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Age determination, reproduction, growth and population analysis of the harbor seal, Phoca vitulina richardi gray Bigg, Michael Andrew

Abstract

Studies have been made on age determination, reproduction, growth and population structure in the harbour seal (Phoca vitulina richardi). They are based upon gross and histological examinations of reproductive tracts, growth measurements, and associated canine teeth of 42 prenatal and 319 postnatal specimens collected between 1964 and 1965. In postnatal seals cementum annulations were used as the criterion of age. Two annulations are deposited annually, a translucent "dense, inorganic" band during the winter and spring and an opaque "less dense, more organic" band during the summer and fall. Their formation may result from an internal rhythm. In southeastern Vancouver Island (southern British Columbia) parturition occurs primarily during July, lactation then lasts 6 weeks with ovulation shortly thereafter in September. Implantation is delayed for 2 to 2½ months and birth is about 230 days later. The male's main breeding season begins at least 4 months in advance of the female's but ends shortly after. Extensive clines in the timing of the reproductive cycle of both sexes are found in the Pacific and Atlantic Oceans. First ovulations occur at 2 to 5 years of age with peak periods at 2 and 3 years. About 88.9% of all adult females produce young each year. Males mature at 5 and 6 years of age. Prenatal males and females grow at similar rates. Postnatal males, however, grow slightly faster and larger being about 10% longer and 36% heavier at asymptotic sizes. Sex ratios are about 1 : 1 from implantation until males reach sexual maturity at 5 years, their mortality rate then increases resulting in a gradually increasing percentage of females in older age groups. Males generally do not live longer than 20 years and females seldom more than 30 years. In a random kill sample the average annual sex ratio is 46.9% males, and 49.6% of all females and 33.7% of males are mature. From bounty return records and reproductive and population data a minimum population estimate of 75,000 seals is made for British Columbia. This is a density of about 4.4 seals per mile of coastline.

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