Graduate Thesis Or Dissertation


On the Edge : Assessing Fish Habitat Use Across the Boundary between Pacific Oyster Aquaculture and Eelgrass in Willapa Bay, WA Public Deposited

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  • Estuaries are an important ecological link between terrestrial, freshwater, and marine systems, but are also subject to a variety of human pressures. Along the West Coast of the United States, shellfish aquaculture is one extensive use of estuarine tidelands. Specifically, Pacific oyster (Crassostrea gigas) aquaculture has been practiced for almost 100 years, significantly contributing to the culture and economy of the region. However, this activity does not exist in isolation. Oyster aquaculture commonly occurs in intertidal areas where native eelgrass (Zostera marina) is also present. Seagrasses provide valuable nursery habitat for many commercially-harvested species (e.g. salmonids, English sole, and Dungeness crab) and have recently garnered more conservation interest because they are declining in many locations. For these reasons, eelgrass is protected as “essential fish habitat” under the Magnuson-Stevens Fishery Conservation and Management Act. This protection restricts or prohibits oyster aquaculture within or near eelgrass. To help inform management decisions around this issue, questions were addressed regarding the use of both aquaculture and eelgrass as habitat for fish and crabs. Specifically, I was interested in quantifying and distinguishing whether the edge between these two habitats supported a different number of fauna. Information about edge effects could help elucidate potential consequences of aquaculture expansion at the scale of the whole estuary. Due to a recent shift towards off-bottom culture methods, in part to protect seagrasses, this comparison was made in both long-line and on-bottom aquaculture. Direct (underwater video) and indirect (e.g. predation tethering units) measures of fish and invertebrate community composition and behavior were used to quantify effects along a transect that ran between aquaculture and eelgrass in each type of aquaculture. Results suggest that species use long-line oyster aquaculture and eelgrass habitats similarly with minimal effect of the edge. However, habitat use of the on-bottom aquaculture was less than both the long-line aquaculture and eelgrass beds. This is consistent with an expected positive relationship between faunal abundance and the amount of vertical structure within a habitat. These conclusions add to the best available science regarding aquaculture and eelgrass interactions and provide valuable insight to managers and permitting agencies as they consider requests to expand culture operations.
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