particularly the moon snail ( Polinices duplicata ) , and crabs (especially the 

 green crab) are known to feed heavily on soft-shell clams. 



Another factor affecting abundance is high water temperatures that tend to 

 increase the abundance of the predatory green crab. For example, populations 

 of soft-shell clams were low from the late 1940s to the mid-1950s, and during 

 the mid-1970s, when water temperatures were highest and green crabs most 

 abundant (personal communication from W. R. Welch, Maine Department of Marine 

 Resources, Augusta, ME; November, 1977). Boring gastropods are suspected to 

 have increased mortality (based on bore holes in shells of dead clams) among 3 

 to 5 year old clams in Washington County (personal communication from J. A. 

 Commito, University of Maine, Machais, ME; April, 1979). 



Other factors that may affect soft-shell clam abundance are competition for 

 space from blue mussels and possibly the gem clam. Aggregations of mussels 

 that form reefs over clam populations on sand and mud flats may increase clam 

 mortality (Newcome 1935). Gem clams are not often found in abundance with 

 soft-shell clams (Bradley and Cooke 1959; Sanders et al. 1962; and Larsen and 

 Doggett 1978a). Large numbers of gem clams may interfere with the settlement 

 of clam larvae in some locations. 



Shifting sediments, salinity extremes (<15 ppt or >32 ppt) , and temperature 

 extremes (<17°C or >23°C; 63°F or 73°F) also affect larvae adversely (Stickney 

 1964a). 



Human Impacts 



Potential dangers to clam populations in coastal Maine are destruction of 

 habitat and excessive commercial removal. Excessive exploitation apparently 

 is a greater threat to the clam industry than habitat destruction. According 

 to scientists at the Maine Department of Marine Resources, clam populations 

 are severely depleted and they expect that the record high harvests of 1976 

 and 1977 will probably not reoccur (personal communication from W. R. Welch, 

 Maine Department of Marine Resources, Augusta, ME; November, 1977). 



Other factors potentially affecting clam abundance and survival are oil 

 spills, channel dredging, shoreline construction, and the discharge of 

 contaminants. However, little evidence of the effect of these factors on 

 clams is available. The practice of digging clams with a clam hoe can 

 increase mortality rates in clams through breakage of shells and burying of 

 resident clams (Dow and Wallace 1961). 



Importance to Humanity 



The soft-shell clam strongly supports the commercial and sport-food fisheries 

 of coastal Maine. The commercial industry began in the mid-19th century as a 

 bait fishery for cod trawlers on the Grand Banks. From 1900 to the mid-1940s 

 the clams usually were packed and sold in cans. Currently, fresh or frozen 

 clams dominate the market (Hanks 1963). 



The commercial catch has fluctuated greatly in the last 25 years. Highest 

 catches occurred in 1950, 1976, and 1977, when about 7 million pounds were 

 landed (for 1968 to 1978 catch statistics and values see figure 12-1). The 

 catch was under 2 million pounds in 1960. Annual and seasonal fluctuations in 



12-4 



