Species

Harbor seal

May 21, 2011, 10:33 am
Content Cover Image

Harbor seal. Source: Commander John Bortniak, NOAA Corps (ret.)

Also known as the Common seal, the Harbor seal (Phoca vitulina) is one of 19 species of marine mammals in the family of true seals. Together with the families of Eared seals and Walruses, True seals are part of the classification of marine mammals known as pinnipeds. In appearance this gregarious animal is often described as having a head shaped like a dog. The conservation status of this species is listed as Least Concern, but this designantion is misleading with respect to a complete assessment of biodiversity risk, since some of the individual subspecies are threatened.

caption Harbor seal. Source: Petr Baum/BioLib/Encyclopedia of Life
Conservation Status

Scientific Classification

Kingdom: Animalia (Animals)
Phylum:--- Chordata
Class:------ Mammalia (Mammals)
Order:-------- Carnivora (Carnivores)
Family:-------- Phocidae (True Seals)
Genus:--------- Phoca
Species:--------Phoca vitulina Linnaeus, 1758

There are five subspecies, each associated with a well defined coastal geographic region:

  • Eastern Atlantic Harbor seal - Phoca vitulina vitulina (Linnaeus, 1758)
  • Western Atlantic Harbour seal - Phoca vitulina concolor (De Kay, 1842)
  • Ungava seal or Hudson Bay Har  - Phoca vitulina mellonae (Doutt, 1942)
  • Eastern Pacific Harbor seal - Phoca vitulina richardii (Gray, 1864)
  • Western Pacific Harbour seal or Kuril seal - Phoca vitulina stejnegeri (J. A. Allen, 1902) 

 Distribution

The Harbor Seal is the most widely distributed of pinniped species. This organism is found in temperate, sub-Arctic, and Arctic coastal areas on eastern and western sides of the North Atlantic and North Pacific Oceans. Each of the five discrete subspecies is associated with a specific coastal province:

  • Eastern Atlantic Harbor Seal occurs off the coast of Northern Europe from Brittany to Russia along the Barents Sea. The United Kingdom supports 45% of the European population (5% of the world population); with major centres of population in the Moray Firth, The Wash and the Tay Estuary, and moderately widespread around west coast of Scotland, the Northern Isles and the Hebrides. 
  • Western Atlantic Harbor seal occurs from the mid-Atlantic states of the United States to the Canadian Arctic and east to Greenland and Iceland.
  • Ungava seal is found in freshwater lakes and streams of the Ungava Peninsula in northeastern Canada.
  • Eastern Pacific Harbor Seal occurs along the Pacific coast of North America from central Baja California in Mexico to theAlaskan Peninsula and possibly to the eastern Aleutian Islands.
  • Western Pacific Harbour seal (or Kuril seal) is found from Aleutian Islands, along the Russian Pacific coast, and the Kuril Islands to the Japanese Island of Hokkaido

 Morphology

Harbor seals can attain a length of up to 1.9 meters and body mass of up to 170 kilograms, males characteristically being slightly larger than females. The rounded, fusiform body 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 surface is usually more densely covered with spots or rings than the ventral surface.

The limbs of the Harbor seal have been adaptively modified into flippers. The fore-flippers (pectoral flippers) are composed of five digits of similar length and webbed together. Claws on the fore-flippers 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. The hind flippers propel the seal forward by moving side to side. On land, the harbor seal moves by undulating in a caterpillar-like motion.

Distinguishing diagnostic attributes include a size small in relation to other members of the family phocidae. The face of species Phoca vitulina members feature V-shped nostrils and a dog-like visage, while the color of this taxon may vary from white to tan to dark brown to red, often in a mottled combination.

Reproduction

P. vitulina has a polygynous mating system, with males often selecting several females in succession. Pre-mating behaviors are exhibited by both males and females;  such characteristic courtship actions include rolling, bubble-blowing, and mouthing the neck of the opposite gender. These behaviors typically cease once mating begins. Males initiate the courtship ritual by chasing, playfully biting, and embracing females. Females respond, and the act of copulation usually takes place in the water. Males frequently engage in underwater displays and fights around this time and can lose up to 25 percent of their body weight. Harbor seals usually return to the same breeding ground every year.

It is believed that males become sexually mature once a weight of around 75 kg is achieved; with females maturing at about 50 kg. Sexual maturity is reached between age three and seven for the male and from two to six years of age for the female. While the mating season varies among the various subspecies, it generally occurs from late spring through fall. About six weeks after the female gives birth, she again come into estrus. The gestation period lasts between nine and 11 months, and usually a female produces only one pup per year, typically in June or July. The pups weigh 11 to 12 kilograms at birth and are able to swim and crawl almost immediately. Pups are nursed for about four weeks, after which they may disperse over long distances. Around the time that the pup is weaned, females become receptive and copulation occurs. Juvenile Harbor seals commonly feed on shrimp before progressing to the adult diet.

This species is gregarious; forming a breeding colony on land,  where individuals also moult. These groups are typically small in number, but may occasionally number up to 1000 individuals. P.vitulina is primarily a benthic feeder taking cod, flounder and numerous cephalopods; most often preying in waters of ten meters or less in depth. (Encyclopedia of Life) While a variety of fish are the chief prey, squid, whelk, crab and mussels are also taken.  When feeding, common seals travel up to 50 kilometres from haul-out sites to feed and may stay out at sea for days. The adult can dive for up to 10 minutes, and attain water depths of 50 meters or more.

Behavior

Harbor Seals are usually solitary animals, with reproduction and haul-out being the chief exceptions. Seals haul out onto land for various reasons including resting, thermo-regulating, courtship, giving birth, nursing, molting, and facilitating digestion. Another possible reason for haul outs is protection from predators by being in a group. During haul outs, adult individuals do not make physical contact with each other and respond to being touched with anger. Younger seals interact with each other along the edges of the group and stay away from the adults.

