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 (Carr‐Harris 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.