Phoca vitulinaharbor seal

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.