Clam aquaculturists have suspected low water flow over clam grow-out areas as a principal explanation for decreased growth and yield of bivalves; however, empirical data from field studies to support these observations are rare and in some cases contradictory. I conducted two experiments in Back Sound, NC, to examine the effects of water flow and juvenile density on hard clam survivorship and growth. The first experiment assessed differences in early growth and survivorship of juvenile seed clams under three manipulated water flows (enhanced [0–32 cm sec-1], reduced [0–12 cm sec-1], and ambient tidal flows [0–22 cm sec-1]) and three initial stocking density (250, 500, or 1000 m-2) of juvenile clams (shell length [SL] = 9 mm). The second set experiment examined the effects of two different water flow regimes (ambient [0–22 cm sec-1] and low [ 0–11 cm sec-1]), and three stocking densities (250, 500, or 1000 m-2) on growth of seed clams (SL = 12 mm) to market size. Unlike the first experiment, in which no effort was made to exclude predators to examine survivorship, the second experiment mimicked aquaculture operations and used bottom netting to cover seed clams. Neither planting density nor flow regime affected growth or survivorship of juvenile seed clams in the first experiment. Overall survivorship was high (> 75%) and clams grew to an average SL of 24 mm during the 4 months of grow-out. In the second experiment, clam growth and, consequently, time to marketable size was affected by water flow; however, differences in clam growth were relatively small (< 4 mm SL). After 12 mo, 69% of clams were of harvestable size in the ambient flow (0–22 cm sec-1) compared with 42% in the low flow (0–12 cm sec-1). Planting density did not affect this relationship; no interaction between planting density and flow regime was evident in either experiment. This result suggests the positive effect of water flow on clam growth increases with individual clam size, but is not density dependent on the square meter scale of our experiments.
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Vol. 28 • No. 4