Short-term effects of rapid salinity reduction on seed clams

Journal of Shellfisheries Research, Jan, 2005 by Shirley M. Baker, Patrick Baker, David Heuberger, Leslie Sturmer

ABSTRACT 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.

KEY WORDS: Mercenaria mercenaria, clam, salinity, mortality, condition index, Florida

INTRODUCTION

Florida ranks third in the United States in aquaculture production values, and the culture of the northern quahog (=hard clam), Mercenaria mercenaria (Linnaeus, 1758), represents the fastest growing segment of the state's aquaculture industry; between 1989 and 1999 revenue from farm-raised hard clams increased 15-fold (USDA 1990, USDA 2000a). Florida farm-raised clams are now a recognized commodity on the national market, with Florida's crop accounting for more than 40% of the nation's total aquacultured production (USDA 2000b). Today, approximately 400 active shellfish growers farm over 1800 acres of sovereign submerged state lands in coastal waters, producing a crop worth $18.2 million with an economic impact of about $55 million (USDA 2002, Philippakos et al. 2001). The Florida Department of Agriculture and Consumer Services (FDACS), Division of Aquaculture, has designated tracts of submerged lands suitable for aquaculture, called high-density lease areas (HDLAs), which are located in coastal counties including Franklin, Dixie, Levy, Charlotte, Lee, Indian River, and Brevard. Over half of the statewide production of clams is attributed to the highly productive HDLAs in Dixie and Levy counties on the west coast of Florida. The phenomenal production of clams in Florida is attributed to the high natural productivity of subtropical waters that allow for almost year-round growth.

In 2000, clam growers became the first aquaculturists in the United States eligible to purchase crop insurance. The US Department of Agriculture (USDA) Risk Management Agency developed a program under which hard clam growers are able to buy subsidized Cultivated Clam Crop Insurance. Eligible HDLAs are located in Florida's Dixie, Levy, Brevard, and Indian River Counties, as well as counties in South Carolina, Massachusetts, and Virginia. The clam crop insurance program covers losses due to "unavoidable damage" such as storms, low oxygen, and changes in salinity (USDA 2000c). To date, approximately 95% of the clam growers in the four eligible counties in Florida have purchased crop insurance.

On implementation of the insurance program, there were inadequate provisions for correlating water quality or weather events with crop loss. That same year, however, we had the opportunity to begin deploying multiparameter monitoring systems (YSI 600XLM Sonde or YSI 6600 Sonde, YSI, Yellow Springs, Ohio). In cooperation with the FDACS, monitoring sites were established on the Horseshoe HDLA in Dixie County and the Gulf Jackson HDLA in Levy County. The monitoring data revealed important details of temporal variability, previously unresolved from monthly water samples obtained by the FDACS Shellfish Environmental Assessment Section for management of shellfish harvesting areas. For example, in 2001, mean daily salinity fluctuation at Gulf Jackson HDLA was 5.3 ppt, with a maximum 24-h change of 24.5 ppt. Salinity values for the year ranged from a low of 5.4 ppt to a high of 37.2 ppt (Fig. 1).