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Harbour Seal
Science
The
harbour seal is the most numerous and widespread marine mammal
on the north Pacific Coast. It is found close to shore from
Baja California to Japan.
The subspecies of harbour seal on the north Pacific Coast of
North America is the Pacific subspecies Phoca vitulina
richardsi that ranges along the coast from Baja California
to Japan (Baird 2001). Pelage colouration, dentition patterns,
seasonal time of pupping and mitochondrial and microsatellite
DNA evidence suggests that there might be several discrete
populations on the Pacific Coast (reviewed by Baird 2001).
In recent
years, the harbour seal has become a component of a
flourishing whale watching industry but only a few decades
ago, a bounty was in place to cull seals. Baird (2001) has
reviewed the status of the harbour seal in Canada.
There was an estimated 37,300 seals in the Strait of Georgia
and slightly over 100,000 in British Columbia in 1996 (Olesiuk
1999). These estimates were made by counting seals at
haul-outs and applying a correction factor to account for
seals underwater during the census. The choice of correction
factor introduces considerable error in the estimates but the
conclusion that seals are far more numerous than in the 1970s
is undeniable. Culling of seals occurred in Canada under a
bounty system that ended in 1969. Since then, the population
grew at about 12% annually and it is probably now at its
pre-cull level (Olesiuk et al 1990).
Most harbour seals mate as monogamous pairs but some
polygynous mating is also possible (Bigg 1981, Sullivan 1981,
Riedman 1990). Mating occurs in the water making observations
difficult. Pups are born earlier in the south than the north.
In Washington and southern British Columbia, females give
birth to a single pup in May or June, while in northern
British Columbia and southeast Alaska pupping occurs in June
and July. A pup about 80 cm long at birth will attain a length
of 1.9 m as an adult and males are about 13% larger than
females. The pups follow their mothers into the water soon
after birth. They are weaned at about three weeks of age and
females enter estrus a few weeks later. Females first breed
between 3 and 6 years of age (most at 5 years; Bigg 1969a,b).
Seals eat mostly fish but the diet includes many species such
as herring, salmon, midshipman, ling cod among others and
likely reflects local availability (Olesiuk 1993).
Harbour seals are eaten often by ‘transient’ killer whales
(Baird and Dill 1996, Watts 1993) and they are important
predator of fish. For example, seals formed a line across the
mouth of the Puntledge River at Courtenay to catch salmon
departing for the ocean (Yurk and Trites 2000).
References
Baird, R. W.
2001. The status of the harbour seal Phoca vitulina in
Canada. Canadian Field-Naturalist 115: 663-675.
Baird, R. W.
and L. M. Dill. 1996. Possible indirect interactions between
transient and resident killer whales: implications for the
evolution of foraging specializations in the genus Orcinus.
Oecologia 89: 125-132.
Bigg,, M. A.
1969a. The harbour seal in British Columbia. Fisheries
Research Board of Canada Bulletin 172, Ottawa.
Bigg, M. A.
1969b. Clines in the pupping season of the harbour seal,
Phoca vitulina. Journal of Fisheries Resaerch Board of
Canada 26: 449-455.
Bigg, M. A.
1981. Harbour seals Phoca vitulina Linnaeus, 1758,
Phoca largha Pallas, 1811. Pp. 1-27 in Handbook of Marine
Mammals. Vol. 2 Seals, (ed. S. H. Ridgway and R. J. Harrison).
Academic Press.
Frost, K. J.,
L. F. Lowry, E. H. Sinclair, J. V. Hoef and D. C. McAllister.
1994. Impacts on distribution, abundance, and productivity of
harbour seals. Pp. 97-118 in Marine Mammals and the Exxon
Valdez (ed. T. R. Louglin). Academic Press, San Diego, 395 pp.
Olesiuk, P.F.
1993. Annual prey consumption by harbour seals (Phoca
vitulina) in the Strait of Georgia, British Columbia.
Fisheries Bulletin 91: 491-515.
Olesiuk, P. 1999. An assessment of the status
of harbour seals (Phoca vitulina) in British Columbia.
Canadian Stock Assessment Secretariat Research Document -
1999/33.
Available at
http://www.dfo-mpo.gc.ca/csas/Csas/publications/ResDocs-DocRech/1999/1999_033_e.htm
Olesiuk, P. F.,
M. A. Bigg and G. M. Ellis. 1990. An assessment of the feeding
habits of harbour seals (Phoca vitulina) in the Strait
of Georgia, British Columbia, based on scat analysis. Canadian
Technical Report of Fisheries and Aquatic Sciences 1730,
Nanaimo, BC.
Riedmann, M.
1990. The pinnipeds: seals, sea Lions, and walruses.
Univeristy of California Press, Berkeley, USA.
Ross, P., R. De
Swart, R. Addison, H. Van Loveren, J. Vos and A. Osterhaus.
1996. Contaminant-induced immunotoxicity in harbour seals:
wildlife at risk? Toxicology 112: 158-169.
Ross, P. S., R.
L. De Swart, H. Van Loveren, A. D. M. Osterhaus and J. G. Vos.
1997. The immunotoxicity of environmental contaminants to
marine wildlife: a review. Annual Review of Fish Diseases
6:151-165.
Sullivan, R. M.
1982. Agonistic behavior and dominance relationships in the
harbor seal Phoca vitulina. Journal of Mammalogy 63:
554-569.
Swart, R. L.
de, P. S. Ross, J. G. Vos and A. D. Osterhaus. 1996. Impaired
immunity in harbour seals (Phoca vitulina) exposed to
bioaccumulated environmental contaminants: review of long-term
feeding study. Environmental Health Perspectives 104.
Watts, P. 1993.
Possible lunar infleunece on hauling-out behavior by the
Pacific harbour seal (Phoca vitulina richardsi). Marine
Mammal Science 9:68-76.
Yurk, H and A.
W. Trites. 2000. Experimental attempts to reduce predation by
harbor seals of out-migrating salmonids. Transactions of the
American Fisheries Society 129:1360-1366.
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