PS 10-114 - The relative impacts of both native predators and the invasive European green crab (Carcinus maenas) on seeded soft shell clams (Mya arenaria) in southern Maine tidal mudflats

Monday, August 7, 2017
Exhibit Hall, Oregon Convention Center
Andrew P Davidsohn, Marine Sciences, University of New England, Biddeford, ME

Soft shell clams (Mya arenaria) hold 4% of the seafood market value in Maine, or 3% of the total poundage of seafood taken annually. In Maine, annual soft shell harvests have declined over 40 years by $30 million. At the same time, populations of invasive green crab (Carcinus maenas) have been increasing and are known predators of soft shelled clams. Recent work in eastern Maine demonstrates high predation rates of unprotected clams on mudflats by green crabs. However, little is known about the relative impacts of these invasive species compared to other native predators in southern Maine mudflats. A field experiment was conducted to quantify survivorship of clams in the presence and absence of predators using different treatments: netting, flashing, and a control. For each of these three treatments, four plots of five replicate tubes (10 cm PVC) were arranged in Biddeford Pool mudflat, Biddeford, Maine. Netting, a typical method of protecting clams from crabs for commercial harvest, may also deter other predators, such as birds or fish. Aluminum flashing was used to create a 0.5 meter wall around four plots thereby excluding crabs but still allowing access to flying and swimming predators.


Results show that netting was more effective in promoting clam survivorship than flashing with 84% clam survivorship compared to 63%, respectively. It was also calculated that 45% of all juvenile soft shell clam deaths came from crabs. A 12.5% mortality in the closed plots proves that worms smaller than the 3mm landscape mesh were able to get through and eat the juvenile clams. These results point to the fact that a host of all sizes of worms and crabs were able to feed on the seeded juvenile soft shell clams. When clammers seed mudflats with high densities of soft shell clams, they have the potential to be increasing the mudflat’s biodiversity by adding to a recently low food source and attracting both native and invasive predators. This increase in population size will continue up the food chain to larger predators like fish and birds.