3-D Territoriality and Shadow Competition within Schools of Juvenile Chinook Salmon

Riverine salmonid populations are often influenced by competition for food, which can affect population structure in many ways depending on the territorial behavior of individuals. This dependence has motivated many studies of salmonid territoriality, but most such work focused on adult fish in medium-sized rivers and juvenile fish in small streams, with large, sparsely distributed territories easily represented in 2 dimensions (2-D). Such knowledge may not transfer to small fish in large rivers, such as young-of-the-year Chinook salmon that inhabit the main stems of rivers in the Yukon River drainage. In the Chena River in Alaska, predation risk compels salmon to feed in 3-dimensional (3-D) schools, with some fish directly above or below others. We sought to understand competition within these schools, where the well-known salmonid propensity for territoriality conflicts with risk-induced schooling, juxtaposing two ostensibly opposite behaviors. We used 3-D video techniques to measure salmonid foraging attempts and conflicts in situ, and extended 2-D kernel home range analysis techniques into 3-D to represent potential feeding territories and their interactions. We found that juvenile Chinook salmon in the Chena River defend feeding territories within their 3-D schools, and that shadow competition from upstream competitors affects fish positions within the school.