Effects of Seawalls and Piers on Fish Assemblages and Juvenile Salmon Feeding Behavior

Abstract

Shoreline modifications, such as seawall armoring and piers, are ubiquitous along developed waterfronts worldwide, and recent research suggests that their ecological effects are primarily negative. We utilized snorkel surveys to quantify the effects of seawalls and piers on fish in nearshore habitats of an urbanized estuary in Puget Sound, Washington. We observed 17 species of fish and 4 species of crab during April–August 2012 at sites modified by seawalls and piers and at reference beach sites with minimal anthropogenic structures. Species assemblages at modified sites were significantly different from those at reference beaches. At modified sites, fish distribution and assemblage structure varied with proximity to the shade cast by piers; overall fish abundances were reduced under piers, and the greatest abundances were observed at high tides in areas directly adjacent to piers. Juvenile Pacific salmon Oncorhynchus spp. were the dominant fish species, and piers reduced their presence and feeding, indicating that areas under piers provide less-valuable habitat to salmon species. Piers may interrupt movements of juvenile salmon when they use shallow waters along shorelines to migrate from freshwater to marine habitats, as juvenile salmon tend to avoid shade under piers, especially at high tides. Our results show that shoreline modifications can alter species assemblage structure, thus potentially creating novel combinations and abundances of species, and can reduce habitat function for species that utilize these and similar habitats elsewhere.

Received December 27, 2013; accepted March 27, 2014

ACKNOWLEDGMENTS

Mike Caputo, Beth Sosik, P. Frank Stevick, and Claire Levy of the University of Washington's Wetland Ecosystem Team performed data collection and assistance in the field. Funding for this research was provided by the Seattle Department of Transportation. S.H.M. was also supported by the National Science Foundation's Graduate Research Fellowship Program. Charles Simenstad and three anonymous reviewers provided helpful critiques on previous versions of this paper.