What Sea Otters Eat as Their Numbers Grow in Glacier Bay

A sea otter eats a crab. Image from the National Park Service's page.

Sea otters were nearly wiped out in the North Pacific during the 1700s and 1800s by the maritime fur trade, and by the early 1900s they were gone from much of their range. Legal protection and the transplanting of animals from surviving populations have allowed them to recover across much of that range, and they have done especially well in Southeast Alaska. Sea otters were moved to the outer coast of Southeast Alaska in the 1960s and first arrived in Glacier Bay in the late 1980s. Since then they have spread rapidly into the far reaches of the bay, a protected tidewater glacier fjord with rich, productive nearshore habitat, giving scientists a rare chance to watch how a rapidly colonizing population feeds.

Two long-term records

Since 1993, biologists from the U.S. Geological Survey Alaska Science Center, the U.S. Fish and Wildlife Service and the National Park Service have recorded what sea otters catch, including the number, species and size of their prey, and have surveyed sea otter abundance and distribution from the air. The study combined these two records in a multilevel Bayesian model to see how the otters' diet at each place changed with the number of otters that had been there over time and the year the place was first occupied.

What changed

As sea otters settled in and their numbers grew, their diet shifted in stages:

1. first, large urchins, crabs and clams;
2. then Modiolus mussels and small urchins;
3. finally small clams and Mytilus mussels.

The diet at newly occupied sites also changed as otters spread along the main channel and into the bay's arms. By 2019, sea otters throughout Glacier Bay were mostly eating small prey, no matter how long a place had been occupied.

The lack of any difference in prey size from place to place late in the study suggests that the combined effects of otters eating down their prey, the way otters disperse, and local differences in habitat productivity may have evened out the size of available prey across the bay. Put another way, what sea otters eat reflects both their preferences and what prey is left, which depends on environmental conditions and on the otters' own predation over time.

Why it matters

Sea otters are a keystone predator, and their return across the North Pacific offers a chance to learn how they shape nearshore communities. The study also shows the value of long-term monitoring of a keystone predator and the prey it depends on.

Sources

• National Park Service, Glacier Bay National Park and Preserve, "Sea Otter Prey Selection in a Rapidly Expanding Population." https://www.nps.gov/articles/000/sea-otter-prey.htm
• 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. 2024. "The dynamics of sea otter prey selection under population growth and expansion." Ecosphere 15(12): e70084. https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2.70084
• Related: Leach and others (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.