Shellfish aquaculture helps clean coastal waters

Michael Rice, Professor and Chairman of the URI Department of Fisheries, Animal and Veterinary Science announced on 16 February that the natural feeding method of bivalves like clams, mussels, oysters and quahogs serves as “an estuarine filter, increasing the clarity of coastal waters and facilitating the removal of nitrogen and other nutrients.”

By increasing the quantity of shellfish in local waterways through the establishment of aquaculture facilities, these waters can be continually cleaned naturally at little or no cost to the state or community, and could even derive a profit in the form of harvesting shellfish for commercial use.

All of the economically important bivalve shellfish derive most of their nutritional needs by filtering particles from the water, actively sorting particles according to their nutritional value, ingesting food particles and depositing rejected particles into the seabed. “Healthy assemblages of shellfish can filter a substantial fraction of the water in coastal estuaries on a daily basis,” said Rice. “Aquaculture operations may mitigate the effects of coastal housing developments or other activities that promote excessive coastal eutrophication.” Eutrophic waters have low levels of dissolved oxygen and increased plant life, especially algae, at the expense of other life forms.

Rice estimates that the 26,400 metric tons of northern quahogs living in the Providence River section of Narragansett Bay filter 21.3% of the water at that location during each complete tide cycle. On average, 16.8 grams of nitrogen is removed from the water for every kilogramme of shellfish meat harvested. If just 10% of the standing stock the quahog, or hard clam, was harvested annually, eight metric tons of nitrogen would be removed as well, Rice says, although adds that this constitutes a small percentage of the 4,580 metric tons of nitrogen discharged into the Providence River annually.

The remaining quahogs deposit large quantities of particulate matter from the water on the bottom where chemical processes in the sediments may convert the nitrogen and other nutrients to less biologically active forms.

The input of inorganic nutrients like nitrogen and phosphorous into coastal waters is seen as a significant environmental problem that is expected to intensify as coastal development continues, a famous example being the so-called Gulf of Mexico ‘dead zone’ (see related story). These nutrients come primarily from sewage treatment facilities, individual sewage disposal systems, lawn fertilisers and various agricultural practices, resulting in eutrophication.

In addition to advocating increased private aquaculture development, Rice said that publicly funded shellfish restoration projects may be a cost-effective complement to coastal wastewater management strategies and other projects designed to mitigate the impacts of coastal eutrophication.

“Since shellfish beds filter particulates from the water column and increase water clarity, projects to restore eelgrass and other submerged aquatic vegetation may be improved if there were parallel efforts to restore shellfish in adjacent areas,” he said. “Bivalve aquaculture combined with shellfish fishery enhancement may in fact be an effective mitigation practice to counteract the effects of excessive nutrient enrichment.”