What Is the Smallest Marine Mammal?

Sea Otters, River Otters and Cetaceans

Sea Otter (Enhydra Lutris) Swims On It's Back
Brian Guzzetti/Design Pics/First Light/Getty Images

What is the smallest marine mammal in our waters? Like many questions surrounding the oceans, there isn't a real quick answer to the question of the smallest marine mammal -- there are a few contenders, actually.

In the world of marine mammals, the sea otter has the smallest weight. Sea otters range from 35 to 90 pounds (females are in the range of 35 to 60 pounds, while males can be up to 90 pounds.) These mustelids can grow to about 4.5 feet in length.

They live in coastal waters of the Pacific Ocean off the coasts of Russia, Alaska, British Columbia, Washington, and California.

There are 13 different species of otters. They have slim, long bodies but relatively short limbs compared to the rest of their bodies. They use their webbed feet to swim and can hold their breath while diving underwater, similar to seals. On their feet, they have sharp claws. Sea otters, which live in saltwater, have muscular, long tails. 

On the flip side, river otters are much smaller. They can be about 20 to 25 pounds. They can live in water that is salty, such as bays, but typically stick to rivers. These otters are good runners and can move on land better than sea otters. River otters eat their food on land and sleep in dens, while sea otters are the ones commonly seen floating on their backs and eating off their bellies and sleep in beds of kelp.

As for what they eat, sea otters typically nosh on crabs, clams, sea urchins, mussels, and octopuses.

These creatures almost never leave the water. 

The fur trade has threatened their existence. In the 1900s, the numbers decreased down to about 1,000 to 2,000 otters; today, they have revitalized and there are about 106,000 sea otters across the globe (about 3,000 of them are in California.) 

Other Small Marine Mammals

Here's where it gets a little murky to determine which marine mammal is the smallest.

There are some cetaceans that are around the same length as the otter. 

Two of the smallest cetaceans:

  • Commerson's Dolphin, which grows up to 189 pounds and is about 5 feet long. This species lives in waters off southern South America and in southern parts of the Indian Ocean.
  • Vaquita, which weighs up to about 110 pounds and grows to almost 5 feet. This species, which numbers around 250 individuals, only lives in the Sea of Cortez, Mexico.

The Biggest Marine Mammal Is...

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