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Phoca vitulinaharbor seal

By Kristan Cale

Geographic Range

Harbor seals are the pinniped with the largest geographical range, encompassing coastal areas of the east and west Pacific and Atlantic Oceans in the Northern Hemisphere. Their range spans from the western Pacific Ocean coast north towards the Gulf of Alaska and to the southeastern Bering Sea. The western Atlantic Ocean harbor seals are distributed from the French coast to the North Sea and northward to the Barents Sea, as well as along the Atlantic coast of North America. Although they inhabit a large range of coastal and insular regions, the Pacific and Atlantic colonies are separated by large intervals with uncertain boundaries and five subspecies are recognized. (Boness, 2004; Burns, 2008; Riedman, 1990)

Habitat

Harbor seals populate the shallow waters of coastal areas, bays, rocky islets, estuaries, and even freshwater lakes. They are typically seen near piers and beaches, as well as on intercoastal islands. They are more common in relatively cold marine waters that lack annual sea ice coverage. However, tidewater glaciers are utilized for shelter, breeding, and occasionally transportation. They forage at an average depth of 91 m, but can go as deep as 427 m. (Boness, 2004; Burde and Feldhamer, 2005; Burns, 2008; Riedman, 1990; Spies, 2007)

  • Range depth
    427 (high) m
    1400.92 (high) ft
  • Average depth
    91 m
    298.56 ft

Physical Description

Harbor seals are physically adapted to maximize propulsion while diving. Their large round heads that lack external ears, although their middle ear bones are large. Rather than exhaling before diving, they essentially stop breathing underwater and their heart rate slows, resulting in conservation of oxygen in their bodies. They have narrow nostrils that easily close during swimming. Their flippers are long and flat, each made up of five webbed digits. Speed and agility in the water is enhanced by use of their hind limbs and flexible side-to-side movements of their trunks. Adult males are typically larger than females, measuring 160 to 190 cm in length, and weighing 80 to 170 kg. Females range from 160 to 170 cm long and weigh 60 to 145 kg. (Berta, et al., 2006; Burns, 2008; Nowak, 2003)

Although various regions differ slightly, harbor seals are monomorphic. Individuals with a yellowish coat covered in small pale ringed black spots are characterized as being in the “light phase”. The “dark phase” seals have a black coat containing dark spots with light rings found primarily on their dorsum. Their varying color patterns are a result of differing concentrations of melanocytes in the epidermis. Their fur protects their skin from damage while on land, and is kept moist by secretions from sebaceous glands. (Berta, et al., 2006; Burns, 2008; Nowak, 2003; Riedman, 1990)

Thick layers of subcutaneous fat provide energy and insulation, giving them a higher resting metabolism rate that is 1.7 to 2.2 times higher than other terrestrial mammals their size. Another trait that separates harbor seals from other mammals is their vibrissae (whisker) structures, whose follicles are surrounded by 3 blood sinuses as opposed to 2. (Berta, et al., 2006; Riedman, 1990)

  • Sexual Dimorphism
  • male larger
  • Average mass
    80 to 170 kg
    lb
  • Average mass
    115000 g
    4052.86 oz
    AnAge
  • Average length
    160 to 190 cm
    in
  • Average basal metabolic rate
    73.29 W
    AnAge

Reproduction

Harbor seals mate in aquatic habitats, and males attempt to attract potential mates in a variety of ways. These include showing off vocal or diving capabilities and direct competition between males. The latter involves intense fighting near haul out sites and areas of high female abundance. Males may also establish territories in areas of high female traffic to maximize contact with potential mates. Actual mating occurs in water, as is typical of seal species, which makes it difficult to observe and fully understand their mating systems. Harbor seals are considered mostly monogamous, but there have been reports in literature of slight polygamy with males mating with multiple females, but with a maximum of five successful fertilizations. (Boness, 2004; Burns, 2008; Nowak, 2003)

Female harbor seals typically give birth to a single pup every season with pregnancy rates that are about 85%. Twins have occasionally been reported. Once a female becomes fertilized, there is a delay in implantation that lasts about 2.5 months. Then implantation resumes and develops for 8 months before birth. The total gestation period lasts about ten and a half months. Birthing occurs close to the shore or on land, which appears to be an adaptation to isolate females and pups from aquatic predators. (Burns, 2008; Coltman, et al., 1998; Nowak, 2003)

