Assessing manipulations of larval density and culling in hatchery production of the hard clam, Mercenaria mercenaria

Studies of the hard clam, Mercenaria mercenaria, indicate that the hatchery practice of larval culling may be counterproductive because of an inverse relationship in growth between larvae and postsettlement juveniles. The effects of larval culture manipulations were explored with two parallel studies: one set up in a commercial hatchery and the other in the laboratory. Both laboratory and hatchery studies started with the same cohort of larvae produced from spawning hatchery broodstock. Except for culling, these larvae were raised using standard practices in the hatchery for the first 10 days. On the tenth day of development, the cohort was sieved through 105-mu m mesh and separated into two treatments, large larvae and small larvae. In the hatchery, these samples were followed through settlement, and growth was monitored for 174 days postfertilization. In the laboratory, the two samples of larvae were raised under high-density (20 larvae/mL) or low-density (4 larvae/mL) conditions and thus assigned to one of four larval treatments: large/high-density, large/low-density, small/high-density, and small/low-density. These treatments were replicated (10 times each) to yield a 2 x 2 factorial, randomized block, repeated-measures analysis of variance (ANOVA) experiment. Growth in the laboratory was monitored for 276 days. Both the hatchery and laboratory studies indicate that larval culling does not increase productivity and may be counterproductive. Furthermore, larval density has an effect on subsequent juvenile growth. Larvae raised at low density produced larger clams in both the small and large larval treatments.