Drivers of population dynamics and juvenile mortality in northwest Atlantic harp seals

Human-induced threats to terrestrial and marine wildlife are on the rise, and while some species may face a single major threat that is easily identifiable, others face multiple concurrent threats. Harp seals, an abundant pinniped in the north Atlantic that was historically depleted by human harvest, are one such species: while commercial and subsistence harvests remain a significant source of mortality, in recent decades their environment has undergone significant changes which could also impact population dynamics. Inferring the relative importance of various threats as drivers of population dynamics can be challenging, particularly for marine species where monitoring abundance is difficult: the use of integrated population models (IPM), which leverage multiple data sources to parameterize process-based models of population dynamics, provide one solution to this challenge. We developed a hierarchical Bayesian IPM with which to explore the shifting roles of anthropogenic and environmental factors in driving population trends. We used a competing hazards formulation for survival, enabling the partitioning of mortality into multiple discreet causes (hazards), and allowing us to assess how these different hazards varied over 7 decades (1952 – 2019). We fitted the model to available data on pup production, fecundity, age structure, human removals and environmental conditions, and used the fitted model to produce annual estimates of pup production and total abundance. We conducted a Bayesian life stage simulation analysis (LSA) to compare the relative contributions of various hazards to variation in population growth. We found that harvests of young of the year and adults were the primary contributors to variation in population trends from 1951-1982; however, after 1983 the relative importance of harvest mortality decreased while the impacts of natural mortality increased, especially for young of year (YOY), and since 2000 the impacts of YOY mortality from ice cover anomalies have become one of the strongest drivers of trends. Based on current climate models, which project warmer water and decreasing ice cover, we expect continued high levels of YOY mortality from environmental factors such as deteriorating ice conditions. These climate-related hazards are likely to become the dominant drivers of population dynamics in coming decades, which will in turn affect sustainable harvest levels for both Canada and Greenland. Our model will provide a useful tool for exploring future scenarios of climate impacts and management strategies.