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 and in a variety of habitats. We compile available survey data for Washington State sea otters (Enhydra lutris kenyoni) and fit a model in a Bayesian state-space framework to evaluate a range of realistic scenarios of range expansion into unoccupied habitats and estimate equilibrium abundance at multiple spatial scales. In addition, we investigate habitat type and sea otter population status as drivers of sea otter diet 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
Population size increased on average from 26 independent sea otters in 1977 to 2,355 in 2019. The observed rate of range expansion was 0.90km/year to the northeast and 0.60km/year to the south. Simulations using these observed rates of range expansion (“low movement” scenario) estimated that the northeastern and southern range edges would move 40km east and south of their current positions, respectively, by 2044. We compare these results to a “high movement” scenario reflecting a typical rate of range expansion, of 2-5 km/year. The estimated mean sea otter equilibrium density in Washington was 1.69 ± 0.80 independent sea otters/km2. Densities within the current range correspond on average to 93% of sub-regional equilibrium values (range = 77-107%). The projected equilibrium abundance for all of Washington is 7,170 sea otters (95% CI = 3,765-10,574). 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 density. We observed higher energy intake rates than we would have expected based on population status. Our energy intake rate and equilibrium abundance estimates may be indicative of the Washington coast’s ability to support a higher number of sea otters than previously thought.