Nearshore Forage Fish Populations in the Context of Elwha River Dam Removals

Abstract

Removal of two dams on the Elwha River, Washington, is expected to help restore natural sediment processes to the coastal environment near the river mouth. Since 2006 we have been collecting data on shallow subtidal (nearshore) fish communities near the Elwha River and at reference sites in the Strait of Juan de Fuca to assess fish response to sediment changes resulting from dam removal. Ecologically important forage fish spawn and rear, and juvenile salmon migrate through these areas. Beach seine samples to date include 5 years pre-removal, 2 high impact years, and one year post-removal. Trends in species richness and abundance were consistent throughout, with reference areas possessing more species and overall abundance of fish than sites near the river mouth. Given the important role of forage fish in the ecosystem, both as plankton predators and as prey for marine fish including listed salmonids, we focused this analysis on population patterns within the forage fish community. Forage fish dominate our samples across years, but the influence of individual species varies, and can drive fish assemblage structure between sampling regions. We explored patterns of abundance in relation to dam removal, environmental variables, site characteristics, and season using a Bayesian hierarchical modeling framework and multivariate analyses. Forage fish abundance has increased at sampling sites since dam removal; Pacific Herring (Clupea pallasii), Night Smelt (Spirinchus starksi), and Pacific Sand Lance (Ammodytes hexapterus) have shown the greatest increases over all areas. Though variable, overall abundance of forage fish increased more following dam removal within the impacted region than within the reference regions. We will continue to monitor nearshore fish populations in this region, including use of genetics and stable isotopes to explore forage fish population structure, as the system evolves towards a more natural sediment regime and material distributes from the Elwha River watershed