Research Document - 2012/148

Estimating carrying capacity and population trends of Northwest Atlantic harp seals, 1952-2012

By M.O. Hammill, G.B. Stenson, T. Doniol-Valcroze, and A. Mosnier

Abstract

A population model was used to examine changes in the size of the Northwest Atlantic harp seal population between 1952 and 2012. 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 periodic estimates of pup production from 1952 to 2008, and to annual pregnancy rate data collected between 1954 and 2012. Pup production declined throughout the 1960s reaching a minimum in 1971, and then increased to a maximum in 2008. Estimated pup production in 2012 was 1.5 million animals (95% CI=1.0-2.1million); the total estimated population size in 2012 was 7.1 million (95% CI=5.9 to 8.3 million). Fitting the model to both the aerial survey data and the reproductive rate data (age classes 8+ only), resulted in estimates of K=10.0 million. Different formulations were used to describe future population trends. If the harvest rates in Greenland and future reproductive rates are fixed, then an annual harvest of 300,000 animals would respect the management objective. If future catches in Greenland, future reproductive rates and juvenile survival are linked to changes in population size then annual harvests of up to 400,000 animals would respect the management objectives. The effect of variable harvest levels was also examined. Ice conditions, reproductive rates and removals from the Greenland harvest continue to be important factors affecting the dynamics of this population. Modifications to the assessment model have provided a means of estimating environmental carrying capacity assuming a certain functional relationship between total population size and juvenile survival, and between population size and reproductive rates.

Complete document (PDF, 1109 KB)

Accessibility Notice

This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.