On the edge: assessing fish habitat use across the boundary between Pacific oyster aquaculture and eelgrass in Willapa Bay, WA

Estuaries serve as ecological links between terrestrial, freshwater, and marine systems, but are subject to diverse anthropogenic stressors. Along the US West Coast, shellfish aquaculture is one potential stressor as it involves extensive use of estuarine tidelands. Pacific oyster (Crassostrea gigas) aquaculture is a century-old practice that significantly contributes to the regional culture and economy. Native eelgrass (Zostera marina) also commonly occurs in intertidal areas where oyster aquaculture is practiced. Seagrasses provide nursery habitat for many commercially harvested species and have recently garnered more conservation interest because of widespread decline. Eelgrass is federally protected as "essential fish habitat", restricting aquaculture activities within or near eelgrass. To inform associated management decisions, we sought to characterize habitat use of aquaculture and eelgrass by fish. Two perspectives shaped the investigation: 1) seascape ecology, highlighting the role of edge effects, and 2) aquaculture method where. Aa recent shift towards off-bottom culture methods, in part to protect seagrasses, precipitated a comparison between long-line and on-bottom aquaculture. Direct (underwater video) and indirect (e.g. predation tethering units) measures of fish distribution and behavior were used to quantify differences along a transect between aquaculture and eelgrass in each type of aquaculture. Results suggest that most fish species use long-line oyster aquaculture and eelgrass habitats similarly with minimal edge effects, but on-bottom aquaculture was used less than the other two habitat types. These results are consistent with previously observed positive relationships between fish abundance and vertical habitat structure but appear to reflect individual species behavior and use of these intertidal habitats.