Feeding grounds definition and validation
While for some of the stocks, such as Chilkoot, Okanagan and Wenatchee, the method resulted in well-defined feeding ground estimates, estimates for other stocks showed more uncertainty with larger areas showing high correlation and/or multiple regions with high correlation. Several approaches were used to assess these areas and to further resolve the feeding grounds. Data from tagging studies were used to define the limits of where the salmon are likely to be distributed (Figure S1.1). However, these limits encompassed large areas as many stocks were pooled, and so these data were mainly useful to discard high correlation spots that were located outside the potential distribution range. For example, for Red Lake and Upper station stocks (Kodiak Island), two secondary high correlation spots were located eastward of the range determined by the tagging studies and were therefore not considered. The EOF analysis of SST data was useful to understand connectivity between high correlation areas. The Chilkat stock distribution was a good example, with a high correlation area covering a spot in the middle of the Gulf of Alaska and extending northwest to Alaska. This may have been due to the SST of these water masses varying in a similar way during the time period considered for this stock (Figure 7). Based on the tagging study, the central Gulf of Alaska region was estimated to be more realistic.
Another way to validate the inferred stock distributions is to integrate information provided by the δ15N values. We would expect that the stocks with similar foraging areas would also show correlation between their δ15N time series, assuming that they feed at the same trophic level. However, very few stocks showed significant correlation of δ15N values (Figure S1.4). One explanation for this is that there was little overlap between stock distributions. Differences in return timing may also have been a factor, since the δ15N values of prey change rapidly over the productive season. Based on average levels of δ15N and δ13C (Figure 3), Bristol Bay stocks were separated from the others by their higher SI ratios. This was in agreement with the distributions of the stocks for which the correlation was significant (Ugashik, Naknek and Wood river). These stocks were distributed closer to the coast of the Aleutian Islands or onto the Bering Sea shelf, and both of these areas are more productive and display elevated SI ratios (Pomerleau, Nelson, Hunt, Sastri, & Williams, 2014). Similarly, we found a correlation between the δ15N of salmon prey and salmon for the stocks with feeding grounds identified in the Gulf of Alaska. This also allowed estimation of the trophic level for these stocks (Table 3). Salmon are reported to feed on a large variety of prey such as copepods, squids, euphausiids and amphipods (Kaeriyama et al., 2004), and, therefore, one would except their trophic level (TL) to range between 3 and 4, which is consistent with our estimates (TL = 3.3 to 3.8) and those from Qin and Kaeriyama (2016) (TL = 3.9).