Evaluating Causes of Decline of Pacific Herring

Pacific herring (Clupea pallasii) are key prey for threatened salmonid species in Puget Sound. Puget Sound herring spawning stock biomass has been declining locally over the past 40 years, including the San Juan Island (SJI) spawning populations. At present, there is little agreement on the primary cause of declines or the best management actions for recovery. We convened an expert elicitation workshop to evaluate key threats to San Juan Island (SJI) herring. One such identified threat included the loss of spawning habitat, and specifically eelgrass, as Pacific herring rely on submerged vegetation, including eelgrass, in shallow subtidal habitats for spawning and egg incubation. The expert workshop also identified the lack of regular herring spawn surveys in the San Juan Islands as a barrier to monitoring their abundance and recovery in the area. Following on the expert workshop, we used a unique 25+ year fine-scale historical dataset to quantify changes in the abundance and distribution of eelgrass and other herring spawning substrates in the San Juan Islands, as an extension of previous analyses focused on other Salish Sea herring spawning sites. Last, in follow-up to another potential threat to herring recovery identified in the workshop, increased predation, we compiled all known and available datasets describing the diets of herring predators in the Salish Sea, and developed Bayesian generalized additive models to estimate trends in the contribution of herring to the diets of various predators, and multiple potential predictors of those trends, including oceanic conditions, herring spawner abundance, and the abundance of predators. The original scope and workplan of this project was dramatically revised in the middle of the project period. Specifically, field work to estimate egg loss rates in the San Juan Islands, based on the hypothesis that declines in San Juan herring were associated with egg mortality rates, potentially owing to predation by birds, was not completed and instead the modeling project was added. The field work was attempted but unsuccessful owing to exceedingly low herring spawn and personnel challenges associated with monitoring for spawn. Transitioning to a modeling exercise resulted in a minor cost shift, as we engaged a subcontractor to lead the modeling. In addition, the shift away from a field project resulted in reduced salary expenses, especially for the PI, and a balance remaining at the end of the project. Last, these workplan shifts, the Covid-19 pandemic, and back-to-back family tragedies resulted in the need for multiple project extensions.

The University of Washington used this grant to evaluate the causes of the decline of Pacific herring, a key food for salmon in Puget Sound.