Gulf of Alaska | Mammals
Contrasting responses of Steller sea lions during a marine heatwave and resulting implications for fisheries management Presenter: Elizabeth Mchuron , emchuron@uw.edu, University of Washington Cooperative Institute for Climate, Ocean and Ecosystem Studies Kathryn Sweeney , Katie.Sweeney@noaa.gov, Marine Mammal Laboratory, Alaska Fisheries Science Center, NOAA Brian Fadely , brian.fadely@noaa.gov, Marine Mammal Laboratory, Alaska Fisheries Science Center, NOAA The 2014 – 2016 marine heatwave in the Gulf of Alaska (GOA) resulted in changes in the biological community that persisted well past 2016, including shifts in biomass and distribution across multiple trophic levels. For Steller sea lions (SSL), a top predator in the system, documented responses included decreased abundance in the eastern GOA and increased abundance in the western GOA. The goal of this study was to use count data from aerial surveys of terrestrial sites to provide a more in-depth understanding of SSL responses to the heatwave and couple those responses with a bioenergetic model to predict resulting shifts in SSL predation pressure on fish communities. Pup production and non- pup counts were anomalously high in 2015 relative to predicted estimates from population trends, particularly in the eastern GOA and Southeast (SE) Alaska. This trend reversed in 2017 with anomalously low pup production in all regions except the western GOA, particularly in the eastern and central GOA. Non-pup counts in 2017 largely followed these same trends, except in the central GOA where there was a large influx of primarily juvenile SSLs. The western GOA also exhibited increased non-pup counts during 2017, primarily due to adult females. In 2019, juvenile SSL counts in the eastern, central, and western GOA declined below pre heatwave levels, indicating that in addition to reduced pup production, the heatwave likely impacted pup survival and potentially survival of other age groups. Coupling SSL count fluctuations between 2015 and 2017 with age-specific energy intake from the bioenergetic model indicates there was likely significant redistribution of predation pressure across the GOA and SE Alaska, potentially on the order of thousands to tens of thousands of metric tons of prey. While SSL diet estimates are not available during the heatwave years, decreased biomass of key prey species like Pacific cod coupled with shifts in SSL distribution likely exerted unexpected and potentially high predation pressure on other fish species. Our results highlight that the responses of SSLs to the 2014 – 2016 marine heatwave were more complex than previously indicated. In the context of ecosystem-based fisheries management, they illustrate how marine heatwaves can result in relatively large and rapid shifts in consumptive effects on prey populations from mobile endothermic predators.
Alaska Marine Science Symposium 2023 253
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