Linkages between life history type and migration pathways in freshwater and marine environments for Chinook salmon, Oncorhynchus tshawytscha

Chinook salmon, Oncorhynchus tshawytscha, are commonly categorized as ocean-type (migrating to the ocean in their first year of life) or stream-type (migrating after a full year in freshwater). These two forms have been hypothesized to display different ocean migration pathways; the former are hypothesized to migrate primarily on the continental shelf whereas the latter are hypothesized to migrate off the shelf to the open ocean. These differences in migration patterns have important implications for management, as fishing mortality rates are strongly influenced by ocean migration. Ocean-type Chinook salmon predominate in coastal rivers in the southern part of the species' range, whereas stream-type predominate in the interior and northerly rivers. This latitudinal gradient has confounded previous efforts to test the hypothesis regarding ocean migration pathways. To address this problem, we used a pair-wise design based on coded wire tagging data to compare the marine distributions of stream- and ocean-type Chinook salmon from a suite of rivers producing both forms. Both forms of Chinook salmon from the lower Columbia River, Oregon coast, lower Fraser River, and northern British Columbia rivers followed similar migration paths, contradicting the hypothesis. In contrast, recoveries of tagged Chinook salmon from the upper Columbia River, Snake River, and the upper Fraser River revealed migration patterns consistent with the hypothesis. These findings have important implications for our understanding of these life history types, and also for the conservation and management of declining, threatened, or endangered stream-type Chinook salmon populations in the US and Canada. (C) 2012 Elsevier Masson SAS. All rights reserved.