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Canadian Science Advisory Secretariat
Research Document - 2005/067
Estimation of grey seal population size and trends at Sable Island
By Trzcinski, M.K., R. Mohn, and W.D. Bowen
Abstract
An age-structured population model with density-dependence parameterized using a theta-logistic function was used to estimate population size of grey seals at Sable Island through 2004 based on estimates of pup production, juvenile and adult survival and age-specific fecundity. The model incorporates parameter uncertainty in two ways. Estimates of pup mortality and population size at Sable Island were obtained by minimizing an objective function that is the sum of the negative log likelihoods for the pup count data assuming lognormal error structures which was carried forward to our final estimates. Birth rates and adult mortality rates were taken from other studies and uncertainty in these parameters was incorporated by adding the negative log of the probability density function to the objective function. In 2004, population size ranged between 208,720 (SE = 29,730) and 223,220 (17,376) depending on assumptions about carrying capacity and the strength of density dependence.
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(14 pages; 154K)
