Sea otter population collapse in southwest Alaska: assessing ecological covariates, consequences, and causal factors

Abstract

Sea otter (Enhydra lutris) populations in southwest Alaska declined substantially between about 1990 and the most recent set of surveys in 2015. Here we report changes in the distribution and abundance of sea otters, and covarying patterns in reproduction, mortality, body size and condition, diet and foraging behavior, food availability, health profiles, and exposure to environmental contaminants over this 25-yr period. The population decline, which resulted in densities on the order of 5% of environmental carrying capacity, ranged from Attu Island in the west to about Castle Cape (on the south side of the Alaska Peninsula) in the east. Remaining sea otters moved closer to shore and into shallow, protected habitats. Reproductive rates appeared unchanged with the decline. Although the demographic cause of the decline was clearly elevated mortality, stranded carcasses were rare or absent. The net rate of energy gain by foraging sea otters, body length and condition, and prey biomass density, all increased after the decline and varied inversely with sea otter population density beyond the area of decline. Sea otters within the area of decline showed no increases in health anomalies, disease, contaminant exposure, or abnormal gene transcription patterns as compared to animals outside the area of decline. These collective findings are inconsistent with nutritional limitation, disease, or environmental contaminants, and consistent with predation (or possibly some other density-independent factor) as the reason for the sea otter population decline. Our approach and analyses provide a broad conceptual template for thinking about and assessing the causes of wildlife population declines.

Recovery of a marine keystone predator transforms terrestrial predator–prey dynamics

“…Therefore, while wolf consumption of sea otters has enabled apparent competition and disruption of terrestrial food webs, the effects in the marine system are likely negligible indicating a donor-controlled shift in the recipient system ( 1 ). This example contrasts with orca predation in the Aleutians, which has changed the trajectory of the sea otter population ( 45 ).…”

“…Wolves have been observed consuming sea otter carcasses and predating sea otter pups on the Alaska Peninsula ( 14 ), including one direct observation we made of three wolves ambushing a sea otter pup at a haul-out during summer 2021. Sea otters have occasionally been hunted on land by coyotes ( Canis latrans ) and brown bears ( Ursus arctos ), and by white sharks ( Carcharodon carcharias ) and orcas ( Orcinus orca ) at sea ( 44 , 45 ), but in this case, sea otters become the primary food source for a terrestrial predator.…”

Baseline Gene Expression Levels in Falkland-Malvinas Island Penguins: Towards a New Monitoring Paradigm

“…Miller et al [6] identified a mortality-specific gene expression signature in ocean-tagged sockeye salmon (Oncorhynchus nerka), enabling predictions of spawning success. Tinker et al [7] included gene expression in an analysis of sea otter population collapse, offering substantive data supporting a controversial predation hypothesis. Broad-scale identification of gene expression patterns can provide mechanistic proxies of health [52].…”

“…The earliest observable signs of health impairment are altered levels of gene transcripts that are evident prior to clinical manifestation [3]. The amount of a particular gene that is transcribed is physiologically dictated by a number of intrinsic and extrinsic factors, including stimuli such as infectious agents, contaminant exposure, nutritional deficit, or trauma [4][5][6][7]. The use of novel molecular techniques may help identify these and other factors contributing to host susceptibility, which is critical to developing strategies to mitigate the effect of stressors on wildlife populations [8].…”

“…These assays can be adapted globally and across species, and ultimately may provide early-warning indicators and help us better understand the susceptibility of individuals, populations, and ecosystems to risks from changing conditions. Similar gene expression profiling has been adapted for use with multiple and divergent species and has also been used successfully as a predictor of mortality [4][5][6][7].…”