Seed clam,Mercenaria mercenaria,culture in an experimental-scale upflow nursery system

Upflow nursery culture of bivalve seed, while in widespread use in France and England, has experienced only limited application in commercial-scale nurseries in the United States. Reported studies on appropriate flow rates, stocking densities, and carrying capacities for upflow culture either have used water from fertilized ponds or have provided supplemental feeding with cultured algae. There have been no published results of upflow use in natural waters. This study was conducted to evaluate growth of seed clams, Mercenaria mercenaria, in an upflow nursery culture system in South Carolina relying on natural productivity as the only food source. Experimental-scale passive upflow cylinders were stocked with small seed clams (initial mean size ∼ 4.0 mm) at various densities (2.5, 5.0, 10.0, 20.0, 30.0, and 40.0 kg/m²) in trials initiated at different times of the year (∼ quarterly). Cultures received a continuous uniform water flow rate of 2.5 l/min from an adjacent estuary, at ambient phytoplankton concentration, temperature and salinity. Most rapid growth was obtained with seed stocked in April and October, when water temperatures were between 18 and 22°C. Monthly biomass increases as high as 267 g/100 g were achieved. Growth was positively correlated with flow rate in all seasons except winter. A flow:biomass ratio of 15:1 resulted in a doubling of biomass in 30 days, while a ratio of 30:1 resulted in a tripling over the same period. Under favorable environmental conditions, a biomass doubling could be achieved at stocking densities as high as 20 kg/m². Maximum production over a 3-month period was 495 g (309 g/100 g) of 7 mm seed, corresponding to 62 kg/m². Although water requirements were similar to those previously reported for raceway culture in South Carolina, results indicated much greater biomass carrying capacities per unit area with upflow culture systems.