Measures of effective population size in sea otters reveal special considerations for wide‐ranging species

Abstract

Conservation genetic techniques and considerations of the evolutionary potential of a species are increasingly being applied to species conservation. For example, effective population size (N e) estimates are useful for determining the conservation status of species, yet accurate estimates of current N e remain difficult to obtain. The effective population size can contribute to setting federal delisting criteria, as was done for the southern sea otter (Enhydra lutris nereis). After being hunted to near extinction during the North Pacific fur trade, the southern sea otter has recovered over part of its former range, but remains at relatively low numbers, making it desirable to obtain accurate and consistent estimates of N e. Although theoretical papers have compared the validity of several methods, comparisons of estimators using empirical data in applied conservation settings are limited. We combined thirteen years of demographic and genetic data from 1,006 sea otters to assess multiple N e estimators, as well as temporal trends in genetic diversity and population genetic structure. Genetic diversity was low and did not increase over time. There was no evidence for distinct genetic units, but some evidence for genetic isolation by distance. In particular, estimates of N e based on demographic data were much larger than genetic estimates when computed for the entire range of the population, but were similar at smaller spatial scales. The discrepancy between estimates at large spatial scales could be driven by cryptic population structure and/or individual differences in reproductive success. We recommend the development of new delisting criteria for the southern sea otter. We advise the use of multiple estimates of N e for other wide‐ranging species, species with overlapping generations, or with sex‐biased dispersal, as well as the development of improved metrics of genetic assessments of populations.

Location‐specific factors influence patterns and effects of subsistence sea otter harvest in Southeast Alaska

“…, Gagne et al. ). Therefore, we examined harvest patterns and potential impacts of harvest at both the stock and subregional scales.…”

“…Harvest occurs throughout most of this region with the exception of Glacier Bay National Park, ❖ www.esajournals.org where U.S. National Park Service regulations prohibit it. While the USFWS manages sea otters at the stock level, a number of recent studies and reviews have highlighted that demographically important processes in sea otter populations, including density-dependent resource limitation, occur at much smaller scales because of the low mobility and high site fidelity of mature sea otters (Bodkin 2015, Tinker 2015, Tinker et al 2017, Gagne et al 2018. Therefore, we examined harvest patterns and potential impacts of harvest at both the stock and subregional scales.…”

“…We adopted the same subregions used by Tinker et al (2019a) to estimate carrying capacity of sea otters in SEAK. The authors delineated these subregions in order to track population trends in SEAK at an appropriate spatial scale based on sea otter life history and ecology and on recent findings of fine-scale demographic structuring of sea otter populations (Bodkin 2015, Tinker 2015, Gagne et al 2018, Johnson et al 2019, Tinker et al 2019b). Specifically, each subregion encompassed an area of sea otter habitat approximately 100 times the size of a typical adult home range, which ranges from 1.0 to 11.0 km 2 (Garshelis andGarshelis 1984, Tarjan and, bounded by the low tide line inshore and the 40 m depth contour offshore ( Fig.…”

Non‐trophic impacts from white sharks complicate population recovery for sea otters

“…The sea otter population is not spatially homogeneous, but densest at the range center on the Monterey Peninsula (Nicholson et al, ). There it is either approaching, or already at, carrying capacity (Gagne et al, ; Smith, Newsome, Estes, & Tinker, ). As a result, any appreciable and sustained population growth must come from the range peripheries.…”

Future Directions in Sea Otter Research and Management

“…Previous samples were collected from widely dispersed locations and shared among studies, and they suggested that sampling on a microgeographic scale should be considered in future genetic analysis. More recent highresolution sampling of the southern sea otter population in California did find evidence for isolation by distance and cryptic genetic structure with implications for estimates of effective population size (Gagne et al, 2018). From a conservation genetics perspective, it is interesting to note that genetic diversity is highest in translocated populations derived from two source populations (Larson et al, 2002b).…”