Group size affects predation risk and foraging success in Pacific salmon at sea

Abstract

Grouping (schooling, flocking, herding) is broadly distributed across taxa and environments, and is particularly common in marine fishes. A rich body of theory outlines ways in which grouping can enhance individual fitness, especially by improved predator avoidance and foraging efficiency. However, such theories are difficult to test in the wild, especially in marine environments where observing individuals is challenging, and quantifying predation risk or foraging success is often impossible. To overcome these difficulties, I analyzed a multi-decadal data set from sampling of Pacific salmon (Oncorhynchus spp.) in the ocean. Across all species, individuals in larger groups had lower risk of predator attack, inferred from wounds on surviving fish. Within groups, outliers (smaller and larger fish) were disproportionately attacked by predators, suggesting that collective predator avoidance resulted from a confusion effect. For slower growing species, individuals in larger groups had lower foraging success, indicating that schooling presented a trade-off between predator avoidance and resource consumption. In contrast, for faster growing species, individuals in larger groups had higher foraging success, indicating that this trade off may not exist and individuals in larger groups may even benefit from collective foraging. These results support long-standing theories on the benefits of group living that have rarely been tested in wild populations, and challenge earlier assertions that adult salmon do not school in the marine environment. Ultimately, these results indicate that survival and growth are group-size dependent and thus understanding the relationship between group-size distributions and population size may be critical to unraveling ecology and population dynamics for these and other marine fishes.