Predation risk breaks size-dependent dominance in juvenile coho salmon (Oncorhynchus kisutch) and provides growth opportunities for risk-prone individuals

Abstract

Groups of coho salmon (Oncorhynchus kisutch) fry in stream tanks formed size-determined dominance hierarchies, which were upheld through aggressive interactions and led to smaller fish occupying inferior feeding positions. Under simulated predation risk, the frequency of agonistic interactions was reduced, but more intensive aggressive behaviours were performed. This allowed small fish to gain access to better feeding positions. The effect of the predation threat on coho behaviour seemed to extend across a riffle into a second pool that served as a refuge. Smaller fry that chose to be in the exposed pool had greater growth rates than those that mainly occupied the refuge, while large fry that exposed themselves to predation did not grow better than their risk-avoiding counterparts. Differences in risk taking and growth among small coho fry in this experiment may support reports for Atlantic salmon (Salmo salar) of a split into different life history trajectories. The observations suggest that the presence of predators creates opportunities for the expression of alternative behavioural strategies that are absent under size-dependent dominance hierarchies.