Implications
A broad regional approach, assuming a generalized distribution for all
salmon stocks in the North Pacific, has been demonstrated to be useful
in linking salmon productivity with climate indices (Malick, Cox,
Mueter, Dorner, & Peterman, 2017; Mantua, Hare, Zhang, Wallace, &
Francis, 1997). However, other studies have stressed the importance of
stock specific analysis, as stocks can have different productivity
trends despite having geographically close spawning grounds (Quinn,
Rich, Gosse, Schtickzelle, & Grant, 2012; Rogers & Schindler, 2011),
and there is a need to differentiate between factors affecting salmon at
local to regional scales (Ohlberger, Scheuerell, & Schindler, 2016).
The unique SI signatures for each stock implies different feeding
locations, and this study used a recently developed approach based on
SST to infer stock-specific maturing sockeye distributions.
Stock-specific at sea distributions is an important knowledge gap that
may contribute to understanding of stock specific productivity trends.
Changes in environmental conditions due to hydrographic or atmospheric
processes impact the physical environment experienced by salmon and the
quality and / or quantity of prey that they encounter. Because of the
heterogeneity in their prey distribution, the high seas distributions of
salmon can affect their productivity.
At a large spatial scale, the difference in trends of productivity
between Bristol Bay stocks and most of the NE Pacific stocks during the
last decades may be explained by differences in high seas distributions.
In this study, we found Bristol Bay stocks to be primarily distributed
around the Aleutians Island and Bering Sea, while the NE Pacific stocks
were distributed in the off shelf area of the Gulf of Alaska. These
stocks would thus have experienced different environmental conditions
with respect to both annual conditions and long term trends in factors
such as SST (Mueter, Peterman, & Pyper, 2002) and ocean currents
(Malick et al., 2017).
At a regional scale, stock-specific distributions can help explain
divergence in productivity of stocks with geographically close spawning
grounds. Peterman and Brigitte (2012) investigated similarity in
temporal variation of productivity of several sockeye stocks and found
that among Bristol Bay stocks, Wood River, Naknek, Togiak and Igushik
stocks grouped together while Egegik and Ugashik formed another group,
breaking the usual south/north geographical separation of the stocks.
However, this grouping is consistent with the pattern of salmon at sea
distributions found in this study, with Wood River, Naknek and Igushik
distributed along the south coast of the Aleutian Islands and Egegik and
Ugashik in the Bering Sea. This highlights the uncertainty associated
with assuming that stocks with similar spawning ground locations have
similar at-sea distributions, and the need for at-sea stock ID data to
explicitly connect stocks to high seas conditions.
Another aspect of the high seas life phase that is currently unknown is
what drives salmon at-sea distributions. Feeding ground estimates for
stocks relative to their origin allows the development of a schematic
view of the potential migration sequence determining high-sea salmon
distributions. In the NE Pacific, juvenile sockeye salmon enter the
ocean at the beginning of summer and typically migrate following the
general counter clockwise circulation, northward or westward, depending
on stock location, at a speed of about 14 km d-1(David W. Welch et al., 2011), although this seems to vary depending on
stock (Tucker et al., 2009). The time of ocean entry is influenced by
numerous factors, including distance between the rearing lake and the
ocean (CarrāHarris et al., 2018) and the seasonal pattern of lake water
warming. The timing of ocean entry and the smolt size will define the
distance that they are able to cover before their first ocean winter.
This, in combination with the entry location, will result in juveniles
from different stocks being distributed in different parts of the coast
of the Gulf of Alaska when winter starts. We suggest that this will
define their high seas distribution for the rest of their marine life
assuming that they are then restricted to seasonal offshore/onshore
migration in fall/spring (Burgner, 1991). This would explain why
Columbia stocks, which have a relatively late ocean entry time due to
the long distance from rearing ground to the river mouth, were
distributed in the northern part of the Gulf of Alaska. The SE Alaska
and Kodiak juvenile salmon seem to move westward and then eventually
southward if time allows. For example, Chilkoot smolts, which rear in a
glacial lake, likely enter the sea later than Chilkat smolts and were
also measured to be significantly smaller in size (Bergander, 1988). As
a result, they end up in the area surrounding the Aleutians Islands,
while salmon from the nearby clearwater systems such as Chilkat, and Red
Lake / Upper station stocks (Kodiak Island) travelled further and were
distributed further south. This strategy may allow sockeye salmon to
more broadly colonize much of the marine domain, utilizing the ocean
carrying capacity over a large area, and minimizing density dependence
and risk of food shortage.