Pink salmon (Oncorhynchus gorbuscha) migratory energetics: response to migratory difficulty and comparisons with sockeye salmon (Oncorhynchus nerka)

Pink salmon (Oncorhynchus gorbuscha) are generally considered weak upriver migrants relative to sockeye salmon (Oncorhynchus nerka), though this assertion is largely anecdotal. To assess energy-use patterns during migration, we collected pink salmon from two major Fraser River stocks (Weaver and Seton in British Columbia, Canada) in 1999 at three times and locations: (1) at the start of freshwater migration, (2) at the end of migration before spawning, and (3) immediately after spawning. We calculated the energy content of somatic and reproductive tissues, recorded several body measurements, and conducted both intraspecific (between pink stocks) and interspecific analyses with co-migrating Fraser River sockeye salmon collected during the same season. We found that between pink salmon stocks, there were no significant energetic or morphological differences either at river entry or upon arrival at spawning areas regardless of the level of migratory difficulty encountered. When compared with sockeye salmon, however, we found that pink salmon began upriver migration with significantly smaller somatic energy reserves but made up for this deficiency by minimizing absolute transport and activity costs, presumably by seeking out migratory paths of least resistance. This energetic efficiency comes at a cost to reproductive output: relative to sockeye salmon, pink salmon diverted less absolute energy to egg production, producing smaller ovaries and fewer eggs. We speculate that fundamental differences in behaviour shape the migratory energetic tactics employed by pink salmon.