Research Document - 2014/058

Density-Dependent and Density-Independent Factors Influencing Reproductive Rates in Northwest Atlantic Harp Seals, Pagophilus groenlandicus

By G.B. Stenson, D. Wakeham, A. Buren and M. Koen-Alonso

Abstract

Obtaining accurate estimates of reproductive rates are critical for estimating the population dynamics of a species. Recent estimates of late term pregnancy rates, fecundity and abortion rates of Northwest Atlantic harp seals were obtained from samples collected off the coast of Newfoundland and Labrador. The declining, but highly variable, reproductive rates reported previously have continued with the pregnancy rate of mature females (2010 and 2011; <0.3) falling to the lowest level since data was first collected in the 1950s. Using a fixed-dispersion, beta regression model to explore the importance of biological and environmental conditions, we found that reproductive rates were influenced by both density dependent and independent factors. While the general decline in fecundity is a reflection of density-dependent processes associated with increased population size, including the late-term abortion rates captured much of the large inter-annual variability. Changes in the annual abortion rate could be described either by a model that incorporated ice cover in late January or a model that incorporated ice cover and capelin biomass obtained from the previous fall as a proxy for prey availability. Using these models, we predicted the 2012 fecundity rate to be 0.44 or 0.54, depending upon the model used which is lower than the estimate (0.643) obtained from the small sample of reproductive tracts.

Key words: harp seal, Pagophilus groenlandicus, reproductive rates, fecundity, abortions, density dependent, density independent, beta model.

Density-Dependent and Density-Independent Factors Influencing Reproductive Rates in Northwest Atlantic Harp Seals, Pagophilus groenlandicus (PDF, 685 KB)

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