Washington sea otter (Enydra lutris kenyoni) foraging ecology and population dynamics

Jessica R. Hale and Kristin L. Laidre, School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA, Steven J. Jeffries, Wildlife Science Program, Marine Mammal Investigations, Washington Department of Fish and Wildlife, Lakewood, WA, Jonathan J. Scordino, Makah Fisheries Management, Neah Bay, WA, Deanna Lynch, Washington Fish and Wildlife Office, US Fish and Wildlife Service, Lacey, WA, Ronald J. Jameson, Western Ecological Research Center, United States Geological Survey, Sacramentro, CA, Shawn E. Larson, Seattle Aquarium, Seattle, WA, James L. Bodkin, Alaska Science Center, U.S. Geological Survey, Anchorage, AK, M. Tim Tinker, Department of Ecology and Evolutionary Biology, University of California, Center for Ocean Health, Santa Cruz, CA, M. Tim Tinker, Nhydra Ecological Consulting, Head of St. Margaret’s Bay, NS, Canada

Background/Question/Methods
Many marine mammal populations are recovering from population depletion after overharvest. The depletion and subsequent recovery of these species presents researchers with natural experiments to study the drivers of their population dynamics and function in the marine ecosystem. Sea otters (Enhydra lutris ssp.) are a particularly interesting recovering marine mammal to study, as their presence or absence can dramatically influence marine community structure as a keystone species, and sea otter populations are recovering from extant or translocated populations at varying rates in different regions in a variety of habitats. We compile available survey data for sea otters (Enhydra lutris kenyoni) in Washington State since their translocation (1977–2019) and fit a Bayesian state-space model to estimate past and current abundance, and equilibrium abundance at multiple spatial scales. We then use forward projections of population dynamics to explore potential scenarios of range recolonization and as the basis of a sensitivity analysis. Finally, we investigate sea otter population status and habitat type as drivers of sea otter diet composition, diet diversity, and rates of energy intake to gain a more complete picture of the processes regulating the Washington sea otter population and the function of sea otters in the nearshore marine environment.
Results/Conclusions
Modeled population size increased on average from 21 independent sea otters (95% CI = 13–29) in 1977 to 2,336 (95% CI = 1,467–3,359) in 2019. The estimated mean sea otter equilibrium density was 1.71 ± 0.90 independent sea otters/km2. Estimated densities within the current range correspond on average to 87% of mean sub-regional equilibrium values (range = 66–111%). The projected equilibrium abundance for all of Washington is 6,080 sea otters (95% CI = 2,861–9,300). Sensitivity analysis of diffusion model simulations suggested that mean equilibrium density estimates in currently occupied sub-regions had the largest impact on predicted future population growth (r2 = 0.52). We found that sea otter diet diversity was positively correlated with cumulative sea otter density, while rate of energy gain was negatively correlated with cumulative sea otter density. However, we found that habitat type explained 1.77 times more variance in sea otter diet composition than sea otter cumulative density. When the results of the population model and foraging study are taken together, we found that the relationship between foraging success and population density and growth rate is generally consistent with food resources regulating population growth for sea otters in Washington State.