Evaluating Responses of Nearshore Fish to Removal of the Elwha River Dams

Removal of two dams on the Elwha River began in late 2011 and will restore sediment processes in the near coastal environment adjacent to the river's mouth. Since 2005, we have been collecting data on intertidal/sub-tidal fish communities near the mouth of the River where we expect sediment changes to occur. We have also sampled in reference areas. Samples were collected by beach seining in the spring and summer. Our primary objective has been to determine if attributes of the nearshore fish community (notably species assemblage structure and size distribution) changed in response to sediment restoration. Potential shifts in fish assemblage structure and size distribution of ecologically important species such as forage fish and juvenile salmon are of particular interest because sediment changes will likely be significant in these intertidal and sub-tidal habitats. Trends in species richness and abundance were consistent prior to and following dam removal (2012 is thus far the only year where we have post dam removal information) with reference areas generally possessing more species and a greater overall abundance of fish than treatment areas. Forage fish were the numerically dominate species group in all areas. Using multivariate analysis, we found considerable overlap in fish community composition between years but there was some separation in fish assemblage structure between the different areas prior to dam removal. Regional differences were primarily a result of several forage fish species (notably Pacific sandlance, and surf smelt) and juvenile salmonid species (notably chum salmon). There were also seasonal differences in all regions with salmonids and forage fish the dominate fish in the spring and flatfish, sculpins, perch, and greenlings the primary species occurring in summer. Inclusion of post-dam removal data from 2012 did not significantly change these observed patterns. We plan to continue monitoring in the future. However, our ability to detect responses of fish communities to sediment changes will ultimately depend on both biotic factors (such as species and life stages being considered) and abiotic factors, such as when sediment reaches the coastal environment; the quantity, composition and distribution of the material that reaches the Salish Sea; and how long it takes material to distribute from the river’s mouth.