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Harbor Seals Species Description


HARBOR SEAL Species Description

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When hauled out, their posture almost exudes a sense of regality—arching back with nose and tail pointed to the sky. Underwater and these animals are agile and streamlined, but to watch them move on land or ice, perceptions may change. Whether underwater or on land—graceful or graceless—harbor seals, Phoca vitulina, are perhaps one of the most commonly recognized marine mammals along the North American coastline. With five recognized subspecies of P. vitulina, harbor seals have the most extensive geographic distribution of any seal in the world. Harbor seals, like sea lions, are pinnipeds but are grouped in a different family called Phocidae (meaning “true” or “earless” seals for their lack of visible ear pinnae). There are 14 species of phocids that are more closely related to harbor seals belonging to the subfamily Phocinae, also known as the northern phocids.

Although harbor seals are commonly found throughout the Northern Hemisphere, it is the Alaska population that the Alaska SeaLife Center has been studying since 1998. Their global population is quite healthy (>200,000 individuals), but the Alaska population—especially south-central and south-western—has suffered dramatic declines in recent decades. From 1984-1997, numbers declined by 63% in Prince William Sound, and on Tugidak Island numbers diminished by 85% during 1976-1988. Tugidak Island, near the larger Kodiak Island, was perhaps the largest harbor seal haulout in the world prior to the decline where 15,000-20,000 animals would haul out. More recent declines have been observed in Glacier Bay National Park in southeast Alaska where approximately 58% of the population has declined from 1992-2001. Glacier Bay numbers are alarming because of the enhanced Federal protection measures, and the overall trend in southeast Alaska has been stable or slightly increasing during the reported decline.

It is still unclear why populations of harbor seals, among other pinnipeds, have declined in Alaska in recent decades. To date, there is no single factor that has been identified as the cause(s) of the harbor seal decline in these geographic areas. It is likely that a combination of factors may have contributed to the decline which include but are not limited to commercial harvest, predation, changes in food, contaminants, disease, environmental change etc. Part of the challenge has been the shear magnitude of Alaska’s coastline—it has been very difficult to accurately enumerate harbor seal populations in the Gulf of Alaska and Bering Sea over time. Harbor seals inhabit the coastal waters of southeast Alaska, Gulf of Alaska, the Aleutian Islands, and throughout Bristol Bay in the Bering Sea.

Harbor seals have been known to haul out on both terrestrial sites and glacial ice from calving tidewater glaciers. In Alaska, harbor seals often use ice calved from tidewater glaciers to rest, molt, and give birth. As current oceanic and atmospheric temperatures rise, these tidewater glaciers have been receding and thinning. With only about 50 tidewater glaciers remaining throughout Alaska, available ice habitat for harbor seals has become increasingly limited. When hauled out on ice or land, harbor seals tend to be more intolerant of contact than other pinnipeds.

.Unlike many pinnipeds, harbor seals do not exhibit strong sexual dimorphism. Adult males tend to be slightly larger than adult females but sizes differ among geographic location. In the Gulf of Alaska, they appear to be smaller than the other subspecies of harbor seals with adult males weighing about 190 lbs (87 kg) and a length slightly over 5 ft (160 cm). Adult females weigh about 148 lbs (65 kg) and are about 5 ft (148 cm) in length. Females will have given birth to a single pup—averaging 22 lbs (10 kg) and 2.5 ft (82 cm)—typically during a 10-week pupping season, although the season varies geographically. In Alaska, particularly Aialik Bay, pupping begins in early May and ends in late June with peak pupping in mid-June. Adult females have been known to birth twins although this phenomenon is rare. There is about a 4-week nursing period after which pups will start catching their own food—called weaning. At about the same time, males will mate with females in the water. Implantation is delayed, like other pinnipeds, for about 2.5 months at which point the embryo will become implanted. Gestation is typically 10.5 months from fertilization to birth.



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