Sea Otters

A raft of sea otters in Glacier Bay. Sea otters rest together in groups, sometimes holding on to one another so they don't drift away.

Sea otters are a keystone species in nearshore marine environments. A keystone species dramatically affects the structure and complexity of their environment. For example, when sea otters are present in rocky habitats, they eat sea urchins and urchin density decreases as a result. Lower urchin density reduces grazing pressure on kelps, resulting in a more diverse nearshore ecosystem. And sea otters are voracious; to survive in their cold-water environment, sea otters must consume 20-30% of their body weight in food each day.

By 1900, sea otters had been exterminated from nearly the entire North Pacific Ocean. No populations existed from Prince William Sound, Alaska to Big Sur, California. In 1911, the International Fur Treaty provided the few hundred remaining sea otters with their first measure of protection. In the late 1960s, 413 sea otters were transplanted to several locations in Southeast Alaska and the population began a remarkable recovery. Sea otters began to colonize Glacier Bay in 1988, and they have expanded rapidly.

The methods used for monitoring include high-quality digital images from aircraft flying transects. The images are analyzed to count individuals and distinguish between adults and immature animals and models are used to estimate population numbers and distribution. The primary monitoring objectives are to:

• Monitor the abundance and spatial distribution of sea otters in Glacier Bay National Park and Preserve.

• Provide a dynamic spatio-temporal model using a flexible statistical framework that can accommodate multiple data types.

• Implement optimal dynamic survey designs to increase sampling efficiency to maximize program sustainability, increase safety, and improve the precision of parameter estimates.

• Provide reliable quantitative information regarding the spatial distribution and abundance of sea otters to assess their status in Glacier Bay National Park and Preserve and inform decisions regarding management actions that may have the potential to impact sea otters.
  

We monitor sea otters in Glacier Bay National Park and Preserve (read the monitoring protocol).

Contact: Jamie Womble

Find all articles, reports, and data here.

Also see: Revealing the extent of sea otter impacts on bivalve prey through multi-trophic monitoring and mechanistic models

Abstract

1. Sea otters are apex predators that can exert considerable influence over the nearshore communities they occupy. Since facing near extinction in the early 1900s, sea otters are making a remarkable recovery in Southeast Alaska, particularly in Glacier Bay, the largest protected tidewater glacier fjord in the world. The expansion of sea otters across Glacier Bay offers both a challenge to monitoring and stewardship and an unprecedented opportunity to study the top-down effect of a novel apex predator across a diverse and productive ecosystem.
2. Our goal was to integrate monitoring data across trophic levels, space, and time to quantify and map the predator–prey interaction between sea otters and butter clams Saxidomus gigantea, one of the dominant large bivalves in Glacier Bay and a favoured prey of sea otters.
3. We developed a spatially-referenced mechanistic differential equation model of butter clam dynamics that combined both environmental drivers of local population growth and estimates of otter abundance from aerial survey data. We embedded this model in a Bayesian statistical framework and fit it to clam survey data from 43 intertidal and subtidal sites across Glacier Bay.
4. Prior to substantial sea otter expansion, we found that butter clam density was structured by an environmental gradient driven by distance from glacier (represented by latitude) and a quadratic effect of current speed. Estimates of sea otter attack rate revealed spatial heterogeneity in sea otter impacts and a negative relationship with local shoreline complexity.
5. Sea otter exploitation of productive butter clam habitat substantially reduced the abundance and altered the distribution of butter clams across Glacier Bay, with potential cascading consequences for nearshore community structure and function. Spatial variation in estimated sea otter predation processes further suggests that community context and local environmental conditions mediate the top-down influence of sea otters on a given prey. Overall, our framework provides high-resolution insights about the interaction among components of this food web and could be applied to a variety of other systems involving invasive species, epidemiology or migration.

Leach, C. B., B. P. Weitzman, J. L. Bodkin, D. Esler, G. G. Esslinger, K. A. Kloecker, D. H. Monson, J. N. Womble, and M. B. Hooten. 2023. Revealing the extent of sea otter impacts on bivalve prey through multi-trophic monitoring and mechanistic models. Journal of Animal Ecology 92(6): 1230-1243.