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.