Clams consume phytoplankton and detritus to form an important link between food webs in the water column and the sediments. Clamming also provides recreational opportunities and food. Finally, softshell clam condition and abundance can offer insight into the environmental health of a system.

Increase the number of adult softshell clams in Hampton-Seabrook Estuary to 5.5 million clams.

In 2020, the estimated population of adult (>50 mm, or almost 2 inches, in shell length) softshell clams (Mya arenaria) in Hampton-Seabrook Harbor was 5.1 million, below the PREP goal of 5.5 million. Softshell clams are present in the Great Bay Estuary although recreational clamming is not as prevalent. Softshell clams are only monitored in the Hampton-Seabrook Estuary at this time. The standing stock of softshell clams at the Hampton-Seabrook Estuary clam flats (Figure 18.1) has fluctuated over the years (Figure 18.2). In some cases, the stock increases were most likely caused by the closure of the flats to harvesting. For example, the clam flats were closed to recreational harvesting from 1989 through 1993 due to a lack of sanitary surveys of the growing areas. The Common Island and Browns River flats were reopened intermittently beginning in 1994. The Confluence and Middle Ground flats were reopened in 1994 and 1998. All flats were and still are periodically closed due to coliform pollution following heavy rains and paralytic shellfish poisoning (red tide) outbreaks. Since the reopening of the flats in 1993 – 1997 and the resulting increase in harvesting, the standing stock of clams has generally declined. Since 2012, it has remained below the PREP goal of 5.5 million (Figure 18.2; Table 18.1).

Standing stock is estimated as the density (number per square meter) of adult clams multiplied by the acreage of the flats. As a result, standing stock is affected by changes in the density of clams as well as the acreage of the flats themselves. Clam flats are an extremely dynamic environment due to sediment erosion and accretion, both driven by storms and human activities such as dredging. PREP and partners are considering changing the metric from standing stock to density of softshell clams to avoid variability caused by changes in the acreage of the flats.

Softshell clams might also be limited by a type of contagious cancer (hemic neoplasia) that affects marine bivalves but is not dangerous to humans. The disease is transmitted directly among softshell clams through the transfer of cells called neoplastic hemocytes.^60,61,62 There are several factors that make softshell clams more susceptible to this disease, including pollution (mainly heavy metals and hydrocarbons) and warming water temperatures.⁶³ The percentage of softshell clams infected with neoplasia increased from 2002 through 2015 and has averaged between 60% and 79% since 2015 (Figure 18.3). A review of data collected from 2002 – 2018 indicates that the advanced forms of neoplasia are significantly increasing in the Hampton-Seabrook Estuary.⁶⁴

Green crabs prey on softshell clams, especially newly settled individuals, reducing softshell clam population size; therefore, studies have noted a significant negative relationship between green crab abundance and density of newly settled softshell clams.⁶⁵ Green crab abundance in Hampton-Seabrook Harbor was highest in 1992 and 1996 and has generally declined since then (Figure 18.4). Cold winter water temperatures often limit green crab abundance. Given the trend of warming waters in our region, one concern is that green crab abundance will increase as we look towards the future, negatively impacting softshell clams.

Acknowledgments and Credit

Paul Geoghegan (Nomandeau Associates) with contributions from Kalle Matso (PREP) and Trevor Mattera (PREP).

Softshell Clams – Extended Report
Includes methods, data, and additional discussion not included in the Printed Edition.