Area restriction and result validation
Initial plotting of the correlation coefficient between δ13C and SST can display potential spurious correlations due to inappropriate time series (small dataset or irregular sampling) or synchrony in SST variation across the North Pacific. A range of methods were explored to narrow down the area with highest correlation and propose the most realistic and accurate salmon distributions. The main source of information on salmon distributions in the literature comes from the tagging studies conducted as an international collaboration between Japanese, Canadian and American fishery institutes from 1956 to 1970 (French et al., 1976). More than 60 000 sockeye salmon x.2 (two years spent at sea) or older were caught and tagged over this time period. The data were used to produce distribution maps of maturing sockeye, a caveat being that since the fish were caught and tagged between late April to June, they were potentially already heading to their natal river system. We digitalized these maps and included them as supplementary materials (Figure S1.1). Another approach to indirectly validate the proposed feeding grounds is to compare the average level of δ13C and δ15N values for stocks supposedly distributed in the same area and/or to test for δ15N correlation between stocks (assuming that stocks with the same at-sea distribution will have correlated δ15N series). A pre-requirement for the latter is to check that δ13C and δ15N values are not correlated, which is stock dependent. Finally, we used an empirical orthogonal function (EOF) analysis to describe the spatial mode of SST variability. This provided insight into the connectivity between areas with high correlation values.
The mean δ15N of salmon stocks found to be distributed in the NE Pacific were compared with zooplankton SI values published in Espinasse, Hunt, Batten Sonia, and Pakhomov (in press). The zooplankton SI values were extracted from the isoscapes at a location centered on the highest likelihood distribution of each stock and averaged over one degree latitude and longitude. Stock specific trophic level was estimated assuming a trophic enrichment factor of 3.4‰ (Post, 2002).