Trends and status of harbor seals in Washington State, USA (1977–2023)

Abstract

Harbor seal (Phoca vitulina) numbers in the United States rebounded after the implementation of conservation measures associated with the Marine Mammal Protection Act of 1972 (MMPA). Stock estimates are a critical tool for assessing species status and management options under the MMPA and for modeling efforts designed to evaluate the role of pinnipeds in marine ecosystems. We conducted aerial surveys of harbor seal haul‐out sites between 1977 and 2023 to estimate population size and trends in four Washington State, USA, stocks: (1) Hood Canal, (2) Southern Puget Sound, (3) Washington Northern Inland Waters, and (4) the Washington portion of the Oregon‐Washington Coastal Stock. We fit a Bayesian state‐space model to estimate population trends for all stocks, as well as the Washington Coast region, and present carrying capacities and maximum net productivity levels for all stocks. The most recent population estimates for the three inland water stocks are within their optimal sustainable population range, with some evidence that the Southern Puget Sound Stock is continuing to grow.