Differences in group size and behavior of juvenile salmonids from complex and simple reaches of a cold-water Wisconsin stream

We report here the findings of a study that explored habitat-specific behaviors in juvenile salmonids from a northern Wisconsin stream. Over three summers, we made underwater observations on foraging group size, feeding and aggression of more than 1000 young-of-year coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) from open and complex reaches of the Cranberry River, WI. In both species, foraging groups were consistently and significantly larger in the simple reach. Most notable was the pattern in coho salmon, where group size peaked at 10 fish in the open reach and around two fish in the complex reach. A field experiment revealed that both feeding and aggression in coho salmon also peaked at intermediate group sizes of 10 fish; a pattern consistent with theoretical expectations concerning costs and benefits of group membership. It is noteworthy that the group size associated with maximal foraging was identical to the peak group size that we observed for coho salmon in the open reach. Together, our research findings provide rare field support for the view that foraging success is affected by group size, and further, that group foraging is more likely under conditions of elevated predation risk (i.e., open stream habitats).