Growth of Mercenaria mercenaria and Mercenaria mercenaria texana seed clams planted in two Texas, USA bays

Survival and growth of Mercenaria mercenaria imported from Massachusetts, and M. mercenaria texana, the Texas subspecies, were studied in two Texas bays. Mesh-covered trays were used to limit predation. Survival was high in protected trays; unprotected trays reached 100% mortality within 6 months. In Christmas Bay, M. mercenaria growth averaged 1.8 mm/month. Most growth occurred during the summer months. Periods of no significant growth corresponded to periods when salinities under 20 .per.mill. were recorded. In Redfish Bay, growth was significantly different between stations, being fastest at a station located along the Aransas ship channel, where circulation was the highest of all sample sites (2.8 mm/month vs. 0.9-1.5 mm/month in the grass flats of the bay). Significant differences in growth between bays or between sample sites during some seasons were not correlated with either temperature or salinity regimes. Native clams, M. mercenaria texana, grew faster than imported clams in both the field and the laboratory. It is possible that differences in circulation and circulation-related parameters (e.g., food supply and waste flushing rates) account for much of the variation in growth in the bays and that periods of no growth may be determined by limiting salinities, and less frequently, temperature. Measurements made during and after this study suggest that a 2- to 3-year growout period may produce market-size clams.