Aggression is shown by growling, snorting, threateningly waving a fore-flipper, and head-thrusting, the last of which actions is a sharp, rapid retraction of the neck. Vocalization occurs when P.vitulina is communicating with other species members, in certain courtship occasions and as an alarm when threatened.

Harbor seals are characteristically prey to great white sharks and killer whales.

 Conservation status

While the species has healthy population numbers estimated between 400,000 to 500,000 individuals, and is considered a species of Least Concern (IUCN), however, its various subspecies have experienced significant historical fluctuations, and in some cases taxon subspecies may be considered threatened. More specific subspecies summaries are as follows:.

  • Eastern Atlantic Harbour seal: In the 1970's harbor seals seemed close to disappearing altogether from the Baltic Sea, however the subspecis has made a slow recovery in that region, albeit to numbers that are still just in the hundreds. The previously large populations around the British isles were approximately halved because of cycles of disease outbreak of the Phocid Distemper Virus in the North Sea; Icelandic populations amount to approximately 28,000. Moreover, populations of this taxon were locally extirpated in the southern Netherlands due to water pollution, land reclamation]] and exploitation. Pollution in the Wadden Sea has been a significant threat to the species in that marine basin. (Reijnders) The Baltic population is a mere 200 individuals. (Helle)
  • Western Atlantic Harbor seal - population declines halted in the 1970s as a result of legal protections and, from the 1980's steady increases in the numbers of seals have been observed in most locations along with a slight expansion of their range.
  • Ungava seal: This subspecies is the most threatened,  with the total subspecies population numbing in the range of just 120 to 600 animals. The species was severely exploited by Cree Indians in the 1800s and earlier, with some taking continuing into the 20th century. (Lowe) There is particular concern about the possible effects of hydro-electric projects in the region, since this taxon may likely be under the minimum viable population size viewed at the subspecies taxon level.

  • Eastern Pacific Harbor seal: The long-term decline of Harbor seals in the Gulf of Alaska appear to have abated and may even be reversing. Farther south, numbers have increased over the last decades.
  • Western Pacific Harbour seal or Kuril seal: The numbers in the southern most part of their range, Japan, are small; estimated to be just 350 in the late 1980s. In other parts of their range numbers have increased since the 1960s. 

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 in order to protect fisheries or fish farms. Protection in Britain occurs under the Conservation of Seals Act 1970 (with a closed season from 1 September to 31  December), and schedule 3 of the Conservation Regulations (1994). It is listed as a protected species under Annex II and Annex V of the European Community's Habitats Directive. The eastern Atlantic population is listed under Appendix III of the Bern Convention. Subpopulations in the Baltic and Wadden Seas are listed under Appendix II of the Bonn Convention.

Many fish species taken by harbor seals are also commercially fished, with the outcome that P. vitulina individuals often are entangled in fishing nets and apparatus, with resulting outcomes varying from deprivation of productive energy to outright mortality.

Relation to humans

Harbor seals have historically been hunted primarily for their skins, oil, and meat. They have also been exploited in the production of jewelry and trinkets, and as meat for mink feeding. Phoca vitulina are widely enjoyed by human voyeurs both in the wild and at aquaria. The species has been used in experimental medical research, but increasingly injured and sick Harbor Seals are, instead, rescued, tested for clues to marine chemistry and pathogen content and thereafter rehabilitated and released..An example of such systematic use of Harbor seals and other pinnipeds is at The Marine Mammal Center, in Marin County, California. Fisherman sometimes regard P vitulina as an interference with their commercial fisheries, since this seal will sometimes be observed eating fish that have been entrained in nets, although the Harbor Seal frequently meets with its own demise in such cases by becoming trapped in the nets.(Seal Conservation Society)

Sources

  1. Encyclopedia of Life Phoca vitulina Linnaeus, 1758  (accessed July 20, 2010)
  2. M. Steinway. 2003.Phoca vitulina, Animal Diversity Web (accessed July 20, 2010)
  3. Seal Conservation Society. Harbor seal,  (accessed December July 20, 2010)
  4. IUCN Red List of Threatened Species: Phoca vitulina,  (accessed July 20, 2010)
  5. Marianne Riedman. 1991. The Pinnipeds: Seals, Sea Lions, and Walruses, , University of California Press  ISBN: 0520064984
  6. Bernd Wursig. 2002. Encyclopedia of Marine Mammals, , Academic Press ISBN: 0125513402
  7. Suzanne Montgomery and Timothy J. Ragen. 2005. Marine Mammal Research: Conservation beyond Crisis, edited by John E. Reynolds III, William F. Perrin, Randall R. Reeves, , Johns Hopkins University Press  ISBN: 0801882559
  8. Ronald M. Nowak. 1999. Walker's Mammals of the World, , Johns Hopkins University Press, 1999 ISBN: 0801857899
  9. Harbor Seal, MarineBio.org (accessed July 20, 2010)
  10. P.J.H. Reijnders. 1986. Reproductive failure in common seals feeding on fish from polluted coastal waters. Nature 324:456-457
  11. E. Helle. 1983. Hylkeiden elamaa. Kirjayhtyma Helsinki. 171 pp
  12. A.P. Lowe. 1898. Report on a traverse of the northern part of the Labrador peninsula from Richmond Gulf to Ungava Bay. Geol. Surv. Can. Part I. Ann. Rep, 9, 1-43

 

Glossary

Citation

Saundry, P. (2011). Harbor seal. Retrieved from http://www.eoearth.org/view/article/153251

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