Sudden salinity drops in Gulf Coast clam growing areas in Florida have been suggested as a cause of seed (juvenile) clam mortality. Laboratory experiments were used to assess short-term impacts of rapid salinity drops on hatchery-produced juvenile northern quahog (=hard clam), Mercenaria mercenaria, in two separate trials. Mortality and condition index (CI) were measured as response parameters. In Trial I, clams were exposed to a salinity drop of 5 ppt, 15 ppt, or 24 ppt over a span of 24 h, and the duration of this exposure was either 3 or 6 days. In Trial II, clams were either immediately immersed or dry-stored for 24 h prior to immersion, representing common treatment patterns by clam growers. In this trial, clams were exposed to acute salinity drops of either 10 ppt or 20 ppt for up to 7 days. Both trials were conducted at ambient seasonal temperatures.
Juvenile hard clams were surprisingly robust and resilient to changes in salinity, experiencing less than 5% mortality after relatively abrupt reductions in salinity of 10 to 15 ppt. Nonetheless, salinity declines of the magnitude occasionally observed at clam culture sites, up to 24 ppt, resulted in significant mortality; 17% (Trial I) and 100% (Trial II). Condition index (CI) was an insensitive response parameter. Dry storage of clams did not appear to have an effect on their ability to withstand changes in salinity; storage decreased final survival by <2.5%. Our results suggest that the salinity declines typically experienced at the Gulf Coast aquaculture sites are not of a magnitude or speed to account for the particular seed clam mortality events that spurred this research. However, long-term effects of salinity changes remain to be tested. In addition, reduced salinity may be indicative of a variety of other stressors, such as increased temperature and turbidity, or decreased phytoplankton concentration, which compound the effects of salinity on clam health and survival.