Sea Otter vs River Otter: How to Tell Them Apart
The fastest tell: a sea otter floats on its back in salt water and rarely comes ashore, while a river otter swims belly-down and spends lots of time on land. Sea otters are also much bigger and bulkier, with a short, flat tail; river otters are long and slim with a thick, pointed tail. Below we go trait by trait, add a side-by-side table and a quick checklist for spotting them from shore, then point you to two live cams where you can compare them yourself.
Sea otter or river otter? The quick answer
If you only remember one thing, make it the belly test. A sea otter lies on its back at the surface and uses its chest like a table while it eats, as the Monterey Bay Aquarium describes. A river otter swims belly-down.
The second clue is where you are. If you are beside a freshwater river, lake or inland marsh, you are almost certainly looking at a river otter. On the open coast, either one is possible, because river otters also forage in estuaries, harbors and saltwater bays such as Puget Sound.
The third clue is land. Fisheries and Oceans Canada notes that sea otters rarely come ashore and are clumsy out of the water, while river otters climb onto docks and floats, den on land and usually bring their food ashore to eat.
Size, shape and tail: what to look for
Body shape gives a lot away, even from a distance. Sea otters are heavy and rounded, with a broad head and big, flipper-like hind feet. River otters are sleek and stretched out, built for slipping along riverbanks and through the water.
- Sea otter size: the Monterey Bay Aquarium lists southern sea otters at up to about 4 feet (1.2 m) long. Fisheries and Oceans Canada gives sea otters a weight of 27 to 45 kg (roughly 60 to 100 pounds), about the size of a Labrador retriever.
- River otter size: Smithsonian's National Zoo lists North American river otters at about 2.5 to 5 feet (76 to 152 cm) long and 10 to 33 pounds (4.5 to 15 kg).
- Tails: a sea otter's tail is short and flattened. A river otter's tail is long, thick at the base and tapers to a point; the National Zoo says it can make up as much as 40 percent of the animal's total length.
So a river otter can be nearly as long as a sea otter, but much of that length is tail.
How each otter swims (and eats)
Sea otters spend most of their lives at the surface on their backs. They eat there, groom there and rest there, often with head, paws and hind flippers held up out of the water, which the Monterey Bay Aquarium says can make a resting otter look like a clump of kelp.
River otters swim belly-down nearly all the time. The National Zoo says they power along with an undulating motion of the tail, helped by their feet, and can reach speeds of up to 8 miles per hour. From shore you often see just a low, dark head moving forward.
Cam tip: watch what happens after a dive. A sea otter pops up and settles onto its back with food on its chest. A river otter usually heads for the bank, a log or a rock to eat.
Fur, warmth and grooming
Sea otters have no thick layer of blubber, so they depend on fur to stay warm. River otters also rely on their fur coats for warmth, and both species groom often to keep that fur clean and water-resistant.
Sea otters take this to the extreme. The Monterey Bay Aquarium says their fur reaches more than one million hairs per square inch at its thickest, and that they spend hours each day grooming to keep that coat working. That is a big part of why you see so much rolling, rubbing and fluffing on the sea otter cams.
River otters have a dense, short underfur covered by longer, coarser guard hairs that help shed water, according to the National Zoo. The Seattle Aquarium notes that river otters groom often too, to keep their coats insulating.
Diet and tool use
Sea otters are shellfish specialists. The Monterey Bay Aquarium lists crabs, snails, sea urchins, clams, abalone and mussels on the menu, with northern sea otters also eating fish. They sometimes use a rock to crack hard shells, and they stash prey in a loose pouch of skin under each forearm while they dive for more.
River otters mostly eat fish. The National Zoo also lists frogs, crayfish, turtles, insects and the occasional small mammal. Fisheries and Oceans Canada notes that river otters typically bring their prey ashore to eat, so an otter cracking shells on its belly is a strong sign you are watching a sea otter.
Life on land and in groups
Sea otters live almost their whole lives in the water. The Seattle Aquarium notes that they breed, give birth, hunt and sleep at sea. They usually have a single pup, which rides on mom's belly, according to Fisheries and Oceans Canada. Resting sea otters often gather in floating groups called rafts.
River otters are equally comfortable on land. The National Zoo says they dig dens into stream banks or move into old beaver lodges, and may travel many miles over land to find food. They are usually seen alone, in pairs or in small family groups, and mothers can have several young at a time.
