Eastern oyster and hard clam culture for the reduction of RAS effluent nutrient composition

Closed recirculating aquaculture systems (RAS) are a technological innovation that reduces the amount of water needed for culture by treating and reusing >90% of the total water volume daily. Such systems consequently produce substantially smaller volumes of effluent, but this smaller volume is substantially more enriched in nutrients than effluent from flow-through or open systems. For RAS to emerge as a viable culture strategy, an economical and efficient method must be developed to reduce effluent nutrients concentrations before this effluent is discharged. As part of a larger project evaluating biofilter effects on RAS effluent nutrient composition, this study focused on the effect of bivalve culture on the composition of effluent from a southern flounder RAS. This system produces 1270L/day of enriched effluent, with nutrient and suspended solids concentrations 20-100x that of ambient levels. This effluent was introduced with seawater to an upweller system stocked with bivalves at two densities for 4-6 weeks. Oysters (Crassostrea virginica, (average shell height = 63.38 plus or minus 0.86) and clams (Mercenaria mercenaria, average shell height = 16.65 plus or minus 0.48) were used in separate experiments. Effluent nutrient composition (total nitrogen, total phosphorus, and total suspended solids) was analyzed from samples (collected twice weekly) taken upstream and downstream of each upweller. Bivalve growth was quantified by measurement of shell height at the start and end of each trial. Analysis of preliminary data suggests that both oysters and clams exhibited growth over the course of the experiments. Water samples are currently being processed to determine the impact of bivalve activity on effluent nutrient composition.