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River Otter Populations Are Expanding in the Bay Area

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The otters dense whiskers are used for hunting underwater. (Courtesy of Brenden Collett-Grether)
The otters’ dense whiskers are used for hunting underwater. (Brenden Collett-Grether/ROEP)

Cavorting along stream banks, sliding on their bellies, munching crayfish with relish, river otters seem to approach the necessities of life with playful abandon. After decades of absence in the San Francisco Bay Area’s coastal habitats, river otters appear to be back, according to a census published in Northwestern Naturalist, the journal of the Society for Northwestern Vertebrate Biology.

The two-year study covering 2012-13 found otters reported in eight of the nine Bay Area counties, including what co-authors Paola Bouley and Megan Isadore say are the first river otter sightings in decades in Alameda, San Francisco, and Santa Clara counties.

Bouley and Isadore are co-founders of the River Otter Ecology Project, or ROEP. Project staff monitored a focal study area in Marin County with over 200 km. of coastline and bay habitats, collected documentation from wildlife cameras and used information from citizen scientists who recorded over 1,000 sightings on the ROEP website.

“River otters are often perceived as animals that require pristine wilderness to survive,” ROEP executive director Isadore said in a statement. “But our research shows that they are adapting well to urban and semi-urban living in and around San Francisco Bay and the Marin and Sonoma coast.”

The study says otters will likely continue to expand their range southward, unless there are barriers to them doing so. While river otters tend to hunt for food in freshwater habitats like lakes, streams and reservoirs, researchers saw them foraging in the marine environment and returning from the ocean along beaches to their freshwater areas.

River otter groups are usually composed of mothers and their young. This group was photographed in Abbott's Lagoon. (Anthony Brewer/ROEP)
River otter groups are usually composed of mothers and their young. This group was photographed in Abbott’s Lagoon. (Anthony Brewer/ROEP)

Not to be confused with their ocean-loving cousins, the sea otters, river otters are smaller, and have dens for raising their pups and feet adapted to digging and scampering over land. The Aquarium of the Bay website has a good comparison of the two species. If you see an otter in the Bay Area it’s highly likely to be a river otter, even if you see it in the bay. There have been a few scattered sightings of sea otters, even one that hung around in the bay for a while, but they are rare.


The ROEP study also says river otters may be able to play an important role in assessing ecosystem health, especially since they live in the highly urban San Francisco estuary. Authors didn’t look for pollutants in the otters, but noted that a study of harbor seals has shown a correlation between human-caused pollution and disease.

The Bay Area’s healthy, expanding otter population is a testimony to their resilience as a species. The report notes that the first sighting of an otter in Lake Merritt near downtown Oakland occurred after the city re-linked the lake with the bay, allowing tides to surge in. Fresh scat was seen last month at Temescal, near downtown Oakland, so there’s likely an otter in the area. You can check the Otter Spotter map on the ROEP website to see where else they’ve been reported.

“The fact that they are returning is a very hopeful sign, and underscores the importance of restoration and conservation of Bay Area watersheds,” Isadore said.

If you see an otter, you can contribute to the ongoing project on the ROEP website.

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