Wildlife Spotlight: River Otter (Lutra canadensis)

By Peter Winch
Published: October 2007

Two river otters playing. Credit: Bob Landis

A member of the weasel family, the river otter is one of the most skillful and adaptive predators in the Gulf of the Farallones Marine Sanctuary.

Their adaptations for aquatic life include tiny ear and nostril flaps that close when underwater, long whiskers to feel out for flatfish and other food in murky water and a luxuriant warm waterproof fur coat.  They can stay under water for up to three minutes and have a swimming speed of up to seven miles an hour.

The river otter is about a third of the size of its well known cousin the sea otter, and its long tail and pointy snout make it easily distinguishable from its marine relative.  It has a muscular streamlined body with short legs and webbed toes, making it a great swimmer.

As its name suggests, the river otter is primarily found in freshwater or estuarine environments.  Where sea otters remain in the ocean, river otters can be spotted on both land and in the water.  Tomales Bay and Bolinas Lagoon are two locations in the sanctuary where they can be seen.

Feeding

Notorious opportunists, river otters eat fish, crabs, frogs, insects, mollusk eggs, chicks, adult birds and have even been linked to the disappearance of geese from a pond near Marin Civic Center in the spring of 2006.  If  you are hiking in Point Reyes and see a brown fury animal in the inter-tidal zone with an eel in its mouth, it’s probably a river otter.

In Bolinas Lagoon, the unfortunate introduction of the green crab—an invasive non-native species—has actually been good for river otters.  There has been a noticeable rise in the numbers of river otters in Bolinas Lagoon, and it is thought that green crabs comprise a substantial percentage of their diet.

Mating

Well known for their playfulness, mature river otters are particularly boisterous during mating season in early spring.  After an extended period of “play”, the female follows the male river otter back to his den where mating takes place.

The gestation period is about five to seven weeks and up to six offspring (kits) are born.   The most common litter size is two or three.  The kits do not open their eyes until three weeks after birth. 

Adult river otters are attentive parents, devoting large amounts of time to teaching their young to swim and catch prey, even catching and releasing fish for them to help them to learn how to forage.

Spotting an Otter

Human demand for river otter fur almost lead to the extinction of the species in the Bay Area at the beginning of the 20th century.  A remnant population survived in the Russian River area and spread south in the last forty years to re-colonize Point Reyes and Bolinas Lagoon.

This summer on a FMSA kayaking trip to Tomales Bay we were lucky enough to see two river otters eating a fish.   We will be leading more kayaking trips in the new year, so please join us for a chance to spot these playful charismatic animals.