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.