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Research Document 2020/057

Updated Estimates of Reproductive Rates in Northwest Atlantic Harp Seals and the Influence of Body Condition

By Stenson, G.B., Buren, A.D., and Sheppard, G.L.

Abstract

Obtaining accurate estimates of reproductive rates is critical for describing the population dynamics of a species. Since the 1950s, fecundity rates of Northwest Atlantic harp seals have declined while inter-annual variability has increased. These highly variable reproductive rates have continued although the very low fecundity rates observed in 2010 and 2011 have not occurred again. High fecundity rates were observed in 2014 and 2015 associated with a period of extensive ice and relatively high capelin abundance. A previous study has shown that while the general decline in fecundity is a reflection of density-dependent processes associated with increased population size, the large inter-annual variability is due to varying rates of late-term abortions which are related to changes in capelin abundance (focal forage species of the system), and mid-winter ice coverage (which reflects environmental conditions that influence a variety of species). We hypothesize that the impact of changing prey availability influences reproductive rates through changes in body condition. We found that the average relative condition of pregnant females was high throughout four decades, whereas that of non-pregnant and immature females varied considerably and showed a general decline since 2000. While condition did not appear to influence fecundity rates directly, it affected the rate of late-term abortions. Relatively small reductions in average condition resulted in much higher abortion rates. This suggests that as overall condition in the population declines, females that are able to attain sufficient energy maintain their pregnancy while those that cannot terminate it prematurely. The relative influences of mid-winter ice coverage, capelin abundance, and body condition could not be resolved as these are different metrics of the amount of energy available for pupping. Capelin biomass is influenced by the timing of the ice retreat from the Newfoundland Shelf. In the short term (i.e., the next two years), capelin biomass is predicted to decline, and this will likely affect reproductive rates. In the long term, there is great uncertainty as to how the predicted changes in timing of ice formation and retreat will impact capelin, and hence reproductive rates of harp seals.

Key words: harp seal, Pagophilus groenlandicus, reproductive rates, fecundity, abortions

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