Research Document - 1999/107

Recent Trends in the Population of the Northwest Atlantic Harp Seals, Phoca groenlandica.

By G.B. Stenson, B. Healey, P.A. Shelton and B. Sjare

Abstract

Trends in the abundance of Northwest Atlantic harp seals (Phoca groenlandica) for the period 1960 to 1998 were estimated using survey estimates of pup production, and annual estimates of pregnancy rates and age structure of the catch. These data were fit to a two parameter age structured population model under two different assumptions of pup mortality. In one model formulation pup mortality was assumed to be equal to that of seals one year of age and older (1+) while in the second pup mortality rate was assumed to be 3 times the 1+ mortality rate. The uncertainty associated with the population trajectory from the first formulation was estimated by randomly resampling from within the sampling error of the available pup production estimates. Replacement and sustainable yields were estimated using both model formulations under differing assumptions of the age structure of the harvest. The impact of assuming different levels of unreported mortality on population trends was also examined.

Based upon the first model formulation, the total population was estimated to be approximately 5.5 million (95% C. I. 4.3 - 6.7) in 1998 while assuming higher pup mortality results in a slightly lower estimate of 5.3 million. Under both models, the population was estimated to have increased from the early 1970s until 1996. Since then the population has been relatively stable, growing at less than 0.5% per year. Using the current age structure of the harvest, the estimated 1999 replacement harvest was estimated to be approximately 407,000 animals under the assumption of constant mortality and 402,000 under the assumption of higher pup mortality. The estimates of population size and instantaneous mortality are reduced slightly if unreported mortality is added. Assuming low or moderate levels of additional unreported mortality result in slightly declining (0.2 - 0.8%) populations. Assuming a high level of unreported mortality resulted in an estimated 2% decline in the population in 1998, but the level of loss for young seals in Atlantic Canada used for this run is greater than the current data supports. Given the uncertainty associated with the model estimates the differences among the various model runs rates are not significant. here (justified-left)

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