What Do Seals Eat? Diet of Harbor, Gray and Other Seals

What do seals eat in the wild?

All 18 or so species of true seals are meat-eaters, and none eat plants. Fish make up most of the diet for the majority of species, followed by cephalopods (squid and octopus) and crustaceans such as crabs, shrimp and krill. A few seals also take seabirds, and a very few hunt other marine mammals.

Seal teeth are pointed and cone-shaped, built for gripping slippery prey rather than chewing it. Small fish are swallowed whole underwater, usually head first. Bigger prey, such as a large salmon or flatfish, is often brought to the surface, held with the front flippers and shaken or torn into chunks. Seals get most of their water from their food, so they rarely need to drink.

What a seal eats depends heavily on where it lives and the time of year. The same species can live mostly on herring in one bay and on flatfish or squid a few hundred kilometers away.

What do harbor seals eat?

The harbor seal, the common seal of European and North American coasts, eats mainly fish caught close to shore, in estuaries and sometimes in rivers. Typical prey includes herring, cod, whiting, hake, sand eels, flatfish such as flounder and sole, sculpins, anchovies, pollock and salmon. Harbor seals also take squid, octopus, crabs and shrimp.

They usually hunt alone and feed on whatever is plentiful and easy to catch. Most feeding dives last only a few minutes and are fairly shallow, although harbor seals can reach depths of about 500 m (1,640 ft). Newly weaned pups, which must learn to hunt on their own, start with crustaceans such as shrimp, which are easier to catch, before moving on to fish.

What do gray seals eat?

The gray seal, a bigger seal of the North Atlantic, eats mostly fish that live on or near the seabed. NOAA lists sand eels, hake, whiting, cod, haddock, pollock and flatfish among its main prey, along with squid, octopus, crustaceans and sometimes seabirds. In Europe, herring, wrasse and skates are locally important too.

According to NOAA, a gray seal eats on average about 4–6% of its body weight a day, but it does not feed every day and fasts during the breeding and molting seasons. Females nursing pups eat nothing for about two to three weeks and can lose a large part of their body weight.

Gray seals can also be predators of other marine mammals. Studies in Scotland, the Netherlands and Germany have found that some gray seals, mainly large males, kill and eat harbor porpoises and harbor seals, and in 2014 a male in the North Sea was filmed killing and eating gray seal pups.

What do other seals eat?

Some seals have unusual diets shaped by where they live.

The crabeater seal, despite its name, does not eat crabs. Its cheek teeth have many small lobes that interlock to form a sieve, so it can take a mouthful of water and strain out the krill. The bearded seal feeds on the seabed using a form of suction, much like the walrus, whose clam diet is described in What do walruses eat?

Do seals eat penguins?

Only the leopard seal hunts penguins as a regular part of its diet. It patrols the edge of the sea ice, mostly submerged, and grabs penguins as they enter or leave the water, then shakes and beats the bird against the surface to tear it apart. Adélie, gentoo, chinstrap and even emperor penguins are taken.

Penguins are only part of the story, though. Young leopard seals eat mostly krill, squid and fish, and krill remains an important food for adults. Around the sub-Antarctic island of South Georgia, Antarctic fur seals are their main prey. Harbor and gray seals in the Northern Hemisphere never meet penguins, which live almost entirely south of the equator.

How do seals find their prey?

Seals hunt with a combination of senses. Their large eyes have a reflective layer behind the retina, like a cat’s, that helps them see in dim and murky water. Their hearing is sharp underwater. Their most remarkable tool, though, is their whiskers.

Seal whiskers are richly supplied with nerves and pick up tiny water movements. In an experiment published in 2001, blindfolded harbor seals followed the invisible trail left by a miniature submarine, showing that they can track the wake a swimming fish leaves behind it. Elephant seals, which hunt in near darkness hundreds of meters down, rely heavily on their whiskers too, helped by the glow of bioluminescent prey. Diving that deep takes remarkable breath-holding, explained in How long can seals hold their breath?

Seals and fisheries

Because seals eat fish that people also want, they have long been in conflict with fishers. Seals take fish from nets and lines, raid salmon farms and gather at river mouths and dams where salmon crowd together. In the past, governments paid bounties for killed seals, and some countries still allow seals to be shot to protect fisheries or fish farms.

Scientists generally find that seals are rarely the main cause of a fish stock’s decline, which is more often due to overfishing and changing ocean conditions. In some places, however, seal predation can slow a recovery. In the southern Gulf of St. Lawrence in Canada, for example, heavy predation by a growing gray seal population has been linked to the failure of collapsed cod stocks to recover.

Wild seals should never be fed. Feeding teaches them to approach people and boats, where they can be injured or bite, and in the United States it is illegal under the Marine Mammal Protection Act.