Clark SC, Tanner TL, Sethi SA, Bentley KT, Schindler DE. (2015) Migration timing of adult Chinook salmon into the Togiak River (Alaska) watershed: Is there evidence for stock structure? Transactions of the American Fisheries Society 144:829-836. DOI: http://dx.doi.org/10.1080/00028487.2015.1031281
Variation in the timing of migration to freshwaters to spawn is one important dimension of the life history variation within individual salmon stocks. That variable has important implications for developing sustainable fisheries that simultaneously exploit multiple populations within the same geographic area. We examined the variation in migration timing of adult Chinook Salmon Oncorhynchus tshawytscha within the Togiak River system of southwest Alaska to determine whether fish that spawned in different habitats migrated into freshwater from the ocean at different times. In particular, we evaluated whether there were watershed-wide effects of body size, sex, spawning location, and year on the timing of spawning-run migration into the Togiak River. Migration data and spawning fate were collected using radiotelemetry and analyzed with mixed effects models to account for among-year variation in migration timing across the 4 years of the study. We found that migration timing of Chinook Salmon in the Togiak watershed is segregated by spawning location and sex and is consistent across body sizes and years. Fish that spawned higher in the watershed had a tendency to enter freshwater earlier than fish that spawned lower in the watershed and in the main stem of the river. Our results indicate clear stock structure in the migration of spawning Chinook Salmon to the Togiak River watershed, suggesting that it is important to distribute harvest over the entire distribution of Chinook Salmon run timing to minimize the risk of overexploiting certain components of the stock complex.