| Harbor, common or harbour seal | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Carnivora |
| Suborder: | Pinnipedia |
| Family: | Phocidae |
| Genus: | Phoca |
| Species: | P. vitulina |
| Binomial name | |
| Phoca vitulina Linnaeus, 1758 |
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| Subspecies | |
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P. vitulina concolor (DeKay, 1842) |
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| Range of Phoca vitulina | |
The harbor (or harbour) seal (Phoca vitulina), also known as the common seal, is a true seal found along temperate and Arctic marine coastlines of the Northern Hemisphere. They are found in coastal waters of the northern Atlantic and Pacific Oceans, as well as those of the Baltic and North Seas, making them the most widely distributed of the pinnipeds (walruses, eared seals, and true seals).
Common seals are brown, tan, or gray, with distinctive V-shaped nostrils. An adult can attain a length of 1.85 meters (6.1 ft) and a mass of 132 kilograms (290 lb). Females outlive males (30–35 years versus 20–25 years). Common seals stick to familiar resting spots or haulout sites, generally rocky areas (although ice, sand and mud may also be used) where they are protected from adverse weather conditions and predation, near a foraging area. Males may fight over mates underwater and on land. Females are believed to mate with the strongest males and generally bear a single pup, which they care for alone. Pups are able to swim and dive within hours of birth, and they develop quickly on their mothers' fat-rich milk. A fatty tissue layer called blubber is present under their skins and helps to maintain body temperature.
Their global population is 5-6 million, but subspecies in certain habitats are threatened. Seal hunting or sealing, once a common practice, is now illegal in many nations within the animal's range.
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These seals are rather curious, so sea kayakers sometimes get the opportunity to see them close up. Seals also sometimes swim along beaches, looking at beach walkers. The seals are wary of humans on land, however, and will enter the water at any opportunity. They do not attack humans, though, whether on land or in the water. The seals can be very vocal, especially in large groups, and are rather social animals.
With each individual possessing a unique pattern of fine, dark spots (or light spots on a dark background in some variants), they vary in color from brownish black to tan or grey; underparts are generally lighter. The body and flippers are short, with a proportionately large, rounded head. The nostrils appear distinctively V-shaped; as with other true seals, there is no ear flap, or pinna. A relatively large (for a seal) ear canal may be visible behind the eye. Including the head and flippers, they may reach an adult length of 1.85 meters (6.1 ft) and a weight of 55 to 168 kg (120 to 370 lb).[2] Females are generally smaller than males.
With an estimated 5 million to 6 million individuals, the population is not threatened as a whole; most subspecies are secure in numbers, with the Greenland, Hokkaidō and Baltic Sea populations being exceptions. Local populations have been reduced or eliminated through outbreaks of disease (especially the phocine distemper virus) and conflict with humans, both unintentionally and intentionally, has also been linked to common seal declines. While it is legal to kill seals which are perceived to threaten fisheries in the United Kingdom, Norway and Canada, commercial hunting is illegal; the seals are also taken in subsistence hunting and accidentally as by catch in fishing nets. By catch by fishing nets (mainly in bottomset nets) along the Norwegian coast accounted for 48% of pup mortality.[3] Seals in the United Kingdom are protected by the 1970 Conservation of Seals Act, which prohibits killing them in most circumstances. In the United States, alternative protection applies and it is illegal to kill any seals or any marine mammals, as they fall under the Marine Mammal Protection Act. On the East Coast of the United States, their numbers seem to be increasing quite steadily as they are reclaiming parts of their range, and have been seen as far south as Florida.
Female common seals have a life span of 30–35 years, while male life spans are usually 20-25.
The five subspecies of Phoca vitulina are:
Characterized as showing a strong degree of site fidelity in their choice of resting sites, they may spend several days at sea and travel up to 50 kilometers in search of feeding grounds, and will also swim some distance upstream into freshwater in large rivers. Resting sites may be both rugged, rocky coasts, such as those of the Hebrides or the shorelines of New England, or sandy beaches.[1] They also inhabit sandy intertidal zones;[1] some seals may also enter estuaries in pursuit of their fish prey. Some have even taken to feeding and playing in New York Harbor and Boston Harbor in recent years. The seals frequently choose to congregate in harbors, lending the animals their other common name. The feeding habits have been studied closely in many parts of their range; they are known to prey primarily upon fish, such as menhaden, anchovy, sea bass, herring, mackerel, cod, whiting and flatfish, and occasionally upon shrimp, crabs, mollusks and squid. Although primarily coastal, dives to over 500 m have been recorded.[4] Common seals have been recorded to attack, kill and eat several kinds of seabirds.[5]
While not forming groups as large as some other seals, they are gregarious animals. When not actively feeding, the seals will haul onto a terrestrial resting site. The seals tend to be coastal, not venturing more than 20 kilometers offshore. Both courtship and mating occur underwater. The mating system is not known, but thought to be polygamous. Females give birth once per year, with a gestation period of approximately nine months.
Birthing of pups occurs annually on shore. The timing of the pupping season varies with location,[6] occurring in February for populations in lower latitudes, and as late as July in the subarctic zone. The mothers are the sole providers of care, with lactation lasting four to six weeks. Researchers have found males gather underwater, turn on their backs, put their heads together and vocalize to attract females ready for breeding.[7] The single pups are born and well developed, capable of swimming and diving within hours. Suckling for three to four weeks, pups feed on the mother's rich, fatty milk and grow rapidly; born weighing up to 16 kilograms, the pups may double their weight by the time of weaning.
Common seals must spend a great deal of time on shore when moulting (shedding their fur), which the seals undergo shortly after breeding. This onshore time is important to the life cycle, and can be disturbed when there is substantial human presence [8] The timing of onset of moult depends on the age and sex of the animal with yearlings moulting first and adult males last.[9] A female will mate again immediately following the weaning of her pup. This pinniped is sometimes reluctant to haul out in the presence of humans, so shoreline development and access must be carefully studied in known locations of seal haul out.
The California population of subspecies richardsi amounted to approximately 25,000 individuals as of 1984. Pacific common seals or Californian common seals are found along the entire Pacific coast shoreline of the state. They prefer to remain relatively close to shore in subtidal and intertidal zones, and have not been seen beyond the Channel Islands as a pelagic form; moreover, they will often venture into bays and estuaries and even swim up coastal rivers.
Frequently, they will haul out in small to medium-sized groups onto rock outcrops, mudflats, sandy beaches or even fishing piers. Some of the best locations for viewing common seals up close are at Cannery Row in Monterey, Moss Landing on Monterey Bay or at Bolinas Lagoon in Marin County. They feed in shallow littoral waters on herring, flounder, hake, anchovy, codfish and sculpin.[10]
In California, breeding occurs from March to May, and pupping between April and May, depending on local populations. There is no indication this species has territorial characteristics in water, and it definitely displays none on land. As top level feeders in the kelp forest, common seals enhance species diversity and productivity. They are preyed upon by orcas and white sharks.
Considerable scientific inquiry has been carried out by The Marine Mammal Center and other research organizations beginning in the 1980s regarding the incidence and transmission of diseases in common seals in the wild, including analysis of phocine herpesvirus.[11] In the San Francisco Bay, some common seals are fully or partially reddish in color. This may be caused by an accumulation of trace elements, such as iron or selenium, in the ocean or a change in the hair follicles.