Research Document - 2000/081

Estimating pup production and population size of the northwest Atlantic harp seal (Phoca groenlandica)

By B.P. Healey and G.B. Stenson

Abstract

Pup production and population size of Northwest Atlantic harp seals (Phoca groenlandica) for the period 1960 to 2000 were estimated using independent survey estimates of pup production, annual estimates of pregnancy rates, and age-structured removals. Removals included reported catch, estimated by-catch and assumed levels of seals killed but not landed (struck and lost). These data were fit to a three-parameter age structured population model that allows for differing assumptions of pup mortality. The two parameters estimated in the model are the pup selection parameter (s) and unaccounted mortality (m). The impact of assuming that the mortality of young seals (age class 0) was greater than that of seals one year of age and older (1+) was illustrated by using a fixed parameter (g) ) as the ratio of age class 0 mortality to that of older seals. Replacement yields were estimated using differing assumptions of the age structure of the harvest. The uncertainty associated with the estimates was determined by randomly re-sampling from within the sampling error of the pup production estimates.

Assuming that the unreported mortality of age class 0 seals is 3 times that of 1+ animals, the total population was estimated to be approximately 5.2 million, with a 95% confidence interval of 4.0 to 6.4 million seals in 2000. Assuming different g -values changes the estimates slightly, but differences were minimal. The population was estimated to have increased from less than 2 million in the early 1970s until 1996; since then the population has been relatively stable. Using the current age structure of the removals (~70% young of the year), the 2000 replacement harvest was estimated to be approximately 533,000, with 95% confidence interval (C.I.) 373,000, to 693,000. Assuming that the levels of by-catch and the Greenland harvest remain at their 1999 levels, and accounting for struck and lost, the corresponding replacement level of seals that can be landed in southern Canada at the proportion of pups observed in 1999 (90%) is 257,000; (95% C.I. 102,000, to 342,000). This level would be reduced slightly if the proportion of young in the harvest decreases.

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