Research Document - 2011/011

Estimating abundance of Northwest Atlantic harp seals, examining the impact of density dependence

By M.O. Hammill and G.B. Stenson

Abstract

A population model was used to examine changes in the size of the Northwest Atlantic harp seal population between 1952 and 2010. The model incorporated information on reproductive rates, reported removals, estimates of non-reported removals and losses through bycatch in other fisheries to determine the population trajectory. The model was fit to eleven estimates of pup production beginning in 1952, including a revised estimate of 2008 pup production of 1.63 million animals. The unusually high 2008 estimate of pup production is due to high reproductive rates for the same year. This is in contrast to a general trend towards a decline in reproductive rates as pup production has increased suggesting that the dynamics of this population are being mediated by density-dependent changes. Under the assumption that the population is continuing to grow exponentially, the total population in 2008 was estimated to be 8.0 million (95% CI =6.8-9.3 million) animals, increasing to 9.1 million (95% CI=7.5 to 10.7 million) animals in 2010. Under the assumption that density-dependent population growth is occurring and the population is nearing carrying capacity (K=12 million), the population in 2008 was 8.1 million (95% CI=7.3-8.9 million animals) increasing to 8.6 million (95% CI=7.8 to 9.4 million) animals in 2010. Under both model formulations a harvest of 420,000 animals over the next three years would continue to respect the management plan to maintain an 80% probability that the population would remain above the precautionary reference level (N₇₀).

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