General description: Harbor seals are mammals, that is they are hairy, warm-blooded, air-breathing animals which suckle their young. They weigh about 24 pounds (11 kg) at birth and gain weight rapidly during a month-long suckling period, perhaps doubling their weight. Average weight for adults is about 180 pounds (82 kg); males are somewhat larger than females. They are covered with short, stiff, bristle-like hair. Coloration varies, but two basic patterns occur: a dark background with light rings, or light colored sides and belly with dark blotches or spots. Harbor seals molt annually, usually in late summer.

Harbor seals are well adapted to life in the sea. They are able to dive to depths exceeding 600 feet (183 m) and can remain submerged for over 20 minutes. Oxygen-conserving adaptations that allow such dives include reduced peripheral circulation, reduced heart rate, and high levels of myoglobin (muscle oxygen binder). Harbor seals are graceful and efficient swimmers as they use their hind flippers for propulsion and foreflippers as rudders. Movement on land, however, is slow and laborious.

 

Life history: In Alaska, single pups are born between May and mid-July. The young pups are able to swim almost immediately after birth. They normally remain with their mothers about one month, after which they are weaned and separate from their mother. At that time over half their body weight may consist of fat, providing them a head start on self-sufficiency. Sexual maturity occurs at between 3 and 7 years. Mature females mate shortly after the weaning of their pups; however, the embryo does not implant in the uterus until about 11 weeks later, a trait called delayed implantation. Active fetal development is about 8½ months.

The sex ratio of harbor seals at birth is approximately equal and remains so until about 5 years of age. Thereafter mortality rates for males are higher, and females become relatively more abundant. Maximum ages estimated from annual rings in their teeth are 26 years for a male and 32 years for a female.

 

Food habits: In Alaska, commonly eaten prey include walleye, pollock, Pacific cod, capelin, eulachon, Pacific herring, salmon, octopus, and squid.

 

Abundance and trends: Harbor seals are a difficult species to census because they can be accurately counted only when they are hauled out. They haul out in thousands of locations in Alaska, and even if seals at all sites could be counted, the proportion of the total population hauled out at any given time is unknown. The total Alaska harbor seal population probably ranges between 200,000 and 300,000 animals. Since implementation of the Marine Mammal Protection Act in 1972, hunting has been restricted to Alaska Natives. In some areas, harbor seals are an important part of the subsistence economy. The annual harbor seal harvest is about 2,500 to 4,000 animals.

The number of harbor seals has declined in several areas of the Gulf of Alaska and Prince William Sound since the mid 1970s. At Tugidak Island near Kodiak, numbers have declined 90 percent from approximately 11,000 seals to 1,000. The reasons are unknown.