River Otters

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Otterly Amazing

The North American river otter is a semiaquatic mammal that lives in and around the rivers, estuaries, lakes and coasts of North America and is common in the Bay Area.  River otters are members of the weasel family and are not closely related to sea otters.

Average size
  • 11-31 pounds (5-14kg)
  • 26-42 inches (66-107 cm) long
  • A river otter’s tail makes up almost half of its body length
  • Female otters, on average, are smaller than males
  • 10-13 years in the wild
  • 15-19 years in captivity
More terminology
  • A group of otters is called a romp, family or raft
  • Otter droppings or scat are called spraints
  • An otter den is called a holt

Geographic range

North America, from the Arctic circle to Mexico
Where in the San Francisco Bay-Delta watershed?
Research is ongoing, but we know that more river otters live in the Delta and Suisun marshes.  They are not as common in Napa marshes and in the Carquinez Strait.
Near clean fresh water, from the mountains to the coast
River otters love fish.  They also eat crabs, frogs, birds and rodents.  As top predators in their ecosystems, river otters rely on a healthy food web. 
  • Family grouping is a female and her pups; males sometimes live in bachelor groups
  • Often play by repeated touching, shoving and tobogganing down snow or mud slides 
  • Superb swimmers, reaching speeds of 6-7 miles per hour in the water 
  • River otters communicate with a variety of noises and smells
Notable adaptations
  • Dense fur for insulation—over 58,000 hairs/cm2 on body; over 2,000 hairs/cm2 on tail
  • Lungs for a life aquatic—right lung is about 20% larger than the left and has four lobes instead of the left’s two
  • River otters also have a thick mucus lining in their intestines to protect from sharp objects, e.g. fish spines
  • Mature by age 2 approximately
  • River otters breed November through May, dependent on latitude 
  • Males mate with multiple females and utilize delayed implantation  
  • Total gestation averages 10-12 months, producing litters of 2-3 pups generally born February through April
Conservation status
Stable in California.  A century ago, river otter populations were in bad shape due to water pollution, habitat degradation and a bustling fur trade.  Since then, the situation has improved.  Hunting/trapping of river otters is not allowed in California.  Regulations to improve water quality and estuarine habitat have allowed river otter populations to recover. The return of river otters to the San Francisco Bay estuary reminds us of what is possible with ongoing commitments to a healthy watershed.

River otters and sea otters have some things in common, such as:
  • They’re both a type of weasel with long, slender bodies
  • They have webbed feet for swimming, and are excellent underwater hunters
  • They have two layers of fur to keep warm in cold water
  • A group of otters in the water is called a “raft.”

River Otters Versus Sea Otters:  What's the Difference?

Here’s how river otters are different from sea otters:

River Otters   Sea Otters
Are about 20-25 pounds   Are about 50-100 pounds
Live in fresh water, like rivers and lakes, and slightly salty water, like in the Bay   Live in salt water, in the ocean
Spend half their time on land and half in the water   Almost never leave the water
Swim on their bellies   Float on their backs
Have little paws and can run quickly on land   Have wide webbed feet, and are really awkward on land, like they’re wearing SCUBA flippers
Eat their food in water, on land, or on a rock or log   Eat their food off their belly as they float on their backs
Sleep in dens underground   Sleep wrapped in a kelp “seatbelt”
Eat fish, frogs, crayfish, bugs, rats, and birds   Eat urchins, crabs, clams, octopuses

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