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).