Why Was This Indicator Chosen?
Large and very active, North American river otters (Lontra canadensis) are easy to see and identify. These apex aquatic predators play an important role in ecosystem health, eating fish, crustaceans, invertebrates, birds, and amphibians. Spending 70% of their time on land, and 30% of their time hunting and traveling in waterways, they are useful indicators for the health of both terrestrial and aquatic habitats. Historically extirpated from the San Francisco Bay Area, the return of North American river otters after a decades-long absence is a true wildlife success story.
What is Healthy?
North American river otters are present in all suitable water bodies in the One Tam area of focus.
What Are the Biggest Threats?
- The residual effects of historic persecution and the current threat of vehicle strikes as the otters move through terrestrial habitats
- Fishery declines, which could have serious impacts on North American river otter population numbers and distribution
- Watershed development that degrades habitats and increases pollution levels
- Diseases such as canine distemper, feline and canine parvovirus, and rabies
What is The Current Condition?
The current status is Good based on both observational and camera trap data from River Otter Ecology Project (ROEP). In 2016, we reported that the status of the North American river otter had shifted from extirpation in the San Francisco Bay Area to being present in most suitable water bodies in the area of focus (Bouley et al., 2015). In 2022, Statistical modeling of ROEP camera trap data shows increasing North American river otter abundance in Mt. Tam’s reservoirs and in the portion of Lagunitas Creek immediately downstream of Kent Lake. While the same modeling suggests that abundance is not increasing in the coastal areas around Bolinas Lagoon and Muir Beach, the overall trend in the area of focus is improving. Additionally, ROEP analysis of observational data shows that the North American river otter is present in approximately 75% of lentic water body areas (e.g., ditches, seeps, ponds, seasonal pools, marshes, and lakes) and 25% of perennial stream reaches. Because the data are based on opportunistic sightings, they likely understate the true presence of the North American river otter in the area of focus.
What is the Current Trend?
The trend is Improving, as this species has come back from being gone from the San Francisco Bay Area, to now being present in most suitable water bodies in the One Tam area of focus.
How Sure Are We?
We are Moderately confident in this assessment. Though current data are encouraging, more work is needed to determine if American river otters are present in all of the suitable water bodies on Mt. Tam.
What is This Assessment Based On?
- Since 2012, the River Otter Ecology Project has been collecting observations of North American river otters and monitoring populations in Marin County through non-invasive camera trapping
- North American river otter pictures have also been captured through the Marin Wildlife Watch
What Don’t We Know?
Key information gaps include:
- Population demographics including home range, dispersal patterns, and the distribution and abundance of prey
- The effects of toxins and pathogens on North American river otter health
resources
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