Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 9024
Title: Contributions of adult mortality to declines of Puget Sound Pacific herring
Author: Margaret C. Siple, Andrew Olaf Shelton, T B Francis, Dayv Lowry, Adam Lindquist, T. E. Essington
Publication Year: 2018
Journal: ICES Journal of Marine Science
Keywords: forage fish, natural mortality, population dynamics, Pacific herring,
  1. Forage fish undergo dramatic changes in abundance through time. These fluctuations, which have historically been attributed to changes in recruitment, may also be due to changes in adult mortality. Pacific herring, a lightly exploited forage fish in Puget Sound, WA, have exhibited shifts in age structure and decreases in spawning biomass during the past 30 years. Here, we investigate changes in adult mortality as a potential explanation for these changes. Using a hierarchical, age-structured population model, we present evidence for increased adult natural mortality for Puget Sound Pacific herring since 1973. We find that natural mortality has increased for every age class of adult (age 3+), especially age 4 fish, whose mortality has doubled over the time series. We demonstrate that long-term shifts in mortality explain changes in age structure, and may explain biomass declines and failure to reach management thresholds for two spawning sites in Puget Sound (Cherry Point and Squaxin Pass). Temporal shifts in natural adult mortality could have negative implications for herring and herring predators. We demonstrate that adult mortality, not just recruitment variation, is an important driver for forage species, which face exceptionally high natural mortality compared to other fishes.

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Theme: Recovery and rebuilding of marine and coastal species
Foci: Characterize the population biology of species, and develop and improve methods for predicting the status of populations.