Foraging and use of cover by juvenile coho salmon (Oncorhynchus kisutch) were affected by predation threat in both seminatural channels and laboratory streams. In the field, coho salmon preferred stream sections with brushy cover only when under threat from hunting common mergansers. The mergansers had their highest capture success in pools without cover. Predation threat also caused coho salmon to use cover more as foraging habitat and to aggregate more in favourable positions at the head of the pool. In the laboratory, under simulated predation threat, fish using a refuge were significantly larger than those in the risky habitat. This pattern persisted for 2 days after the predation threat was discontinued. The average growth of coho salmon under predation threat was depressed and the difference in growth between large and small individuals was less than in control groups. We argue that larger fish were more averse to predation risk than smaller fish and that the smaller fish took advantage of feeding opportunities indirectly provided as a result of the predation risk. We speculate that in natural environments, predation may depress growth rates because of risk-avoidance behaviour but may also serve to reduce growth-rate differences among size classes within a cohort.

Dominance, aggression and predator avoidance were compared among farmed, sea‐ranched and wild juvenile masu salmon Oncorhynchus masou in laboratory experiments. Domesticated fish (farmed and sea‐ranched), which had been exposed to artificial selection, were not dominant against wild fish in pairwise contests, nor did they show greater aggressiveness. Farmed fish did show greater feeding than wild fish. Under chemically simulated predation risk, farmed fish were more willing to leave cover and feed than wild fish, indicating reduced predator avoidance in the farmed fish. Our results indicate that selection for fast growth (domestication) in masu salmon favors fish that respond to food quickly and ignore predation risk.

The feeding behavior of Japanese masu salmon Oncorhynchus masou (also known as cherry salmon) 3–4 months old differed among fish from wild, farmed, and sea‐ranched parents. Between feeding intervals, wild‐derived fry groups stayed lower in their separate stream tanks than farmed or ranched fry. When slow‐sinking food was offered, the wild fry made foraging bouts from the bottom, whereas the domesticated (farmed and sea‐ranched) fry stayed close to the surface. Consequently, the domesticated fry finished their meal faster than the wild fish. Over time, all stocks moved up higher in the water column and fed faster, but a difference between wild and domesticated fish persisted. Levels of aggression were similar among the three stocks and observation periods. Sea‐ranched fish were intermediate between wild and farmed fish in most recorded behaviors. Staying close to the water surface obviously is a successful strategy under standard salmon husbandry conditions, but an innate tendency to seek the surface is probably maladaptive in the wild. New hatchery techniques that teach fish to avoid the surface or prevent selection for surface‐seeking behavior promise to bolster the survival of postrelease ranched salmon.

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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.