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

Growth, survival, and condition index (CI) of the northern quahog, Mercenaria mercenaria (Linnaeus, 1758), cultured in nylon mesh bags (1.2 x 1.2 m) were assessed against density and predator exclusion device (PED: Vexar net with 2.5-cm openings) in the northern Indian River Lagoon at Oak Hill, Florida. Nursery seed [mean +/- SD: 6.0 +/- 0.8 mm shell length (SL)I were stocked 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 May 1997. Two replicates of each treatment were inverted 5 weeks before harvesting 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 between the FED (p =.31) treatments. Nursery seed in the inverted bags were significantly 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 effect 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 inversion significantly (p =.002) increased CI. At the end of the growout seed experiment, SL was not significantly different among the treatments (density, p =.40; FED, p =.17). There was a significant(p =.04) effect of density on percentage of the seed that reached legal harvest size (16 mm in shell thickness). In general, percentage of seed that reached harvest size decreased with increasing density. The effects of density (p =.04) and FED (p =.0009) on survival were significant, but there was no apparent pattern. Density (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 indicated that food was not a limiting factor on growth of the northern quahog at the study site. Our recommendations for northern quahog culture in the Oak 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 fouling on the culture bags, although FED may not improve clam survivorship; and (3) invert culture bags periodically.