Mostly Trapping

Study shows differences in river otter survival on Marin’s coast
Nov 19, 2020 05:55 ET

[Reprinted from original]

Study shows differences in river otter survival on Marin’s coast

11/18/2020
The results of a five-year study on river otter populations along Marin’s coast offer some of the first baseline data on the species’ recovery in the county. Out of 14 sites, four showed increasing populations, including in the Giacomini Wetlands, northern Tomales Bay and Lagunitas Creek and near Mount Tam reservoirs. Three—at Muir Beach, Tennessee Valley and the Bolinas Lagoon—had decreasing populations. For the remaining seven, the data was insufficient to determine a trend. The number of otters remains very low overall, with not one site exceeding 10 otters in a given year. Terence Carroll and Megan Isadore, Forest Knolls conservationists who founded the River Otter Ecology Project in 2012, have been monitoring otters in Marin and eight other Bay Area counties, seeking to fill a gap in data and generate interest in watersheds in general. “Our overarching mission is to engage the public in watershed conservation, and river otters are our ambassador because they are incredibly charismatic,” Mr. Carroll told the Light. River otters are social creatures, traveling in family or friend groups. They thrive in a variety of aquatic environments, including rivers, lakes, estuaries and marshes; unlike sea otters, with which they are often confused, they are agile both in the water and on shore. The study calls them “sentinel indicators of watershed function and health.” After fur trapping wiped out Northern California populations, the first evidence of recovery in Marin occurred in Tomales Bay in the 1980s. The new study showed that population increases over the past five years took place in the northern part of the county, where habitat conditions are favorable. “A common characteristic of the [sites] with increasing population abundance was the availability of relatively large expanses of shallow estuarine water at the confluence of river and bay for foraging with potentially a larger diversity and abundance of prey,” it states. By contrast, at the southern sites where the populations decreased, “Perennial streams that flow to small lagoons on narrow sandy beaches are the main habitat features at those sites. The coast at these sites lacks shallow foraging areas owing to deep nearshore waters and sheer coastal bluffs on either side of the beach.” The study speculated that although the habitat at the Bolinas lagoon is wide open, a landscape the otters seem to prefer, the lagoon is extremely shallow, especially at low tides, limiting forage opportunities. Continued research is important, the study notes: “Further study will help to determine the status of the populations for which we had insufficient data, and to document whether the populations with declining abundance do in fact disappear from the sites they presently inhabit. Given the declining abundance in the southern part of Marin County, it becomes more important to understand the role river otters play in localized ecosystems and food webs.” At three sites—Rodeo Lagoon, Muir Beach and the Giacomini Wetlands—the park service has recently undertaken significant restoration projects. “Gauging the progress of those efforts may be assisted by understanding the interactive effects of river otter populations and the restoration management actions, and their mutual outcomes,” the study states. “As an example, interactive effects may include predation on special-status species such as the federally listed coho salmon.” Share your otter sighting and find out how to become an otter spotter in Marin by visiting the organization at riverotterecology.org.