Translocations maintain genetic diversity and increase connectivity in sea otters, Enhydra lutris

Abstract

Sea otters, Enhydra lutris, were once abundant along the nearshore areas of the North Pacific. The international maritime fur trade that ended in 1911 left 13 small remnant populations with low genetic diversity. Subsequent translocations into previously occupied habitat resulted in several reintroduced populations along the coast of North America. We sampled sea otters between 2008 and 2011 throughout much of their current range and used 19 nuclear microsatellite markers to evaluate genetic diversity, population structure, and connectivity between remnant and reintroduced populations. Average genetic diversity within populations was similar: observed heterozygosity 0.55 and 0.53, expected heterozygosity 0.56 and 0.52, unbiased expected heterozygosity 0.57 and 0.52, for reintroduced and remnant populations, respectively. Sea otter population structure was greatest between the Northern and Southern sea otters with further structuring in Northern sea otters into Western, Central, and Southeast populations (including the reintroduced populations). Migrant analyses suggest the successful reintroductions and growth of remnant groups have enhanced connectivity and gene flow between populations throughout many of the sampled Northern populations. We recommend that future management actions for the Southern sea otter focus on future reintroductions to fill the gap between the California and Washington populations ultimately restoring gene flow to the isolated California population.

Genomic analyses reveal range‐wide devastation of sea otter populations

“…Individuals from each sampling location clustered by genotypes in both PCA (Figure 1b) and faststructure (Figure 1c) analyses, and values of F ST increased with distance between populations (Figure D; Table ), agreeing with previous studies (Larson et al, 2012, 2021; Larson, Jameson, Bodkin, et al, 2002). Sea otters in California are the most divergent population and represent the only surviving remnant of the southern sea otter subspecies ( Enhydra lutris nereis ) (Figure 1b,d).…”

“…We found that the geographically driven population structure of sea otter populations remained after the fur trade bottlenecks, leading to five broad geographical groupings in our sample: the Kuril Islands, Commander Islands, western Aleutian Islands, south central Alaska, and California. This shows that a key element of the successful restoration of sea otters was their survival and numerical increase from small remnant populations distributed throughout their geographical range rather than long‐distance dispersal (Larson, Jameson, Bodkin, et al, 2002; Larson et al, 2012, 2021). This fortuitous survival allowed both rapid range‐wide restoration of former abundance and preserved historical genetic structure and perhaps even local specializations that enhanced survival (Fujii et al, 2015).…”

“…Defining management units for sea otters, whether at fine or broader scales, remains a challenge to conservation efforts (Davis et al, 2019). Genetic structure as defined by PCA and admixture analyses can provide insight into natural genetic groupings, but cannot characterize microscale structuring of sea otter familial groups (Bodkin, 2015; Davis et al, 2019; Gagne et al, 2018; Larson et al, 2021; Tinker et al, 2019). We found that the geographically driven population structure of sea otter populations remained after the fur trade bottlenecks, leading to five broad geographical groupings in our sample: the Kuril Islands, Commander Islands, western Aleutian Islands, south central Alaska, and California.…”

Steller’s sea cow uncertain history illustrates importance of ecological context when interpreting demographic histories from genomes

“…Evidence from sea otter morphological and genetic studies 17 has shown population structuring across their range and led to the recognition of different subspecies for the Commander Islands and the Aleutian islands 18 lending support to the idea of ecological barriers at this geographic location. Although we acknowledge considerable differences in life history traits between these species, sea otters are the only other marine mammal restricted to nearshore coastal waters of the North Pacific, sharing a remarkably overlapping distribution with sea cows 19 .…”

“…To evaluate the ecological influence the Steller's sea cow may have exerted over kelp forests around the North Pacific Rim 11 we have been preparing updated range maps considering the species glacial and interglacial ranges. These nearshore habitat maps are grounded in the fossil record and guided by bioindicators of kelp habitat, oceanographic data, and various lines of ecological evidence [9][10][11][12][13][14][15][16][17][18][19][20] (Fig. 1).…”