Sea otter vs river otter: side-by-side table
| Trait | Sea otter | North American river otter |
|---|---|---|
| Scientific name | Enhydra lutris | Lontra canadensis |
| Habitat | Ocean only: kelp forests, estuaries, harbors and shallow coastal water | Rivers, lakes, ponds and marshes, plus coastal bays and estuaries |
| Swimming position | Floats and eats on its back | Swims belly-down |
| Tail | Short and flattened | Long, thick at the base, pointed |
| Size | Up to about 4 ft long; 27 to 45 kg | About 2.5 to 5 ft long; 10 to 33 lb |
| Staying warm | Extremely dense fur, no thick blubber | Dense underfur plus coarse guard hairs |
| Main foods | Crabs, urchins, clams, snails, mussels, abalone | Mostly fish, plus frogs, crayfish and more |
| Tool use | Sometimes uses rocks to crack shells | Not described in the sources above |
| Time on land | Rarely comes ashore | Dens and eats on land, climbs docks |
| Groups | Rests in rafts | Alone, pairs or family groups |
| Young | Usually one pup, carried on mom's belly | Litters of several |
Sizes are from the Monterey Bay Aquarium and Fisheries and Oceans Canada (sea otters) and Smithsonian's National Zoo (river otters). Individual animals vary.
How to ID an otter from shore: a quick checklist
Not sure which one you are looking at? Run through these in order.
- Is it floating belly-up? Lying on its back, especially while eating, points strongly to a sea otter.
- Where are you? Inland fresh water means river otter. On the coast, keep going down the list.
- Did it come out of the water? Running across rocks, sliding down a bank or eating on a dock suggests a river otter.
- Check the tail. A long, tapering tail trailing behind is a river otter trait; a stubby, flat tail fits a sea otter.
- Look at the feet if it hauls out. Big, flipper-like hind feet and an awkward shuffle on land fit a sea otter; a river otter moves easily over rocks and banks.
- Count the crowd. A floating cluster of resting otters is a sea otter raft. A small, busy group moving together along a shoreline is more likely a river otter family.
Watch from a distance: the U.S. Fish and Wildlife Service asks people to stay at least 60 feet (about five kayak lengths) from sea otters. If an otter looks up at you, you are probably too close, so back away slowly. Binoculars make ID much easier anyway.
Practice on the live cams
Here is the fun part. Almost every cam on seaotter.cam shows sea otters, but the Calvert Marine Museum cam in Solomons, Maryland, shows North American river otters. Open it in one tab and the Monterey Bay Aquarium cam in another, then run through the checklist above.
Watch it liveMonterey Bay Aquarium sea otter camSouthern sea otters doing the back float, the grooming roll and the chest-as-table trick.Frequently asked questions
Can sea otters and river otters live in the same place?
Yes, in some coastal areas. River otters also forage in estuaries and saltwater bays, so along parts of the Pacific coast you might see both. Inland fresh water is river otter territory only.
Which is bigger, a sea otter or a river otter?
The sea otter. The Seattle Aquarium describes sea otters as roughly two to three times the size of river otters, and much of a river otter's length is tail.
Do river otters float on their backs like sea otters?
Not as a habit. River otters swim belly-down and usually take their food to land to eat. Floating on the back while eating is a classic sea otter behavior.
Are there river otters on seaotter.cam?
Yes. The Calvert Marine Museum cam shows North American river otters. All the other cams in our directory show sea otters, so it is easy to compare them side by side.
Do sea otters ever come onto land?
Sometimes, but rarely. Sea otters breed, give birth, eat and sleep in the water and are clumsy on land. River otters, by contrast, den and eat on land regularly.
Watch both, side by side
Open the river otter cam, then the sea otter cam, and see how many differences you can spot in five minutes. Free, live, no sign-up.
Sources
- Monterey Bay Aquarium — Sea otter (Animals A to Z)
- Smithsonian's National Zoo — North American river otter
- Fisheries and Oceans Canada — Sea otter or river otter?
- Seattle Aquarium — River otters
- U.S. Fish and Wildlife Service — A reminder on ethical wildlife viewing: please don’t harass the sea otters
seaotter.cam is an independent fan site and is not affiliated with any aquarium, museum, zoo or agency mentioned here. Facts are drawn from the sources above; check them for the latest information.