Could Sea Otters Return to Point Reyes? A Predator-Prey Model Says Yes

Written from an article by Gavin Healy, science communication assistant with the San Francisco Bay Area Inventory & Monitoring Network, July 2024.

Drakes Estero, one of the two Point Reyes estuaries where the study modeled sea otters' return. NPS / Environment for the Americas / Gavin Healy.

Whether or not you find them cute — and some people are tired of hearing it — southern sea otters are important coastal predators that help hold whole underwater ecosystems together. They are also federally threatened, still recovering from near extinction.

That made them a good test case. A team of scientists, partly funded by the National Park Service and the Point Reyes National Seashore Association, asked whether predator-prey population modeling could show conservation planners where reintroducing a predator would work. Their answer, published in Biological Conservation in June 2024: two estuaries in Point Reyes National Seashore could probably feed a good number of otters.

Why Point Reyes

Sea otters have recovered well in Central California, but over the last decade their progress has stalled. White sharks kill them along parts of the coast, keeping them from spreading back into their old range. Drakes Estero and Tomales Bay are protected, and white sharks don't live in estuaries.

How the model was built

1. New data: with permits, the team flew drones to map seagrass, the otters' habitat, in both estuaries, and surveyed their prey — crabs and clams — there and at Elkhorn Slough, an estuary where otters have already come back strongly.
2. Existing knowledge: the model drew on years of otter research, including how long otters spend foraging and how much energy each kind of prey provides.
3. Testing: the team checked it against Elkhorn Slough, making sure it reproduced how otter and prey numbers actually changed there.
4. Running it: finally, they fed in the prey data from Tomales Bay and Drakes Estero.

The results

Both scenarios looked promising — including one in which Point Reyes clams and crabs replenish only half as fast as at Elkhorn Slough.

• Otters supported: together the two estuaries could hold 80 to 160 otters.
• Statewide: California's sea otter population could grow by as much as eight percent, a real boost to recovery.

What otters could change

Drakes Estero. Otters here could bring both benefits and challenges. NPS / Environment for the Americas / Gavin Healy.

Possible gains:

• visitors could spot otters from the Estero Trail;
• fewer clams and crabs could raise seagrass genetic diversity and make meadows more resilient;
• marsh erosion could slow.

Possible challenges:

• recreational crab and clam harvests could shrink;
• oyster farms in Tomales Bay might need to prepare in case otters take to oysters;
• park biologists wonder how otters would get along with local harbor seals.

What happens next

A reintroduction is a live experiment, with surprises both ways. The team hopes the results help the U.S. Fish and Wildlife Service, which is exploring possible reintroduction around Point Reyes and elsewhere in northern California and up to Oregon. There is no reintroduction proposal yet. Either way, managers can use the tool to predict how other key predators will affect their prey.

Sources

Based on "You Otter be Kidding Me, Sea Otters in Point Reyes? Research Uses New Predator-Prey Model to See if it Could Work," by Gavin Healy, San Francisco Bay Area Network, published by the National Park Service and in the public domain; rewritten in hubnx's own words. The study: Hamilton, S. N. M., M. T. Tinker, J. Jackson, J. A. Tomoleoni, M. C. Kenner, J. L. Yee, T. W. Bell, M. C. N. Castorani, B. H. Becker and B. B. Hughes, 2024, "Modeling coupled dynamics of an empirical predator-prey system to predict top predator recovery," Biological Conservation 294: 110623. Two sea otter photographs credited to Samantha Hamilton are not reproduced.