Culture of Mercenaria mercenaria (Linnaeus): effects of density, predator exclusion device, and bag inversion

Growth, survival, and condition index (CI) of the northern quahog, Mercenar ia mercenaria (Linnaeus, 1758), cultured in nylon mesh bags (1.2 x 1.2 m) w ere assessed against density and predator exclusion device (PED: Vexar net with 2.5-cm openings) in the northern Indian River Lagoon at Oak Hill, Flor ida. Nursery seed [mean +/- SD: 6.0 +/- 0.8 mm shell length (SL)I were stoc ked in February 1997 at densities of 7,500 (5,210), 10,000 (6,944), and 12, 500 (8,680) clams/bag (clams/m(2)) (n = 4) and monitored until the end of M ay 1997. Two replicates of each treatment were inverted 5 weeks before harv esting to smother fouling organisms and examine their influence on growth. Growout seed (mean +/- SD: 21.1 +/- 1.7 mm SL) were stocked in October 1996 at densities of 750 (521), 1,000 (694), and 1,250 (868) clams/bag (clams/m (2)) (n = 4) and monitored until early June 1997. At the end of the nursery seed experiment, the average final SL of clams was significantly different among the density treatments (p =.03) and not significantly different betw een the FED (p =.31) treatments. Nursery seed in the inverted bags were sig nificantly larger (p =.03), and a higher percentage of them reached growout seed size (12 mm in SL). Density (p =.60) did not have a significant effec t on survival; whereas, the bags with FED had significantly (p =.005) lower survivorship than that of the bags without PED. Density (p =.15) and FED ( p =.79) did not significantly affect mean CI at the end of the study, but i nversion significantly (p =.002) increased CI. At the end of the growout se ed experiment, SL was not significantly different among the treatments (den sity, p =.40; FED, p =.17). There was a significant(p =.04) effect of densi ty on percentage of the seed that reached legal harvest size (16 mm in shel l thickness). In general, percentage of seed that reached harvest size decr eased with increasing density. The effects of density (p =.04) and FED (p = .0009) on survival were significant, but there was no apparent pattern. Den sity (p =.29) and FED (p =.88) did not affect mean final CI. Chlorophyll a concentration and water current speed measured in April and May, 1997 indic ated that food was not a limiting factor on growth of the northern quahog a t the study site. Our recommendations for northern quahog culture in the Oa k Hill area are: (1) use a planting density of 7,500 clams/bag for nursery seed and 750 clams/bag for growout seed; (2) could use FED to reduce foulin g on the culture bags, although FED may not improve clam survivorship; and (3) invert culture bags periodically.