The influence of predation risk on the feeding motivation and foraging strategy of juvenile Atlantic salmon

Juvenile Atlantic salmon, Salmo salar, changed their foraging strategy markedly after a brief exposure to a model trout predator, in ways that reduced their conspicuousness and hence risk of being preyed upon. After seeing the predator, the salmon were less likely to orientate to passing food particles, and having orientated, were less likely to attack them. They also reduced the extent of their movements, by only attacking those food particles that came close to them, and by delaying attacks until the food had reached its closest point. They were slower to orientate to approaching food items, possibly because more visual attention was switched to scanning for predators and less to feeding. However, there was no change in the outcome of attacks, indicating that the reduced food intake was not caused by a reduction in appetite. The relative priority given to foraging and predator avoidance varied with time elapsed since the predator was last sighted; as a consequence, intake rates in the 20 min following the predator presentation averaged only 33% of the pre-predator level, but had increased to 57% 20 min later, and had recovered completely within 2 h.