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

Groups of coho salmon (Oncorhynchus kisutch) fry in stream tanks formed siz e-determined dominance hierarchies, which were upheld through aggressive in teractions and led to smaller fish occupying inferior feeding positions. Un der simulated predation risk, the frequency of agonistic interactions was r educed, but more intensive aggressive behaviours were performed. This allow ed 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 e xposed pool had greater growth rates than those that mainly occupied the re fuge, while large fry that exposed themselves to predation did not grow bet ter than their risk-avoiding counterparts. Differences in risk taking and g rowth among small coho fry in this experiment may support reports for Atlan tic salmon (Salmo salar) of a split into different life history trajectorie s. The observations suggest that the presence of predators creates opportun ities for the expression of alternative behavioural strategies that are abs ent under size-dependent dominance hierarchies.