Depending on regional and environmental factors, all sub-species of harbor seal pups are typically born in the same 10 week window lasting from late winter to summer. Newborns weigh 8 to 12 kg. While some seals retain a fine, downy coat of hair called lanugo after birth, harbor seal pups shed their lanugo well before they are born. At birth, the pups' fur is the same texture as that of the adults, which allows them to safely enter cooler waters. However, adult fur coats don't develop until the end of the first summer. Pups are weaned and independent after 4 to 6 weeks, and mating for the following year commences immediately afterward. Female harbor seals reach sexual maturity by age 3 to 4, and physical maturity by age 6 to 7. Male harbor seals don’t reach sexual maturity until age 4 to 5, and physical maturity until 7 to 9 years of age. (Burns, 2008; Coltman, et al., 1998; Nowak, 2003)

  • Breeding interval
    Harbor seals usually breed once per year, normally in late winter to summer.
  • Breeding season
    Breeding occurs during a 10 week period.
  • Range number of offspring
    0 to 2
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    10.5 months
  • Average gestation period
    253 days
    AnAge
  • Range weaning age
    4 to 6 weeks
  • Range time to independence
    4 to 6 weeks
  • Average age at sexual or reproductive maturity (female)
    3-4 years
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    1095 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    4-5 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    1460 days
    AnAge

Female harbor seals invest more effort in parental care than do males. Within the first hour of birth, mothers eagerly bond to establish recognition, which is critical to the success or raising a harbor seal. Pups are generally nursed for 4 weeks with milk containing around 50% fat from their mothers. This high fat content results in rapid growth. For 4 to 6 weeks until the pups are weaned, they cling to their mothers' backs in the water. (Allen, et al., 1988; Burde and Feldhamer, 2005; Burns, 2008; Geraci and Lounsbury, 2005; Nowak, 2003)

  • Parental Investment
  • female parental care
  • pre-hatching/birth
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

Harbor seals in the wild are estimated to reach an average lifespan of 40 years. In captivity, the longest recorded lifespan was 47.6 years. (de Magalhaes and Costa, 2009)

Behavior

Harbor seals are diurnal pinnipeds that are usually solitary. They form small mixed groups of males, females, and pups around the pupping season and during molting, but these groups do not exhibit social organization. They do not migrate, but disperse a couple hundred kilometers away from where they reside to forage and breed, or occasionally to avoid environmental disturbances. (Boness, 2004)

Throughout the year harbor seals haul out, which is the act of moving from aquatic to terrestrial grounds. Depending on the region, the majority of haul outs occur either onto land or icebergs. This activity provides benefits such as sites for birthing, thermoregulation, rest, and is a way to reduce predation. Harbor seals usually molt within 2 to 3 months after the pupping season, which induces high numbers of seals in haul out locations. Pups normally molt first, followed by juveniles, then female adults, and lastly male adults. However, in all species of seals, environmental factors such as precipitation and air temperature most strongly affect their hauling out activity. (Boness, 2004; Burns, 2008)

Harbor seals can dive to depths of 427 m and remain underwater for nearly 30 minutes without resurfacing. However, the average dive lasts a few minutes and is 91 m deep. They perform dives in a variety of shapes. A recent study categorized their dives into variations of V-shaped and U-shaped dives. Although U-shaped dives have been connected to 89% of feeding events across all regions, sexes, and age groups, variations of this dive were seen most often in adult females. As a result of learning behaviors from their mothers, young that still nurse exhibit diving patterns similar to those of adult females. In contrast, already weaned pups displayed diving patterns similar to adult males that were variations of the V-shaped dive. (Baechler, et al., 2002)

  • Average territory size
    25 m^2

Home Range

Harbor seals usually stay within a 50 meter radius of their haul out site. Closer proximity to land during foraging allows for an easier escape from predators, so most of their activity occurs within a 10 meter radius from haul out sites. (Grigg, et al., 2009; Nowak, 2003)

