Harbor seal coloration can vary greatly from white or light gray with dark spots to dark brownish black with light spots depending on where in their range they are found.
Adult male harbor seals weigh between 120 and 300 pounds (55 to 130 kilograms) and are between 5 and 6 feet (1.5 to 1.8 meters) long. Adult females are slightly smaller than their male counterparts weighing between 100 and 190 pounds (45 to 87 kilograms) and averaging between 4 and 5 feet (1.2 to 1.5 meters) long.
Harbor seals have the widest distribution of any seal and can be found in both the North Atlantic and Northern Pacific oceans. On the West Coast of North America, their distribution spans from the Southern Arctic (Yukon to northern Alaska) down the California coastline and on the East Coast from South Greenland, the Hudson Bay and down the coastline to the Carolinas.
Harbor seals can be found anywhere from cool, temperate waters to cold, arctic and sub-arctic coasts. They spend half their time on land resting, breeding and raising their young on both rocky and sandy beaches. Harbor seals do not migrate and will remain in the same general area unless searching for food requires them to move. Some recent tagging studies have shown that juvenile animals may travel greater distances.
Harbor seals spend most of their time staying alert for predators such as polar bear, orca and even sharks. Regardless of whether it is alone or in a group, a harbor seal will let out an alarm call if danger appears to be near and flee the area, usually by diving into the water.
Harbor seals use vocalizations to warn other seals about danger, fend of danger, establish hierarchy, and keep mothers and their pups together.
Harbor seals, like other pinnipeds have adaptations that allow them to dive and conserve oxygen while under water. Harbor seals are generally able to dive to depths of 500 feet (152.4 meters) but depths of up to 1,460 feet (446 meters) have been recorded. They can remain submerged for up to 30 minutes at a time although the average dive lasts less than three minutes given that most prey items live in shallower depths.
Before a deep dive, harbor seals will exhale to reduce the amount oxygen in their lungs, relying instead on oxygen stored in their blood and muscle tissues. Blood is shunted away from the extremities and is concentrated on maintaining the core functions of brain heart, and lungs where it is needed. Harbor seals will also slow their heart rates from upwards of 80 (averaging between 80 and 120) beats per minute to as few as three or four. After surfacing, the seal's heartbeat accelerates rapidly for a short time.
Harbor seals have a higher metabolic rate than other mammals of comparable size. This allows them to generate heat to keep warm—usually about one degree warmer than the surrounding temperature of the water. A thick layer of insulating blubber provides thermoregulation as well as a nutrient reserve during fasting periods. During the winter months, the blubber layer can account for up to 30 percent of a harbor seal's body mass. Other adaptations to prevent heat loss to the environment include the ability to shunt blood vessels in their extremities and skin to conserve heat at their core.
Harbor seals are opportunistic feeders and eat mainly fish: rockfish, herring, cod, mackerel, flounder and salmon. They will also eat squid, clams, octopus, crayfish, crab and shrimp when available. Harbor seals wait until the tide comes in to feed and some may even follow fish runs with the high tides in spring. These seals that follow fish runs for food will wait to return to the coastal waters in the fall. Harbor seals frequently swallow their food whole.
At the Smithsonian's National Zoo, harbor seals are offered herring, capelin, squid, butterfish and mackerel.
Seals molt following the pupping season. During this time, they remain hauled out on the shore to avoid heat and energy loss.
Male harbor seals are generally promiscuous, mating with multiple females in a breeding season. Males initiate mating by chasing, neck and flipper biting, and embracing. When approached, females respond by growling, head thrusting and flipper waving. Copulation usually takes place in the water, which occurs very shortly after weaning young conceived during the previous breeding season. Several hundred animals will move ashore at one time for breeding season which ranges from March to August.
The peak of harbor seal births vary widely between areas. Harbor seals can be born as early as February in Baja California and as late as July in Europe. Mothers will give birth to one pup after an 11-month gestation period. Harbor seal pups average 2.5 to 3 feet (75 to 100 centimeters) in length and weigh average between 22 and 26 pounds (10 to 12 kilograms) at birth. Mothers nurse their pups for about four weeks after birth. Although a pup can immediately swim on its own after birth, a pup will often stay on its mother's back while she dives and remain very close to its mother until it matures. Harbor seals reach sexual maturity between 3 and 7 years of age.
Harbor seals in the wild can live between 25 and 30 years and for over 30 years in human care.
On a global scale, harbor seals benefit from large and stable populations that do not qualify them under a threatened category. However, the International Union for Conservation of Nature still recommends the protection and observation of this species from a conservation standpoint. Historically, harbor seals have suffered population drops due to viral diseases similar to distemper, water pollution and habitat loss. Harbor seals are also threatened by humans through hunting and commercial fishing practices. Harbor seals have learned how to steal fish from fishing nets, resulting in damage to nets angering fishermen and fisheries in these areas. Harbor seals also consume the fish in these areas leaving fisheries with lower yields. Other threats include over-fishing in an area which results in a loss of resources.
In the Unites States, harbor seals are protected under the Marine Mammal Protection Act of 1972. With the exception of licensed hunting, this Act protects from all other threats including importation of parts or products from all seals. From a conservation standpoint, the adherence to legal protection as well as the safe-keeping of our waterways will help harbor seals to survive in their current habitats without fear of hunting, pollution or habitat loss.
One male harbor seal, Luke, lives at the mixed-species seal exhibit along with the gray seals.