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The rounded body of the harbor seal is covered by a coat made of thick, short hairs that range from nearly white with dark spots to black or dark brown with white rings. The dorsal (back) surface is usually more densely covered with spots or rings than the ventral (belly) surface. The foreflippers (pectoral flippers) are composed of five digits of similar length and webbed together. Claws on the foreflippers are used for scratching, grooming, and defense. The hind flippers also have five digits; however, the first and fifth digits are long and stout, while the middle digits are shorter and thinner.
Harbor seals are well adapted to life in the sea. They are able to dive to depth of up to 1,640 feet (500 meters) and can remain submerged for over 20 minutes, although most dives are less than 65 feet and less than four minutes long. Adaptations to conserve oxygen allow such dives include high blood volume, reduced peripheral circulation, reduced heart rate and high levels of myoglobin (oxygen-binding protein in muscle). They are graceful and efficient swimmers as they use their hind flippers for propulsion and fore flippers as rudders. Movement on land is accomplished by laborious, caterpillar-like undulations on their bellies. Harbor seals are usually solitary animals, with reproduction and "haul outs" being the only exceptions. Seals "haul out" onto land for various reasons including resting, thermoregulating, giving birth, nursing, molting, and facilitating digestion.
Harbor seals are opportunistic feeders and take advantage of seasonally available prey resources. In Alaska, commonly eaten prey include walleye Pollock, Pacific cod, capelin, eulachon, Pacific herring, sandlance, Pacific salmon, sculpin, flatfish, octopus and squid.
Mating season varies but usually occurs in late spring through fall, when females come into estrus (“heat” or “season”)
about six weeks after the birth of pups. In Alaska, this would mean a fall breeding season. Development of the embryo is suspended for about 11 weeks, called delayed implantation. In Alaska, single pups are born between May and mid-July. Young pups are able to swim almost immediately after birth. They normally remain with their months for one month, after which they are weaned and separate. At that time, over half their body weight may consist of fat, providing them a head start on self-sufficiency. Mature females mate shortly after weaning their pups.
The U.S. Marine Mammal Protection Act of 1972 made it illegal to hunt or harass any marine mammal in U.S. waters. In Canada, Norway, and the United Kingdom, it is legal to shoot harbor seals to protect fisheries or fish farms. In Alaska, there are approximately 141,000 harbor seals in non-glacial sites and approximately 15,000 in glacial fjords. They are difficult to census because they can only be accurately counted when they are hauled out. Haul-out times vary individually and are scattered across thousands of areas in Alaska, both on land and glacial ice calved from tidewater glaciers.
Harbor seals are a Species of Special Concern in Alaska, meaning they are sensitive to environmental changes and prone to declines. Drastic declines have occured in harbor seal populations in Alaska in the past, with seals in Glacier Bay continuing to decline despite efforts to regulate boat traffic and fishing there. Seals in Alaska appear to be sensitive to declines with slower recovery rates, compared to seals in other parts of the world where populations appear to be more robust to change and recovery.