Performance of a tidal-powered upwelling nursery system for northern quahogs (hardclams) (Mercenaria mercenaria) in South Carolina

Entry into the hard clam aquaculture industry on a small scale has been lim ited by the cost of plantable seed. Growers must either purchase expensive seed at a suitable planting size (usually 8 mm SL or larger) or raise small seed to this size in a nursery. Land-based nurseries foster high survival and rapid growth, but require expensive waterfront property and are energy- and labor-intensive to operate. Field-based nurseries are inexpensive to o perate, but seed survival is often very low and success is site-specific. F loating upwelling systems (FLUPSYs) combine many of the advantages of land- based systems (high survival, rapid growth) with those of field-based syste ms (inexpensive operation). One particular type of FLUPSY, a tidal-powered upwelling system (TPU), is described here. TPU performance was tested in So uth Carolina over a 5-year period. Tidal currents averaging 0.53 m/s produc ed flow rates of 52 Lpm through upwelling bins. The TPU requires a mean cur rent velocity of 0.26 m/s to produce a flow rate of 26 Lpm through the upwe lling units, which is comparable to flow that land-based upwellers provide. Daily growth rates as high as 15% were observed. Growth in the TPU was mor e rapid than published reports of growth rates in land-based systems in Sou th Carolina. The TPU described here can produce 122,000 (12 mm) seed, start ing from I-mm seed, in a 32-week growing season (March through October). Ca pacity is considerably higher (up to 1,074,000) if seed are stocked at a la rger size and/or harvested at a smaller size. The described system, built f or $4,500 and with annual operating costs of less than $5,000, is a cost-ef fective nursery system that smalt-scale growers could implement to produce field-plantable seed.