A series of field experiments to assess the efficacy of enhancing intertidal areas with cultured clam (Mya arenaria L.) seed (mean shell length [SL] = 7-10 mm) was conducted at Willows Flat, Hampton, New Hampshire from November 2004 to May 2005 and from June - October 2005. The first trial examined the interactive effects of size of planting area (4, 8, 12, 18 m2) and predator deterrent netting (none, 4.2 mm, and 6.4 mm aperture [flexible, plastic netting]) on clam growth and survival at one intertidal location. The second trial examined the effect of predator deterrent netting on clam growth and survival at two intertidal locations. From November 2004 to May 2005, clam survival was nearly 90% in plots protected with the smallest aperture netting, and this was three times greater than survival in plots protected with 6.4 mm mesh netting. Few animals were recovered from plots that were not covered fully with plastic netting. Overall, enhancement due to the predator deterrent netting was greater than 100-fold. Clams survival in the smallest size plots was significantly greater (by 30%) than those in the three larger sized plots. Clams reached a mean shell length of 14.6 ± 0.57 mm during this period, an average increase in shell of 4.2 mm. Growth rate of clams was 30% faster in plots protected with the smaller aperture netting. Plot size affected growth rate, but the effects were complex. For example, no differences in clam growth rate were detected between the smallest vs. the other three plot sizes; however, clams grew more slowly in the 8 m2 plots compared to the mean of the two largest plot sizes. This study indicates that 1) it is possible to seed flats in the Hampton River area with cultured soft-shell clam seed in the late fall and be successful (i.e., attain survival rates > 75%); 2) protecting clams with plastic, flexible netting is warranted and necessary to deter predators and retain clams in the seeded areas; 3) if clam sizes are < 10 mm 2SL, using 4.2 mm mesh netting rather than 6.4 mm netting will yield higher recovery rates; and, 4) seeding small areas (< 8 m2), rather than larger ones will result in higher clam yields. The experiment initiated in June 2005 must be repeated in 2006 due to mass mortality shortly after seeding. Animals were seeded on an extremely hot day (11 June 2005) when pre-noon temperatures reached > 32oC. Animals were exposed to the air and heat for several hours before the tide covered the seeded plots and observations made within a week after the seeding event suggested that a massive die-off occurred soon after the seeding event. By 8 October, losses of greater than 1,200 individuals m-2 had occurred in all three treatments at both intertidal locations. Although results were more than disappointing, the study yielded several pieces of valuable information that can be used in future. First, clam numbers were enhanced by using protective netting. In fact, no clams were recovered in benthic cores from plots that were seeded but not covered with netting. Second, plots covered with the smaller aperture netting at both sites produced the highest number of clams – a result similar to the first experiment – suggesting the patterns observed here and in previous trials in this region (see Beal 2002) are generalizable. Clam populations in this region are exposed to intense predation (due mostly to green crabs and bottom feeding fish) that can eliminate entire year classes. Experimental results to date indicate that enhancement can be effective if carried out properly; however, it is unknown whether these activities are cost effective. Only after the field trials of 2006 can this important question be assessed.
New Hampshire Estuaries Project
Beal, Brian F., "Large-scale, manipulative field tests involving cultured and wild juveniles of the soft-shell clam" (2005). PREP Publications. 195.