Design and maintenance of a closed artificial seawater system for long-term holding of bivalve shellfish

To study the potential for transmission of Perkinsus karlssoni, an apicomplexan parasite, among bivalve hosts, a long-term laboratory-scale, closed, artificial seawater system for shellfish was designed. Species of shellfish utilised were as follows: bay scallops, Argopecten irradians; blue mussels, Mytilus edulis; eastern oysters, Crassostrea virginica; European oysters, Ostrea edulis; sea scallops, Placopecten megallanicus; softshell clams, Mya arenaria; and quahaugs, Mercenaria mercenaria. All shellfish used were juveniles with shell heights of 10-25 mm. The design was duplicated for control and experimental systems with a combined population of 1600 individual shellfish. The maximum biomass in each system was estimated at < 1 kg/m super(3). Each system included six, 400 litre rectangular fibreglass tanks, a modular pump and filter unit (particle and activated carbon filters and ultra-violet sterilisers), a biological filter and a refrigeration unit. The total volume of water for each system was 2300 litres of artificial seawater (Instant Ocean super(")). The mean water temperature of 22 degree C was achieved by thermostat-controlled room temperature. Salinity was maintained between 27 and 31 ppt by addition of either freshwater or artificial seawater. Shellfish were fed daily a mixed diet of carboy-cultured algae and spray-dried algae. Mean values for water quality parameters in both systems were as follows: NH sub(3) < 0.004 mg/litre; NO sub(2) super(-) < 0.01 mg/litre; NO sub(3) super(-) < 19-16 mg/litre; and pH 8.0-8.4. The system design was adequate in maintaining healthy bivalves for a period of 22 months, with the exception of the sea scallops which succumbed to warm water. Mean monthly shellfish survival rates were 79.9-100% in the control and 74.8-98.9% in the experimental system. A method of controlling water temperature other than via room temperature should reduce slight seasonal temperature fluctuations.