The study of grouping behavior in fishes has often been conducted in laboratory experiments; comparatively few studies have been done in the wild. Studies of wild populations allow scientists to examine questions concerning grouping behavior, including but not limited to the following: 1) at what group sizes will fish most commonly occur, 2) what are group-size related costs and benefits and 3) why do costs and benefits vary with group size? Stream-dwelling juvenile coho salmon (Oncorhynhus kisutch) can serve as a model for studying grouping behavior in the wild because they occupy small home ranges, are common, and occur in environments that facilitate underwater observation. In this study, we conducted snorkeling observations of coho salmon 1) to determine whether individuals forage in groups of optimal size (defined by foraging success) and 2) to test alternative hypotheses about the factors that may explain group-related variability in foraging success (i.e., reduced predation or increased resource competition). With respect to our first objective, we counted and estimated the sizes of over 400 foraging groups, and quantified individual patterns of foraging and aggression from another 300 groups (from 2 – 40 individuals). As a test of the second objective, we used structural equation models incorporating factors we assumed to be related to predation risk and resource competition. These factors were food availability, microhabitat conditions and body size (i.e., smaller fish more risk prone). Therefore, our study was designed to assess group-related foraging success under different resource and microhabitat conditions, and over two seasons (early and late summer).
Our study generated two principle findings. First, salmon were disproportionately represented in groups of intermediate sizes, and foraging success was maximized at these sizes. Our second finding, derived from the structural equation models, concerned the alternative explanations for group-related foraging success. Model output indicated no effect of food availability on group size or feeding. Group size affected aggression (+) but aggression did not predict foraging success. These results are not consistent with the hypothesis that resource competition is the mechanism behind the link between group size and foraging success. The models do, however, support the predation risk hypothesis. For small fish, predation risk was a significant predictor of foraging success, acting indirectly through group size. This relationship was absent for large fish. To our knowledge, the results from this study are among the first to be documented in a wild fish population.