Research Document - 2011/099

Northwest Atlantic Harp Seals Population Trends, 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. Reproduction rates have continued to decline. Samples collected up to 2011, indicate that adult reproductive rates have declined to as low as 0.22, which is much lower than the estimate of 0.74 observed for 2008, the last year data were available for the 2010 assessment. The model was fit to eleven estimates of pup production from 1952 to 2008, using two different methods of smoothing the reproductive data and assuming carrying capacity can be either 10.8 million or 12 million seals. Estimated pup production in 1952 was 500,000 (95% CI=500,000-600,000) animals. Pup production declined throughout the 1960s reaching a minimum 1971, and then increased to a maximum of 1,600,000 (95% CI=1,400,000-1,800,000) in 2008. Estimated pup production declined to 600,000 (95% CI=500,000-700,000) in 2011 due to the low pregnancy rates observed.  The total population size in 1952 was 2,300,000 (95% CI=2,200,000 -2,400,000) declining to a minimum in 1971 and then increasing to 7.9 to 8.3 million (95% CI=7,300,000-9,000,000) in 2008, depending upon the assumptions. The 2008 estimate is also NMax. The 2012 population is estimated to be 7.3 to 7.7 million. Although the previous assessment indicated that a harvest of 400,000 could be sustained for the remainder of the management period, the maximum harvest that would respect the management plan under this assessment is 300,000 animals, assuming that beaters comprise 97% of the harvest. The difference is due to the significant decline in reproductive rates observed in samples collected since 2008. Increasing catches on one component of the population through a transfer of quota will adversely impact that component unless it is offset by an equal reduction in subsequent years.

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