Return of North American River Otters,Lontra canadensis, to Coastal Habitats of the San Francisco Bay Area, California

Abstract

We present results from the first-ever study of populations of the North American River Otter, Lontra canadensis, in coastal habitats of the San Francisco Bay Area, California. Historically extirpated from the region, wild populations of this sentinel carnivore appear to have made a recovery in recent years. Utilizing a citizen-science network paired with field investigations in 2012 and 2013, we documented 1374 River Otter observations across 8 of 9 San Francisco Bay Area counties. We demonstrate that River Otters are reproducing, and report here on the 1st sightings in decades in Alameda, San Francisco, and Santa Clara counties indicating a possible gradual expansion of the species' range southward. Within our Intensive Study Area in coastal Marin County, conservatively estimated densities ranged from 0.21 to 0.32 River Otters/km, with otters inhabiting a range of habitats from freshwater to marine. A pilot assessment of disease and mortality indicates that otters are being exposed to pathogens such as Vibrio and that observable mortality was largely due to car-strikes. We also report on timing of mating, timing of pup-juvenile emergence, and pup-juvenile production. Despite large-scale ecosystem restoration actions underway across the San Francisco Bay Area, River Otters have been overlooked by resource managers. Being apex carnivores that not only directly benefit from restoration actions but also likely play a significant role in the outcome of recovery actions focused on endangered salmonids and waterfowl, we strongly recommend attention to their potential role as a keystone species in the San Francisco Bay Area.