have important population-level effects on their growth and survival. These studies predict reduced foraging activity with increases in predation risk or food abundance. Thus, food-dependent predation mortality, via increased foraging activity when food declines, represents a novel and very general mechanism for prey mortality. We tested this mechanism using age-0 rainbow trout under risk of cannibalism in a 2x2 factorial experiment. We used nine whole lakes and manipulated food abundance (via whole-lake fertilization) and predation risk (adult trout density) over one growing season. Mortality of age-0 trout increased from 40% at high food abundance to 61% at low food abundance, but did not vary with risk or the interaction. Individual activity and use of riskier (deeper) habitats were also food-dependent. The proportion of time individuals spent moving, the proportion of each cohort that was continuously active, and the velocity of continuously active individuals were all significantly lower in high food lakes. Young trout used deeper habitats to a greater extent in low, as compared to high food lakes. None of the measures of risk-taking were related to risk or the food*risk interaction.
KEY WORDS: behaviour, population, predator-prey, foraging