Juvenile Clam Mortality Study at Three Intertidal Flats in Hampton Harbor, New Hampshire

Soft-shell clams, Mya arenaria L., represent an important recreational fishery along the New Hampshire coast. Intertidal flats in the Hampton-Seabrook Estuary are among the most heavily harvested. As recently as the fall of 1998, the sandy flats in this area supported more than 900 clammers, who, on weekends, easily harvested their 9.5-liter limit (Varney, 1999). Recently, however, quantitative benthic surveys have suggested that the abundance of adult clams (\u3e 50 mm shell length, SL) has dwindled on the three estuarine flats (Brown’s Flat, Common Island, and Middle Ground). Clammers support this contention and many have been disappointed at the relative paucity of harvestable clams and the effort required to obtain them. Surveys have shown that clams in the area are reproducing and that s pat (\u3c 25 mm SL) are abundant, but that populations of yearling clams (i.e., age 7-12 months and 26-50 mm SL) are very low. Staff at the New Hampshire Estuaries Project asked whether the limiting factor for a sustainable fishery could be poor juvenile survival

Large-scale, manipulative field tests involving cultured and wild juveniles of the soft-shell clam

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 \u3e 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 \u3c 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 (\u3c 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 \u3e 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

Juvenile soft-shell clam, Mya arenaria L.research in the Hampton-Seabrook Estuary

A series of field experiments was conducted at two intertidal sites in the Hampton Seabrook Estuary from November 2004-2006 to assess the efficacy of enhancing intertidal areas with cultured clam (Mya arenaria L.) seed (mean shell length [SL] = 7-10 mm). Measurement variables in each experiment included survival and growth of both cultured and wild seed clams. The first of three trials (November 2004 - May 2005) examined the interactive effects of size of planting area (4 m2, 8 m2, 12 m2, and 18 m2) and predator deterrent netting (none, 4.2 mm, and 6.4 mm aperture [flexible, plastic netting]) at the Willows Flat in the Hampton River. The second trial (June - October 2005) examined the effect of predator deterrent netting at two discrete intertidal locations at the Willows Flat. The third trial (April - November 2006) replicated trial two except at two intertidal sites within the estuary approximately 3 km apart

Interactive effects of tidal height and predator exclusion on growth and survival of wild and cultured juveniles of the soft-shell clam, Mya arenaria L., at two intertidal flats in southern Maine

A comparative field experiment was initiated at two intertidal flats in southern Maine (Wells – Webhannet River; Portland – Fore River) in May 2014 to examine the interactive effects of tidal height and predator exclusion on the growth and survival of cultured individuals of the soft-shell clam, Mya arenaria L. (xShell length = 12.95 ± 0.20 mm). Experimental units (0.018 m2) were placed near the upper and lower intertidal and filled with ambient sediments at both sites. Clams were added to units at a density of 660 ind. m-2. Predator exclusion included five treatments: 1) none (controls); 2) flexible netting (4.2 mm aperture); 3) flexible and rigid netting (6.4 mm); 4) Pet screen over the top of the units; and 5) Pet screen over the top and bottom of the units. Netting was designed to exclude green crabs, Carcinus maenas, whereas Pet screen was designed to exclude crabs and nemertean worms, Cerebratulus lacteus. Experimental units at each site were collected in October 2014, after 151 days in the field. Survival did not vary significantly across tidal heights at either site. Less than 5% of clams in control units were recovered vs. \u3e 50% survival in protected units at both sites. Pet screen did not enhance survival at either site compared with flexible netting. Growth was faster at the lower vs. upper intertidal at one site (by ca. 20%), but not at the other, and was depressed between 50-60% in units protected with Pet Screening compared to open and netted units. Mean final shell length in open and netted units pooled across sites ranged from 25-40 mm. Wild, 0-year class recruits of Mya were observed at both sites, and were generally more abundant in lower vs. upper intertidal units. Few recruits occurred in control units, but mean abundance of recruits was an order of magnitude greater in units protected with Pet screen (3843.9 ± 1737.9, n = 44) vs. flexible netting (726.4 ± 400.5, n = 44) pooled across both sites

Tracking ocean wave spectrum from SAR images

An end to end algorithm for recovery of ocean wave spectral peaks from Synthetic Aperture Radar (SAR) images is described. Current approaches allow precisions of 1 percent in wave number, and 0.6 deg in direction