Effects of the North Pacific Current on the productivity of 163 Pacific salmon stocks

Abstract

Horizontal ocean transport can influence the dynamics of higher‐trophic‐level species in coastal ecosystems by altering either physical oceanographic conditions or the advection of food resources into coastal areas. In this study, we investigated whether variability in two North Pacific Current (NPC) indices was associated with changes in productivity of North American Pacific salmon stocks. Specifically, we used Bayesian hierarchical models to estimate the effects of the north‐south location of the NPC bifurcation (BI) and the NPC strength, indexed by the North Pacific Gyre Oscillation (NPGO), on the productivity of 163 pink, chum, and sockeye salmon stocks. We found that for salmon stocks located in Washington (WA) and British Columbia (BC), both the BI and NPGO had significant positive effects on productivity, indicating that a northward‐shifted bifurcation and a stronger NPC are associated with increased salmon productivity. For the WA and BC regions, the estimated NPGO effect was over two times larger than the BI effect for pink and chum salmon, whereas for sockeye salmon the BI effect was 2.4 times higher than the NPGO. In contrast to WA and BC stocks, we found weak effects of both horizontal ocean transport processes on the productivity of salmon stocks in Alaska. Our results indicated that horizontal transport pathways might strongly influence population dynamics of Pacific salmon in the southern part of their North American ranges, but not the northern part, suggesting that different environmental pathways may underlie changes in salmon productivity in northern and southern areas for the species under consideration.

Time‐varying relationships between ocean conditions and sockeye salmon productivity

“…Following Mueter, Peterman, et al () and Malick et al (), I grouped sockeye salmon stocks into three groups based on the large marine ecosystem the stock enters the ocean into (Figure ). Stocks originating in B.C.…”

“…and southeast Alaska were grouped into the West Coast region; Central Alaska stocks (e.g., Alaska Peninsula, Kodiak) were grouped into the Gulf of Alaska region; and stocks from western Alaska were grouped into the Bering Sea region. The inclusion of southeast Alaska stocks in the West Coast region differs from both Mueter, Peterman, et al () and Malick et al (), and was based on evidence that productivity of southeast Alaska sockeye salmon stocks tends to covary more strongly with more southern sockeye salmon stocks than Central Alaska stocks (Peterman & Dorner, ).…”

“…Correlations between environmental conditions and the dynamics of fish populations can also vary across space, which is driven in part, by spatially variable climate–ocean linkages (Malick, Cox, Mueter, Dorner, & Peterman, ; Mueter, Peterman, & Pyper, ). For North American Pacific salmon stocks, correlations between SST and productivity are opposite in direction for stocks in the southern upwelling domain offshore of Washington and B.C.…”

“…and the northern downwelling domain in the coastal Gulf of Alaska (Mueter, Peterman, et al, ; Su, Peterman, & Haeseker, ; Ware & McFarlane, ). Similarly, the relative importance of horizontal ocean transport processes in explaining variability in salmon productivity tends to be stronger in southern areas compared to northern areas (Malick et al, ). This evidence for spatial heterogeneity in pathways linking climate and salmon populations suggests that temporal changes in environmental–salmon relationships in one location may not be indicative of time‐varying relationships in another location.…”

Conservation Risk and Uncertainty in Recovery Prospects for a Collapsed and Culturally Important Salmon Population in a Mixed‐Stock Fishery

“…The recovery plan synthesized the available data, reviewed the evidence for factors potentially limiting survival, and identified actions that could be taken to promote recovery (Connors and Atnarko Sockeye Recovery Planning Committee 2016). Broad-scale declines in Sockeye Salmon survival across British Columbia (Peterman and Dorner 2012) suggest that common mechanisms that are operating at sea such as the changing ocean climate (Malick et al 2017) and competition with increasing abundances of other salmon (Ruggerone and Connors 2015), coupled with disturbances affecting freshwater survival and productivity (including forest fires and severe floods), likely have contributed to the depressed state of Sockeye Salmon in Atnarko (Connors and Atnarko Sockeye Recovery Planning Committee 2016). The recovery plan identified the incidental harvest of Sockeye Salmon in commercial gillnet and seine fisheries that target other species as well as harvest in Indigenous fisheries for food, social, and ceremonial (FSC) purposes as a potential, but poorly understood, impediments to recovery.…”