Annual coastal migration of juvenile Chinook salmon: Static stock-specific patterns in a highly dynamic ocean

Abstract

While recent studies have evaluated the stock-specific coastal migration of juvenile Chinook salmon, it remains unclear if these seasonal patterns are consistent between years, particularly when ocean conditions change dramatically. Here we contrast the abundance, distribution and seasonal stock compositions of juvenile Chinook salmon between years in 3 oceanographic regions of the Pacific from southern British Columbia to southeast Alaska. Between 1998 and 2008, we surveyed salmon in various months from June through March, in different regions along the shelf. Variable conditions in the North Pacific Ocean, as well as large overall shifts in ocean regimes were extensively documented over this decade. We employed genetic stock identification to identify mixed-stock compositions; fish (n = 6274) were allocated to one of 15 regional and 40 subregional stocks. Catch-per-unit-effort and distribution of salmon, as denoted by centre of mass, varied significantly between seasons, regions and years. In a similar manner, fish body size and dryweight varied significantly between years, seasons and regions. Despite these inter-annual differences in catch, distribution, fish growth performance and large variations in ocean conditions encountered by salmon over the time period of the study, we observed no response in terms of shifts in stock-specific distributions. Regional stock composition was similar between years, suggesting migration patterns for all stocks remain consistent despite fluctuations in the marine environment: local stocks remain resident in respective coastal areas during their first year at sea, except for Columbia River salmon, which move quickly into waters north of Vancouver Island in summer. © 2012 Fisheries and Oceans Canada.