Discussion
In this study we estimated the high seas distribution and foraging grounds for 17 sockeye salmon stocks, finding significant correlations for 13 of them. However, the spatial pattern of correlation between SST and δ13C did not always result in the identification of a single well defined foraging region. Aside from the potential influence of factors other than SST on carbon isotopic composition, the accuracy of the results relies on the assumptions that 1) the fish go to a similar place every year and 2) that they do not move across strong SST/prey δ13C gradient during the time period over which the SI values are integrated.
The first assumption is difficult to assess given that there are limited data available on high seas stock-specific locations. The question is probably even more essential for stocks with a strong pattern in age distribution (one predominant age-class), resulting in little mixing between cohorts from different brood years. Another aspect that should be investigated is whether there are differences in SI values between early and late runs for the same stock. While for some stocks, adult salmon return predominantly occurs over a short time window, other stocks have two temporally separated runs identified and it is not currently known if high seas distributions differ between runs.
The second assumption is also difficult to evaluate as no data are available for stock-specific movement. A strong gradient in δ13C values at lower trophic levels is usually located at the transition between the coastal system (subject to fluvial input, resuspension, etc.) and the open ocean system (more oligotrophic conditions). This transition usually occurs above the shelf or continental slope, depending on the width of the shelf, local hydrodynamics and water depth, but in all cases occurs at a relatively short distance from the coast (El-Sabaawi, Trudel, Mackas, Dower, & Mazumder, 2012; Kline Jr, 2009). Maturing salmon will likely only swim through this zone once, just before entering the river system and it is therefore unlikely to influence their SI values. They could, however, target areas of higher productivity while in the high seas. For example, eddies have the potential to enhance local primary production that will promote development of salmon prey (Crawford, Brickley, & Thomas, 2007; Mackas, Tsurumi, Galbraith, & Yelland, 2005) with higher δ13C values (Espinasse et al., in press). Unfortunately, it is not yet documented whether the salmon display such behavior.