Climate, competition, and disease are well-recognized drivers of ecological dynamics. These stressors can be intertwined by animal migrations, leading to uncertainty about the roles of natural and anthropogenic factors in biodiversity, conservation, and resource management. Furthermore, synergistic and antagonistic interactions among stressors may give rise to “ecological surprises” that cannot be predicted based on the additive effect of each stressor on its own. Sockeye salmon (Oncorhynchus nerka) from the Fraser River, British Columbia, whose migrations span freshwater, coastal, and offshore ocean ecosystems, once supported one of the largest salmon fisheries in the world. However, the productivity of many Fraser sockeye populations has declined over the past two decades. Using hierarchical models of sockeye spawner-recruit dynamics we quantitatively assessed the four leading hypotheses for this enigmatic long-term decline in Fraser sockeye productivity: (i) delayed density-dependence, (ii) local oceanographic conditions, (iii) exposure to farmed salmon, (iv) ocean-basin scale competition with pink salmon.
Results/Conclusions
We found no support for delayed density-dependence. Instead, our findings suggest that the long-term decline in Fraser sockeye productivity is primarily explained by competition with pink salmon and a synergistic interaction between exposure to farmed salmon and subsequent competition with pink salmon. In addition, our analyses suggest oceanographic conditions during early marine life mediate the effects of concurrent exposure to farmed salmon. These findings highlight the potential for the long migrations characteristic of Pacific salmon to link natural and anthropogenic stressors across coastal and oceanic environments and suggest oceanic-scale processes that are beyond the reach of current regulatory agencies may exacerbate local ecological process that challenge the coexistence of fishery and aquaculture-based economies in coastal seas.