Possible Impacts of Ice Related Mortality on Trends in the Northwest Atlantic harp seals population

No abstracts are to be cited without prior reference to the author.

The harp seal is a medium sized highly migratory phocid distributed over continental shelf regions of the north Atlantic. The Northwest Atlantic population currently numbers around 5.5 million animals and is harvested commercially and for subsistence purposes. Harp seals use pack ice to haul out on, to give birth and nurse their young. After weaning the young of the year (YOY) remain with the ice, which they use as a resting platform. The harp seal population is assessed approximately every 4 years using a population model that relies upon independent estimates of pup production obtained from aerial surveys. Since the current harvest is focused on YOY animals, the impact of any unusual mortality will not be reflected in the assessment for at least two decades later. In the Gulf of St. Lawrence, poor ice conditions, which are thought to lead to increased mortality among young animals, have been observed in 6 of the last 10 years. A factor to account for increased mortality during poor ice years has been incorporated into the assessment model since 2004, but the impacts of this factor on model predictions has not been evaluated. Under scenarios of a constant harvest, an annual mortality of 30% or higher, due to ice, in a single year would result in significant changes in the population trajectory within a decade, but these changes would not be noticed as detectable changes in pup production for at least 20 years. Repeated ice-related mortality of 10% had limited impact unless it occurred in 6 or more winters within a decade. Changes in the population and pup production due to increased YOY mortality could not be detected until 15 or more years had passed even under high levels of mortality or variability among years, by which time significant changes in the population can occur. For management considerations, taking into account possible changes in natural mortality due to ice would not appear to be important in the short-term, but will have more important longer term implications.