EPIBENTHIC INVERTEBRATES AND JUVENILE CHUM SALMON (ONCORHYNCHUS KETA) PREY AVAILABILITY ALONG AN ECO-ENGINEERED SHORELINE: A CASE STUDY AT THE SEAWALL IN SEATTLE, WASHINGTON

Abstract

Shoreline development can alter coastal processes resulting in habitat loss and species assemblage shifts, but eco-engineered enhancements, or modifications to hard structures that mimic or support ecological processes, may improve ecosystem functioning and benefit shoreline dependent species. Seawalls and piers can reduce the quality and quantity of shallow water habitat available to out-migrating juvenile salmon (Oncorhynchus spp.) by deepening nearshore water and shading the shoreline environment. Previous studies along Seattle’s Elliott Bay waterfront have shown that common prey of migrating juvenile salmon, small epibenthic crustaceans such as amphipods and harpacticoid copepods, are negatively impacted by pier shading, presumably due to the reduced light intensity necessary to support primary production, and that juvenile salmon occurring there feed less on these types of prey. To reduce the impacts on out-migrating salmon and increase epibenthic prey availability a section of the Seattle seawall was replaced with an eco-engineered design including a raised marine bench to reduce shoreline water depths, a textured seawall and ledge to increase habitat diversity, and a cantilevered, glass-paneled walkway to allow light penetration to the water below. In 2018, we sampled epibenthic invertebrates along the raised bench and seawall-ledge, covered (under) and not covered (outside) by piers to compare the availability of epibenthic invertebrate prey among the eco-engineered habitats. The following year, juvenile chum salmon were collected for diet analysis from outside piers along the eco-engineered seawall, the adjacent original seawall with no eco-engineering, a nearby restored pocket beach, and two other riprap armored locations along the Seattle waterfront to compare prey selection among modified shorelines with and without the eco-engineered enhancements.Epibenthic invertebrate assemblages varied by exposure (under-pier and outside-pier) and surface type (seawall shelf and bench) with higher relative densities and taxa richness outside piers and on the bench. Total epibenthic invertebrate densities and densities of juvenile chum salmon prey were higher on the bench positioned outside piers than on all other eco-engineered strata (the marine bench under piers, the seawall shelf under piers, and the seawall shelf outside piers) combined. Taxa richness and total densities along the eco-engineered seawall increased after enhancement and were more like values at a nearby un-armored and enhanced pocket beach site suggesting a positive effect of the eco-engineering. However, juvenile chum salmon consumed mostly planktonic prey and we did not observe a significant difference in the number or biomass of epibenthic or terrestrial prey consumed along the eco-engineered seawall relative to the un-enhanced seawall. Prolonged daytime shading from the cantilevered sidewalk may have influenced juvenile chum salmon to move away from the eco-engineering preventing access to epibenthic prey. However, previously observed changes in behavior, shoreline distribution, and under-pier use of the eco-engineering by juvenile salmon suggest it has enhanced juvenile salmon habitat.