Salmonid distribution and abundance in the context of Elwha River dam removals

Removal of two dams on the Elwha River, Washington from 2011 to 2014 has begun to 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. Juvenile salmon (Chinook, coho, pink and chum salmon) migrate through this region, which also supports ecologically important forage fish and endemic benthic fauna. Beach seine samples collected annually from April through September from over 20 sites span pre-removal, high impact (during dam removal), and post-removal years. Annual catches included 23,093 to 92,677 individuals from 45-55 species. Trends in species richness and abundance were variable over this time period. Forage fish dominated our catches followed by salmonids. In this analysis we explored patterns of juvenile salmonid abundance in relation to dam removal, environmental variables, site characteristics (including community composition), and year/season using a Bayesian hierarchical modeling framework and multivariate analyses. Chinook salmon show a tenfold variation in abundance over the years examined. Catches of Chinook and coho salmon were dominated by locally released hatchery fish with high variability between sampling sites. Throughout this region we have seen a marked decrease in coho salmon catches in recent years. Understanding what biotic and abiotic factors contribute most to this variability in salmon abundance and distribution may help tailor future dam removal processes or reframe management decisions.