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.

New lamprey-friendly fishways feature inclined ramps that facilitate passage of Pacific lampreys ( Lampetra tridentata (Richardson, 1836)) over Bonneville Dam on the Columbia River, USA. We observed the lampreys moving against water at two flow volumes and on two ramps of 45° and 18° angles relative to horizontal. We documented climbing movements using high-speed video (125 frames/s). Lampreys advanced on the ramps by repeated cycles of attaching to the ramps by their sucker mouths (resting phase), bending their bodies into a W shape (stage II), and then, rapidly straightening the body to propel themselves up the ramp, with simultaneous brief (20–140 ms) release of suction (stage III). We inferred that lampreys were using burst swimming to propel themselves up the ramp, because we observed inflection points in the body curvature traveling toward the posterior of the body and the center of mass moving up, during stage III. This climbing behavior is not described for any other fish species. Vertical motion, relative to the ground, during each cycle of movement was greatest in the 45° ramp – low water flow volume treatment (mean of 0.07 L/cycle), but the movement upstream along the ramp plane was greatest on the 18° ramp, regardless of flow volume. These findings can be used to develop ramp designs that maximize lamprey climbing performance.

We compared the vulnerability of naive and experienced coho and chinook salmon to predation by rainbow trout and related these findings to observations of the specific behaviour of each species during an encounter with the predator. Naive chinook were significantly less vulnerable to capture than naive coho in an open field encounter but there was no difference in capture rate of experienced coho and chinook. In videotaped encounters we found that coho and chinook behaved in significantly different ways when attacked by the predator, both when naive and when experienced. Our results suggest that predator avoidance strategies are highly species specific, that both species adjusted their behaviour after experience with the predator, and that the behavioural changes adopted by coho improved their ability to avoid the predator in an open field but those adopted by chinook did not. Considerable recent research has explored the willingness of different species, sexes, or life stages to risk exposure to a predator to obtain some reward (usually food). Our results further suggest that published experiments that purport to demonstrate differences in risk taking among types of prey may, instead, demonstrate differences in predator avoidance capability.

Populations of invasive bighead carp Hypophthalmichthys nobilis in the Mississippi River basin are rapidly increasing in size and range. However, the rate of expansion is not well understood. We used radiotelemetry to document movements of bighead carp within the LaGrange Reach of the Illinois River, Illinois, where populations have been documented since 1993. We surgically implanted radio transmitters into 42 adults in June 2003 and May-July 2004. Successful relocation of individuals decreased over time and ended in August of both years. We analyzed 132 observations from 23 adults and found a mean (6SE) movement rate of 1.70 6 0.74 km/d. The highest movement rate was 14.33 km/d. The maximum distance traveled by an individual was 163 km upstream in 35 d, and the top 10% of movements as observed by boat were between 26.5 and 56.5 km within 3-10 d. Forty-three percent of fish died or dropped transmitters for unknown reasons, but handling, environmental conditions, or both may have contributed to the loss. Our study is the first to document the movement rates and patterns of bighead carp within the United States and shows that adults are capable of moving considerable distances in a short time. Immediate actions to prevent or control their spread are warranted.