Communication and Perception

Harbor seals are extremely alert and appear to be continuously aware of their surroundings, even when in captivity. In comparison to related seals, they are known to be less vocal. Their detected sound frequencies range from 0.1 to 10 kHz. Their relative “quietness” may be an adaptation to avoid predication. The noises they produce have been associated with sounding more like burping, grunting, and yelping. Harbor seals are recognized to be a profoundly playful species in both pups and adults. They often play by themselves and with other objects such as kelp. They have large eyes with corneas that are flattened that allow them to take in more light and see brighter images in dark water. Their sensitive whiskers acoustically detect sound waves, which is useful for catching prey. Another behavior they are known for is slapping their flippers vigorously against the water’s surface; the motive behind such action is still unknown. With such a wide geographical range, behaviors may vary between regions. (Burns, 2008; Nowak, 2003; Riedman, 1990)

Food Habits

Harbor seals are carnivores that consume mostly fish. Despite regional variation in individual diets, the overarching goal of harbor seals is to keep foraging efforts low by eating what is abundant and easily caught. Adults eat mostly fish, consuming the fish whole or head first. Preferred medium-sized fish species include codfish (Gadus), hake (Merluccius merluccius), mackerel (Scombridae), and herring (Clupea harengus). They also eat octopus (Octopoda) or squid (Teuthida), as well as crustaceans (Crustacea) such as crabs (Brachyura) and shrimp. Because newly weaned pups have poorly developed diving abilities, their primary diet consists of crustaceans that are easier to catch. Harbor seals weighing 100 kg eat about 5 to 7 kg of food per day. They will dive up to 427 m for a potential meal. (Berta, et al., 2006; Burns, 2008; Grigg, et al., 2009; Nowak, 2003)

  • Animal Foods
  • fish
  • mollusks
  • aquatic crustaceans

Predation

Predators of harbor seals inlcude larger species like sharks (Selachimorpha), killer whales (Orcinus orca), and polar bears (Ursus maritimus). Haul out sites help reduce predation by shortening the time they spend in water, yet there is still danger on land. For example, coyotes (Canis latrans) may prey on pups when their mothers are foraging. Humans (Homo sapiens) also prey on harbor seals in some areas of the Arctic. (Geraci and Lounsbury, 2005; Nowak, 2003; Riedman, 1990)

Ecosystem Roles

Harbor seals are hosts to many parasites. These include nematodes such as Pseudoterranova decipiens and Contracaecum osculatum and cestodes such as Anophryocephalus and Diplogonoporus. Although many parasites found living on the seals are normally harmless, seals that acquire pathogens when their immune systems are repressed could become deadly. The parasites could also pass bacteria and/or viruses that could make them very ill. For example, Leptospira interrogans is the bacterium responsible for the most recorded deaths in all pinnipeds. (Geraci and Lounsbury, 2005; Herreman, et al., 2011)

Commensal/Parasitic Species
  • nematodes (Pseudoterranova decipiens)
  • nematodes (Contracaecum osculatum)
  • cestodes (Anophryocephalus)
  • cestodes (Diplogonoporus)
  • bacteria (Leptospira interrogans)

Economic Importance for Humans: Positive

Harbor seals are hunted for their blubber, meat, fur, and skin. They are also a potential source of ecotourism value. (Riedman, 1990)

  • Positive Impacts
  • food
  • body parts are source of valuable material
  • ecotourism
  • research and education

Economic Importance for Humans: Negative

Harbor seals interfere with some commercial and local fisheries. They lower the available fish supply for consumption, and can also become entangled in nets. Entanglement usually leads to the nets breaking which releases fish that could have been caught and utilized by the fisheries. (Jefferson, et al., 2008)

Conservation Status

The total population of harbor seals has been increasing since around the 1970s. However, major die-offs have occurred in recent years where thousands of seals have died from diseases not previously known to be a problem. Because they are a coastal species, they are particularly vulnerable to pollution. Their status on the IUCN Red List is "least concern." However, two subspecies are currently on the brink of extinction. First is the subspecies population Phoca vitulina stejnegeri in Japan that has been steadily declining due to excessive hunting since the 1980s. Secondly, the subspecies Phoca vitulina mellonae that lives in the Ungava Peninsula in Canada have very low population numbers of 120 to 600 seals. They are the subspecies most at risk due to low genetic variation and are also negatively impacted from hydroelectric developments. No reports of efforts to conserve these two subspecies have been found.

