Biology of the harbor seal, Phoca vitulina, in eastern Canada

BOULVA, J., AND I. A. MCLAREN. 1979. Biology of the harbor seal, Phoca vit/ma, in eastern Canada. Bull. Fish. Res. Board Can. 200: 24 p. Measurements, samples, and observations of seals (1968-73) are augmented by age determination of bounty kills (1962-71). An estimated 12 700 individuals, in semi-isolated groups, occurred in eastern Canada south of Labrador in 1973. Local abundance in Nova Scotia is related to availability of small islets. Seals move offshore in winter and haul out on Sable Island only when windchill-corrected temperatures exceed I5°C. On Sable Island (1971-73) there were small annual variations in birth size and birth date (mean May 21-27) and larger variations in sex ratio and percent with lanugo. Mothers began to wean at about 20 d and left their pups at about 30 d, after a (probably natural) loss of about 12% of pups. Mating activity, probably polygynous, was seen from early April through July, when molting occurred. Seals on Sable Island haul out mostly near dawn to spend much time on land, but elsewhere tides are important. Embryonic, pre-weaning, and post-weaning sizes (asymptotic weight 90 and 70 kg for males and females) are similar to those of other harbor seals, but smaller than those of seals of the northern and eastern North Pacific. Maturity of males and females is at about 6 and 3-4 yr, respectively, but only half of the females aged 4 and 95% of those 7+ yr were fertile. Adult males were potent between April and July, but most females ovulated around mid-June and embryos were implanted around mid-September. Earlier and recent population estimates, rates of decrease of bounty kill, and age structure of heavily vs. lightly exploited populations all suggest an overall decline of about 4% yr I between 1950 and 1973 in the Maritimes. Best estimates of pre-weaning and post-weaning mortality rates, sex ratios (increasing with age), and fertility rates from generally lightly hunted populations produce near equilibrium in population simulation. If females were fertile 1 yr younger this would only allow a very small sustainable yield. The very high natural mortality rates (est. 17.5%. yr post-weaning), possibly a result of shark predation and overexposed breeding sites in the reduced range, may make eastern Canadian harbor seals particularly vulnerable to hunting. The seals feed mostly on fish, have no apparent fasting season, and evidently eat one meal daily. Small ingestion rates (about 5% body wt d ') mean that the seals have negligible impact on fish stocks. The annual production/biomass ratio of the population is about 0.18, and ecological efficiency (production/consumption in energy equivalents) at almost 6% does not indicate a metabolically demanding life style and environment.