Green Crab Interaction with Softshell Clams

Project Description

The primary focus of this project was to determine the effects of green crab predation on soft shell clams in Biddeford Pool. By using an experimental design for field-based manipulation of predator exclusion, the results of green crab predation on soft shell clams were quantified through survivorship rates between several different experimental scenarios. Three scenarios were assessed: complete predator exclusion, crab-only exclusion, and no predator exclusion (the control). Utilizing these methods, researchers expected to find a higher clam mortality rate in the presence of predators compared to no predators, and that crabs would be responsible for a majority of the clam mortalities in the tests performed. Environmental change is a driver in species invasion. Therefore, Goals 3.2 and 3.3 were also informed by this project. Understanding predator-prey interactions and how aquacultured/harvested species fit into the greater food web of the ecosystem are important for improved management of sustainable ecological aquaculture.

Results and Accomplishments

The results of this project supported both of the original hypotheses. The preliminary hypotheses were that the clams with no predator exclusion placed around them would have the highest mortality rate in comparison to clams with predator exclusion placed around them. The second hypothesis was that green crab predation would be responsible for the most clam death. In the presence of all predators (open plots), there was a 92.5% mortality rate. To compare, the closed plots had a mortality rate of 16.6%. 75.9% of all clam mortality was prevented by simply covering the clams in a fine landscape mesh. It was also found that, in comparison to other causes of death (worms, birds, etc.), crabs were the most effective predator, with 25% of all deaths attributed to green crab predation.

When comparing treatments, it was found that there were significant differences in the mortality rates of clams in tubes with no predator exclusion, all predator exclusion, and crab-only exclusion. All p-values between the treatments were less than 0.01, which translates to significant differences between the mortality rates in each treatment. This supports the hypothesis that predator exclusion would have an effect on mortality rates and that the three treatments would have different mortality rates. The same tests were done between sampling dates to test if the progression of the summer had any effect on the mortality rates of the clams. This set of results were inconclusive, with all p-values for mortality rates between trial dates being greater than 0.01. As such no relationship between clam mortality and progression of the summer was observed.

Summary of Data Being Collected

Data Type Quantity Location
Predation of green crabs on softshell clams Predator-prey 27 replicates monitored every 2 weeks for 6 weeks. 27×3=81 Biddeford Pool, Biddeford ME