Contributors

Kristan Cale (author), Radford University, Karen Powers (editor), Radford University, Kiersten Newtoff (editor), Radford University, Melissa Whistleman (editor), Radford University, Catherine Kent (editor), Special Projects.

Glossary

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

delayed implantation

in mammals, a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months.

diurnal
  1. active during the day, 2. lasting for one day.
ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

embryonic diapause

At about the time a female gives birth (e.g. in most kangaroo species), she also becomes receptive and mates. Embryos produced at this mating develop only as far as a hollow ball of cells (the blastocyst) and then become quiescent, entering a state of suspended animation or embryonic diapause. The hormonal signal (prolactin) which blocks further development of the blastocyst is produced in response to the sucking stimulus from the young in the pouch. When sucking decreases as the young begins to eat other food and to leave the pouch, or if the young is lost from the pouch, the quiescent blastocyst resumes development, the embryo is born, and the cycle begins again. (Macdonald 1984)

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

piscivore

an animal that mainly eats fish

polygynous

having more than one female as a mate at one time

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Allen, S., C. Ribic, J. Kjelmyr. 1988. Herd segregation in harbor seals at Point Reyes, California. Calif. Fish and Game, 74/1: 55-59.

Baechler, J., C. Beck, W. Bowen. 2002. Dive shapes reveal temporal changes in the foraging behaviour of different age and sex classes of harbour seals (Phoca vitulina). Canadian Journal Of Zoology, 80/9: 1569.

Berta, A., J. Sumich, K. Kovacs. 2006. Marine Mammals: Evolutionary Biology. Burlington, MA: American Press.

Boness, D. 2004. True Seals (Phocidae): Harbor Seal. Pp. 417-436 in M Hutchins, A Evans, J Jackson, D Kleiman, J Murphy, D Thoney, eds. Grzimek's Animal Life Encyclopedia, Vol. 14: Mammals III, 2nd Edition. Detroit: Gale.

Burde, J., G. Feldhamer. 2005. Mammals of the National Parks. Baltimore, MD: The Johns Hopkins University Press.

Burns, J. 2008. Encyclopedia of Marine Mammals. Pp. 533-541 in W Perrin, B Wursig, J Thewissen, eds. Harbor Seal and Spotted Seal, Vol. 1, 2 Edition. New York, NY: Academic Press.

Coltman, D., W. Bowen, J. Wright. 1998. Male mating success in an aquatically mating pinniped, the harbour seal (Phoca vitulina), assessed by microsatellite DNA marker. Behavioral Ecology and Sociobiology, 7/5: 627-638.

Geraci, J., V. Lounsbury. 2005. Marine Mammals Ashore: A Field Guide for Strandings. Baltimore, MD: National Aquarium in Baltimore.

Grigg, E., A. Klimley, S. Allen, D. Green, D. Elliott-Fisk, H. Markowitz. 2009. Spatial and seasonal relationships between Pacific harbor seals (Phoca vitulina richardii) and their prey, at multiple scales. Fishery Bulletin, 107/3: 359-372.

Herreman, J., A. McIntosh, R. Dziuba, G. Blundell, M. Ben-David, E. Greiner. 2011. Parasites of harbor seals (Phoca vitulina) in Glacier Bay and Prince William Sound, Alaska. Marine Mammal Science, 27/1: 247-253.

Jefferson, T., M. Webber, R. Pitman. 2008. Marine mammals of the World. Canada: Elsevier.

Nowak, R. 2003. Walker's Marine Mammals of the World. Baltimore, MD: The John Hopkins University Press.

Riedman, M. 1990. The Pinnipeds: Seals, Sea Lions, and Walruses. Oxford, England: University of California Press.

Spies, R. 2007. Long-Term Ecological Change in the Northern Gulf of Alaska. Kidlington, Ox, UK: Elsevier B.V..

de Magalhaes, J., J. Costa. 2009. A database of vertebrate longevity records and their relation to other life-history traits. Journal of Evolutionary Biology, 22/8: 1770-1774.

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