Overview of seed hard clam winter mortality studies in New Jersey, New York and Massachusetts

Hard clam (Mercenaria mercenaria) aquaculture is one of the most widespread forms of marine aquaculture on the U.S. east coast. The industry is based on hatchery seed production and planting of these seed in protected beds for growout. In the Northeast and Mid-Atlantic, seed planted late in the season is subject to variable (up to 50% in some beds) and unpredictable mortalities. Similar observations have been made for manila clams on the Pacific coast. The reasons for the variable success of late plantings of 8-15 mm seed have not been systematically investigated, but are believed to be related to seed "condition" (stored energy reserves) and its interaction with environmental variables or pathogens. One solution to late planting mortality is to hold the clams in nursery systems during winter. Unfortunately, a second, but apparently related, mortality occurs when seed clams are overwintered in nursery systems. Once seed begin to die in these systems, losses can be as high as 5% per day. The current work is developing and testing methods that could be used by culturists to evaluate the "condition" of seed clams prior to planting or overwintering. The goal is to provide a quantifiable means of evaluating alternatives to mitigate losses and to integrate the methodology into an economic/biological decision matrix. The work underway will evaluate whether disease (bacterial or other infections), lack of energy reserves, or their interaction is the primary cause of winter mortalities. The overall experimental protocol is: 1. Evaluate methods for assessing condition on specific size classes of seed. 2. Test seed and experimentally manipulate condition of specific seed sizes. 3. Re-analyze these manipulated seed and plant at one half commercial scale. 4. Place additional seed, at specific temperatures, during the fall in field nursery systems. 5. Test growth, survival and condition of all seed in spring. This is New Jersey Experimental Station Publication No. K-32403-1-96 and a Contribution of the Institute of Marine and Coastal Sciences, Rutgers University. Supported by state funds and grants from the Northeast Regional Aquaculture Center, National Science Foundation, and the Fisheries and Aquaculture Technology Extension Center of Rutgers University.