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).