Sea Otters in Marin

by Parker Pringle

In recent years, river otters and coyotes have recolonized much of southern Marin. Bald eagles are also making inroads into our area: two juveniles spent some of this past winter eating coots in Richardson Bay. Now, a recent study has raised the tantalizing possibility that sea otters could be the next predator species to return to the Corte Madera Creek watershed.

Up until the 1830s sea otters were abundant in San Francisco Bay and the Corte Madera Creek estuary. Miwoks of the Larkspur area hunted otters in the estuary and San Quentin Bay. Sea otter bones (along with those of grizzly bear and condor) were found in the shell mound at the site of the Rose Lane development. Author Adele Ogden, in her California Sea Otter Trade, 1784–1848, writes that Aleut hunters, operating as part of Russian expeditions out of Fort Ross, made illegal visits to Marin’s sea otter fields in the early 1800s by landing at Point Bonita and portaging around the Golden Gate to avoid Spanish lookouts at the Presidio. They would relaunch their kayaks in Sausalito and work up the Marin coast. In 1830 a hunting party from the San Jose mission found 100 sea otters hauled out at Point San Quentin and “lassoed” 30 of them.

Sea otters are larger than river otters, and often float on their back while feasting on shellfish.

Sea otters are larger than river otters, and often float on their back while feasting on shellfish.

Sea otter pelts were exported to China where they were highly prized. In the days of Spanish and Mexican ownership of California, this fur trade was big business. But the 1830s saw the introduction of the gun to sea otter hunting—which until then had been conducted mainly with spears and arrows—and the populations declined precipitously. In 1840, the grantee of the San Quentin Ran-cho received a license to hunt sea otters, but there probably weren’t too many left to shoot. When an international treaty in 1911 banned the killing of sea otters, the California population was estimated to number only 50 otters. Soon after, the sea otter was thought to have been extirpated from California. Then a relict population was found near Big Sur in 1938. All the sea otters in California today are descendants of this small group.

The possibility of sea otters re-colonizing San Francisco Bay stems from the fact that this year the sea otter population has climbed to its highest number since the 1800s. The historical population is estimated to have been 16,000. This year’s population survey, conducted by the USGS, found 3,272 sea otters in California.

While this is good news, the range of the sea otter range is not expanding, largely due to shark at-tacks killing any otter that takes up residence in the Año Nuevo area. Range expansion is crucial to the long-term survival of the species, but it occurs only in small steps as female otters generally do not migrate very far from their birthplaces. With the population having filled the currently available habitat, and sharks killing off females venturing into the Año Nuevo area, resource managers might attempt to move a colony of sea otters around the shark road-block at Año Nuevo to some suitable habitat to the north.

A visiting sea otter was oblivious of the attention he attracted near Richardson Bay bridge in Mill Valley, in 2015. Photos by Parker Pringle

A visiting sea otter was oblivious of the attention he attracted near Richardson Bay bridge in Mill Valley, in 2015. Photos by Parker Pringle

Transplantation of sea otters has been done successfully once before, at San Nicolas Island in Southern California. According to Dr. Tim Tinker, head of the USGS sea otter research program, San Francisco Bay could be a candidate for a sea otter transplant colony. If a breeding sea otter population is established in San Francisco Bay, these hungry predators would transform the Bay ecosystem. For example, one of their favorite foods is crab. Crabs like to eat the slugs that eat algae on eel grass. A decrease in crab populations due to sea otter predation would lead to an increase in these algae-eating slugs, in turn promoting growth of eel grass which is an important habitat for herring. This phenomenon has been documented in Elkhorn Slough, an estuary which is similar to parts of San Francisco Bay.

Male sea otters do occasionally appear in our waters. In June of 2015 a male sea otter took up residence in Richardson Bay, upstream of the Hwy 101 bridge. He could be seen gorging on mussels, clams and crabs along the Sausalito to Mill Val-ley path. Sadly, this otter ultimately died from a combination of domoic acid poisoning (the same toxin which kept crab out of the super-market in 2015) and a parasite, spread by opossums, that causes an illness similar to toxoplasmosis. The domoic acid poisoning was found to have occurred while the otter was in the bay. But had this sea otter not died, he would have soon left the Bay anyway and headed back to the Central Coast region where female sea otters are exclusively located.

There will be much study and consideration before anything hap-pens on this front. But the return of sea otters to San Francisco Bay, one way or another, seems to be getting closer to a reality.

If you are interested in reading more about sea otters, there are two excellent articles at regarding the sea otter that visited Richardson’s Bay in 2015.