EFFECT OF GROWOUT DENSITY ON HERITABILITY OF GROWTH-RATE IN THE NORTHERN QUAHOG, MERCENARIA-MERCENARIA (LINNAEUS, 1758)

Realized heritability for the increase in the rate of growth in the no rthern quahog, Mercenaria mercenaria, was determined under conditions of moderate growout density (<90 per sq. ft.) independently for two li nes by Crenshaw et al. (1993). For one line, termed Group A, a mean es timate of heritability of 0.402 was obtained. This estimate was based on a single generation of selection in which a standardized selection differential (i) of about 1.5 standard deviations was used, representi ng a selection intensity of about 16%. When Select and Control progeny of Group A were maintained in growout at high densities (>350 per sq. ft.), Control progeny grew at significantly greater rates than Select s; thus resulting in negative estimates of realized heritability. Clam stocks were collected in House Creek, Little Tybee Island, Wassaw Sou nd, in coastal Georgia. Same-age cohorts of F-1 progeny were establish ed in April and May 1986. Progeny were reared in the laboratory until December 1986, when they were transferred to growout cages in an inter tidal creek. Selection was carried out for F-1 cohorts in March 1988. Select and Control parental groups were identical in number, the latte r randomly chosen from the entire population before ascertaining the S elect-line parents. Spawnings of the Control and Select Group A F-1 pa rents occurred on July 6 and 7, 1989, respectively. Clam progeny were transferred from nursery to growout cages on September 12, 1990, and d ensity was reduced for Group A moderate-density cohorts on April 1, 19 91. Other Group A cohorts of Select and Control lines were maintained at high density in growout cages similar to those holding moderate-den sity cohorts. For clams reared at moderate density, heritability estim ates were calculated on September 12, 1991, for clams of Group A. Grou p A clams maintained in high-density growout developed more slowly tha n those reared at moderate density, and it was not appropriate to take measurements for estimating heritability until March 16, 1992.