Using Weight of Evidence Characterization and Modeling to Investigate the Cause of the Changes in Pacific Herring (Clupea pallasi) Population Dynamics in Puget Sound and at Cherry Point, Washington

The decline of the Cherry Point Pacific herring stock (CPPHS) has been a puzzle of the upper Puget Sound. In this study, age‐structured population modeling was used to examine the timing and scale of the dynamics of Pacific herring stocks throughout the Puget Sound region. The intrinsic rate of increase and equilibrium age structure was calculated and forecast models were created for the stocks found in Puget Sound. We demonstrate that the causative agent for the decline of the CPPHS and the collapse in age structure existed in the 1974–1975 timeframe. Similarly, Puget Sound Pacific herring stocks at Squaxin Pass, Discovery Bay, and Port Gamble also demonstrate a collapse in age structure as seen in CPPHS during that same period. The data from the mid 1980s to 2006 demonstrate that all stocks share the collapse in age structure. The conclusion is that the causative agent had to affect these stocks at a scale corresponding to the Puget Sound; the effect has lasted over a 30‐year period, and was essentially simultaneous across the region. Forecast modeling of the stocks indicates that given current conditions none of the stocks are likely to show an increase in population size. The results were used to examine the likelihood that the Eastern Pacific decadal oscillation, disease, and persistent organic pollutants are singularly or in concert the causative agents. A research program is suggested to determine the cause(s).