Previous surveys (1996 to 2002) provided distribution and abundance data for soft-shell clam (Mya arenaria) populations in ten areas of the Great Bay and Piscataqua River estuaries identified as potentially good clam habitat. The present study was designed to complete the overall survey by sampling six remaining areas: Weeks Point, Brackett's Point, Squamscott River mouth, Moody Point, Herods Cove, and Upper Little Bay (western shore). The objectives of the present project were to: (1) visually inspect the six study areas for the general distribution of sediment types and soft-shell clams, (2) quantitatively sample the six areas to determine densities of soft-shell clams, (3) produce GIS maps based on the survey data, and (4) assess clam distributions considering data from the present study and previous research. At each of the six sampling areas, the approximate boundary of "potential clam habitat" (=intertidal soft sediments) was determined by visual inspection at low tide. Notes were made on changes in major sediment types, the presence of clam siphon holes, and empty clam shells. At each site, nine to fourteen 0.125 m2 quadrats were haphazardly tossed onto the sediment surface, excavated to at least 20 cm depth using clam rakes, and all excavated sediments washed through a 5 mm mesh sieve. All clams retained on the sieve were measured (shell length to nearest mm with calipers), counted, and returned to the general area. A sample of the upper 5 cm of sediment was collected from each quadrat and stored at Jackson Estuarine Laboratory. Quadrat locations were geo-referenced using DGPS.The general environmental conditions in all six areas appeared suitable as soft-shell clam habitat. However, very few live clams were collected and very few empty shells were observed. From a total of 65 excavated quadrats, only 8 live clams were collected with mean densities ranging from 0.0 to 3.1/m2 at the six sites. It was concluded that none of the six areas were productive clam flats at the time of sampling, and they probably had not been in the recent past. Previous research and the present study indicate that many of the expansive intertidal flats in the Great Bay/Piscataqua River system have not been productive clam habitat for decades, probably since at least the 1940s in some areas. However, moderate to high densities of clams have been reported in some areas, particularly in sandy sediments. Previous research also showed high densities of early post-set clams in some areas, suggesting that spat mortality (probably predation effects) may be an important cause of low densities of larger clams in these areas. Future research should focus on sandy sediments and mixed soft sediments with cobble to better characterize the distribution and abundance of clams in the Great Bay/Piscataqua River system. Future research also should assess the role of predation on newly set spat in controlling clam populations.
New Hampshire Estuaries Project
Grizzle, Raymond E.; Greene, Jennifer K.; and Abeels, Holly, "Soft-Shell Clam (Mya Arenaria) Distribution & Abundance at Selected Sites in the Great Bay Estuary" (2006). PREP Reports & Publications. 168.