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Measuring the Impact of Mussel Rafts

US - With funding from USDA's Cooperative State Research, Education and Extension Service (CSREES), a project team in Maine has measured the environmental impact of mussel rafts, an aquaculture method used to grow shellfish in coastal New England.

Aerial view of mussel rafts in Maine.
Photo: Carter Newell

Carter Newell and John Richardson used a combination of field data and an advanced numerical model to determine how a variety of bivalves, including manila clams, geoducks and Pacific oysters, consume the particles that reach the ocean floor, writes Stacy Kish, USDA CREES staff. The researchers conducted these studies in Puget Sound, Wash., and Ireland.

For the computer model, the project leaders developed a customized user interface that included the following variables: bathymetry (ocean depth), tidal elevations and speed, wind speed and direction, number of rafts, raft dimensions, rope spacing, rope length, rope diameter, mesh size and depth of predator net, particulate matter and water temperature. Output from the model predicts flow patterns through shellfish rafts, particle depletion caused by the shellfish rafts, shellfish biomass and mooring requirements.

The researchers also examined water velocity and direction and modeled how fouling and seaweed might affect these values, which could affect shellfish survival and growth.

The scientists measured the concentration of chlorophyll, the green pigment plants use to collect sunlight, to estimate food variability during tidal cycles. The data was compared to water filtration through the shellfish raft, based on the biomass of the shellfish. Model output using this information also produced a risk analysis related to site conditions and exposure to wave action.

Recent improvements in the models include an aquaculture geographic information system, computer aided-design representation of aquaculture structures and improved software for shellfish growth and particulate matter depletion models.

Since the 1990s, mussel production using rafting aquaculture techniques has increased in Maine and Washington with more than 2 million pounds currently produced. Mussel production has an economic impact of more than $5 million.

The model developed during this study can be used easily by sea farmers to make important decisions on mooring arrangements and raft modifications. The model output provides sea farmers with a new set of tools to choose productive sites and plan management strategies that take environmental sustainability into consideration.