Sustainable salmon aquaculture and tidal flushing in a macrotidal ecosystem: Cobscook Bay, Maine

Cobscook Bay, Maine is located at the mouth of the Bay of Fundy. Cobscook Bay supports an unusually productive and diverse ecosystem including scallops, clams, mussels, sea urchins and macroalgae, all of which are commercially valuable. Salmon aquaculture has been introduced in Cobscook Bay and the adjoining Passamaquoddy Bay, and aquaculture has become an important contributor to the economy on both sides of the border. In 1995, the salmon aquaculture industry of Maine produced about 10 million kg of fish, mostly from Cobscook Bay (DMR, 1996). The increasing density of salmon pens leads to questions about sustainable levels for aquaculture in the bay because of concerns about nutrient loading and the potential for eutrophication associated with point sources of fish wastes and unconsumed fish feed. The central questions relate to the significance of aquaculture-related nutrient sources compared to natural ones, and the capacity of the tides to flush the bay of pollutants and waste products. At present, Cobscook Bay does not appear to be significantly stressed by either land-based or aquaculture nutrient sources, but there are indications of changes of the ecosystem such as the occurrence of green algal mats in the intertidal zone. To address these issues it is necessary to understand the extent to which the tidal circulation is able to flush pollutants from the bays. This paper summarizes results from experiments with a 3D numerical hydrodynamic model which we have used to estimate residual circulation, dispersion and flushing times in Cobscook Bay.