Influence of Movement by Coho Salmon ( Oncorhynchus kisutch) Parr on Their Detection by Common Mergansers ( Mergus merganser)

A common reaction in juvenile salmonids that detect predators is to decrease activity. To test whether there is a survival advantage to reduced movement under such circumstances, juvenile coho salmon ( Oncorhynchus kisutch) were placed with common mergansers ( Mergus merganser) under laboratory conditions that precluded crypsis. Mergansers were more likely to attack fish that moved than fish that remained stationary, and the relation between the lag time for detection by the birds and fish movement rate is best described as inversely exponential. The lag time for detection was not correlated with fish size. The risk of detection of prey by visual predators should be determined by both predator and prey behaviour, but our results suggest that in this case prey behaviour is more important. In the field, territorial coho juveniles do feed more and grow faster than other, non‐territorial fish. They also spend a smaller proportion of their time moving. The present experiment suggests that territorial coho may also suffer less mortality than non‐territorial fish, which would lower their ratio of mortality risk to growth rate.