The impact of changing climate and abundance on reproduction in an ice-dependent species, the Northwest Atlantic harp seal, Pagophilus groenlandicus

As the northern hemisphere continues to warm, declines in sea ice seriously impact species that rely on ice for reproduction and/or feeding. One such species, the Northwest Atlantic harp seal, gives birth on ice and feeds along the southern edge of the seasonal pack ice. Unfortunately, little is known about the impact of climate change on ice-dependent species in sub-Arctic areas, although they are areas of rapid ecosystem change. While climate change has been shown to affect harp seals directly through increased mortality of young, it may also impact indirectly through changes in prey and subsequent reproductive rates. Over the past four decades, harp seals have also undergone a large change in abundance, increasing from <1.5 million seals in the early 1970s to similar to 7.4 million seals today and, since 1987, late-term abortions have been observed. To determine the importance of biological and environmental factors influencing reproduction, pregnancy and abortion rates of harp seals were estimated from samples collected between 1954 and 2014 off Newfoundland, Canada. Since the early 1980s, late-term pregnancy rates among mature females have declined while interannual variability increased, ranging from 0.2 to 0.86. Using a beta-regression model, we found that while the general decline in pregnancy is associated with increased population size, including the rate of late-term abortions captured much of the interannual variability. Changes in abortion rates were described by a model that incorporated capelin biomass and mid-winter ice cover (likely a proxy for ecosystem changes in overall prey abundance). Harp seals appear to respond to relatively small variations in environmental conditions when they are at high population levels. If the observed changes in climate continue, negative impacts on the Northwest Atlantic harp seal population will likely increase due to the predicted warming trend and associated reduction in ice cover.