Estimating population-specific predation effects on Chinook salmon via data integration

Recent success in the conservation of many marine mammals has resulted in new management challenges due to increasing conflict with fisheries. Increasing predation by pinnipeds on threatened salmon is of particular concern. Seemingly, pinniped conservation is now in conflict with the recovery of threatened salmon, creating a dilemma for managers. We use the Lower Columbia River as a case study for examining the relationship between seasonal California sea lionZalophus californianusabundance and survival of threatened salmon. To quantify mortality associated with increasing sea lion abundance, we examined the effect of seasonal sea lion abundance on adult Chinook salmonOncorhynchus tshawytschasurvival during migrations through the Lower Columbia River. We integrated data on survival with data on population-specific migration timing, allowing quantification of the relationship between sea lion abundance and survival in 18 populations of spring-summer Chinook salmon listed as Threatened or Endangered under the U.S. Endangered Species Act. Of the 18 populations examined, earlier migrating populations experienced lower survival in association with increased exposure to higher sea lion abundance. We estimated that in years with high sea lion abundance, the nine earliest-migrating populations experienced an additional 21.1% (95% CI = 16.3-26.1) mortality compared to years with baseline sea lion abundance, while the nine latest migrating populations experienced an additional 10.1% (7.5-13.0). Synthesis and applications. Integrating datasets on seasonal survival and migration timing made it possible for us to estimate population-specific mortality associated with increased sea lion abundance in the Lower Columbia River. This information could not be produced from any one dataset, highlighting the utility of data integration approaches. The mortality experienced by early migrating Chinook salmon suggests the potential for demographic and evolutionary consequences. Management actions such as hazing, relocating, or removing individuals that are frequent predators on salmon have been proposed. Identifying the management actions that will allow for socially and legally acceptable trade-offs between multiple conservation and other social values will be facilitated by development of explicit multi-species management frameworks. Continued monitoring will help to reduce the substantial uncertainty about the effect of pinnipeds on salmon and the predicted outcomes of alternative management actions.

Automated summary

Recent success in marine mammal conservation has led to new management challenges, as conflicts with fisheries, particularly pinniped predation on threatened salmon, have arisen. A study in the Lower Columbia River found that increasing sea lion abundance led to lower survival rates for early-migrating Chinook salmon populations, highlighting the need for management actions to mitigate the impact of pinnipeds on salmon populations.