Shellfish aquaculture farms as foraging habitat for nearshore fishes and crabs in Puget Sound

Abstract

Oyster reefs across North America have declined precipitously over the past 140 years. In Washington State, Olympia oyster Ostrea lurida reefs historically provided water filtration and nearshore structural habitat for fishes and invertebrates but are now functionally extinct across their historical range. In place of these naturally occurring reefs, shellfish farms consisting mainly of non-native Pacific oysters Magallana gigas now occupy patches of nearshore habitat across Washington. These farms modify intertidal substrate by adding structural habitat via suspended oyster grow bags, predator exclusion nets, loose oyster beds, and other shellfish grow-out gear. As interest and investment in shellfish aquaculture has expanded both locally and globally, so has interest in how these farms modify intertidal habitat, and whether the complex structure created by the shellfish and shellfish growing gear provide ecosystem services that are comparable to unfarmed areas such as mudflats and eelgrass meadows. In this study, we sought to quantify how shellfish farms are used as foraging habitat for several common nearshore species of fishes and crabs in Puget Sound, Washington. In the first part of this study, we used direct observations of species-specific behaviors from underwater video to model how habitat type affected observed foraging rates. We obtained a total of 393 crab observations, 431 demersal fish observations, and 1,856 pelagic fish observations across all seven farm sites. Several common species of pelagic fishes such as shiner perch Embiotocidae used aquaculture growing gear more frequently than unfarmed areas as foraging habitat, but Metacarcinus crabs displayed higher foraging frequency in unfarmed mudflats. Species groups such as sculpin Cottidae and small flatfish Pleuronectidae clearly used specific aquaculture growing gear and mudflats in roughly equal proportion. In the second part of the study, we used stable isotope mixing models to estimate, for several species of nearshore fish and crab in two areas of North Puget Sound, Washington, the percent diet originating from eelgrass meadows, pelagic planktonic sources, and oyster farm habitats. Our results indicate that several species of nearshore fish derive a significant proportion of their diets from farm areas, while others derive most of their diets from eelgrass habitat or planktonic sources. Shellfish farms within a larger nearshore habitat mosaic of eelgrass meadows, mudflats, bivalve aquaculture gear, and edge habitat can provide foraging habitat for several species of nearshore fish.