Balancing tradeoffs between predator protection and associated growth penalties in aquaculture of northern quahogs, Mercenaria mercenaria (Linnaeus, 1758): A comparison of two common grow-out methods

Poor survivorship of Mercenarin mercenaria seed clams is the chief obstacle hindering economically successful grow-out operations of hard clams. While much effort has been invested in decreasing such mortality, increasing pro tection of clams often results in a reduction in their growth. We examined how the mortality and growth of seed clams during the first year of culture differed between two common grow-out methods: tented, nylon-mesh bags and mesh-covered bottom areas. We also determined how initial seed size modifie d these relationships. Specifically, three initial seed sizes (SL = 10.9, 1 3.0, and 14.8 mm) were grown separately in 1.2 x 1.2 m (9.4-mm) mesh bags t ented with a 30-cm-long PVC stake in the center. At the same time, two init ial seed sizes (SL = 12.8 and 14.9 mm) were grown in 1.2 x 1.2 m bottom bed s covered with 7.0-mm polypropylene mesh. All treatments were stocked with 700 seed clams. Clam survival in tented bags was enhanced by increasing ini tial planting size. Small seed (10.9 mm) had a mean survival rate of 76.6% and large seed (14.8 mm) had a mean survival rate of 93.1%. Survival was gr eater in nylon bags than mesh-covered bottom beds when similar initial seed sizes were used. When data for medium (13.0 mm) and large (14.8 mm) initia l seed sizes were pooled, the mean survival in tented bags was 90.1%. Survi val in the mesh-covered bottom-beds for similar initial clam sizes (12.8 mm and 14.9 mm) was 71.7%. Clam growth (in SL) was 21.7 % greater within mesh -covered bottom beds than in nylon bags. Increasing initial seed size enhan ced clam growth slightly in both grow-out methods. This study demonstrates a method to increase survival using tented nylon-bags Tor the first year of grow-out as compared to mesh-covered bottom-beds; however, there is a grow th penalty associated with this increased survival. The project also sugges ts a minimum initial seed size (>12 mm SL) that should be attained fi om a nursery system before planting in tented bags or bottom beds because greate r survivorship associated with larger initial clam size more than compensat ed for the increased cost of planting larger seed clams.