Risk of predation and the feeding behavior of juvenile coho salmon (Oncorhynchus kisutch)

Summary During their first 1–2 years of life, juvenile coho salmon (Oncorhynchus kisutch) are stream-dwelling, and feed upon drifting invertebrates. They move upstream from a holding position to intercept individual prey items; the distance moved (attack distance) is an increasing, but decelerating, function of prey size. Since the fish are presumably more visible to predators during such feeding excursions, prey size and risk are associated variables.The effect on attack distance of the presentation of a model predator (a photograph of a rainbow trout) was examined in the laboratory. Attack distances are shortened following presentation of a predator; this is particularly true when the prey are large (Fig. 1). The extent of the reduction of attack distance is directly related to predator presentation frequency, although there appears to be a minimum level to which it will decline (Fig. 2). Hungry fish and fish in the presence of a competitor (simulated by a mirror) are less responsive to the predator, suggesting a trade-off of energetic requirements and risk (Fig. 3 and Table 3). The effect of predation risk should be to reduce the relative proportion of large prey in a juvenile coho's diet, and its net rate of energy intake.