Effects of subsidies from spawning chum and pink salmon on juvenile coho salmon body size and migration timing

Organisms transporting nutrients from highly productive ecosystems can subsidize food webs and alter ecosystem processes. For example, the carcasses and eggs of migratory Pacific salmon ( Oncorhynchus spp.) provide a high‐quality food source that could potentially benefit other species of salmon rearing in fresh water. We investigated relationships between spawning chum ( O. keta ) and pink ( O. gorbuscha ) salmon density, and the body size and age of juvenile coho salmon ( O. kisutch ) in 17 streams on the central coast of British Columbia, Canada. Chum salmon density was the most consistently important and positive correlate of coho body size, in comparison with pink salmon density, juvenile coho salmon density, and numerous characteristics of habitats. This was shown by comparisons both among and within streams, and between sites above and below natural barriers to spawning chum and pink salmon. In addition, streams that had higher chum and pink salmon spawning densities had a higher proportion of age 0 coho (less age 1), suggesting earlier juvenile coho salmon migration to the ocean with increased spawning salmon nutrient availability. Most of the coho salmon sampled had little or no direct contact with spawning chum and pink salmon, which suggests an indirect, time‐delayed influence on coho salmon body size.