Souris & Area Branch of the PEI Wildlife Federation
Activities
Green Crab Research - 2007
As part of his Honours thesis, Garry Gregory has been conducting research in the Souris River and Basin Head estuaries this summer. This project focuses on the feeding habits of the invasive green crab (Carcinus maenas) in different habitat types and at different densities. The green crab is known to feed voraciously on soft shell clam beds, often decimating aquaculture industries. View a video of a large green crab HERE.
As part of a larger effort to learn more about the impacts of invasive species in Prince Edward Island, this project will help initiate the development of an invasive species management plan as stated in Objective 4 of the Souris River Watershed Management Plan. An accrual of data is essential before any effective management strategy can be implemented, and the results of this research will contribute to this collection of information.
For use in the experiments, the green crabs were collected at the Souris River Bridge using eight small traps baited with soft shell clams or mackerel. Initially, the traps were deployed at low tide and left overnight, with the trapped crabs collected the subsequent day.
As the summer progressed, however, crab densities and movements increased substantially, to the point where crab collection required only a few hours.
Only large, male crabs were used in the experiments to ensure that reproduction (eg. carrying of eggs) did not interfere with feeding behaviours. In addition, all experimental crabs were held without food for forty-eight hours before inclusion in the study to ensure that they were sufficiently hungry.
The study site was prepared one day before the experiments were begun. At low tide, eighteen exclusion cages were inserted into the sediment to a depth of approximately 8 cm. These cages are designed to confine experimental subjects within a specified area, while preventing other predators from entering.
Before green crabs were inserted into the cages, preliminary samples were taken around twelve randomly chosen cages. This sampling regime involved using a 25 cm x 25 cm quadrant, which was inserted into the sediment to a depth of 13 cm. Using a spade, all sediment contained within this quadrant was removed for further sampling.
Everything that was not separated by the 5mm sieve was then passed through a 2mm bucket sieve. This method allowed for the separation and subsequent collection of smaller organisms. These were added to a labelled jar for later examination.
After all preliminary sampling was completed, the crabs were inserted into the exclusion cages. Each cage was given one of three treatments, and each treatment was replicated six times in the study design; a control (0 crabs), low density (2 crabs), or high density (6 crabs).
These cages were then left for six days, with Garry returning every other day to remove any biofouling or dead crabs. After six days, the crabs were removed, and sampling commenced again. Using the quadrat, a sample was taken from each cage, and the same methodology was used to collect the organisms from the sample.
With all samples from the experiment collected and preserved, laboratory analysis was begun. Using a dissecting microscope, all organisms were separated, identified, and counted. The results were recorded, and comparisons between different treatments and between preliminary and final samples were made to determine statistical significance.
Three experiments were conducted throughout the summer. The first experiment involved a combination of rock and green crab, and was designed to determine the level of interspecific competition. The second experiment involved only green crabs, and was carried out in a sandy environment. The third and final experiment was conducted in Basin Head, and involved only green crabs but in a muddy environment. Comparisons between the final two experiments allowed for examination of feeding efficiency of green crabs in different habitat types.
Compiling the final results will require much close scrutiny and will take place in the fall of 2007. It is expected that the knowledge gained from this study will broaden our understanding of this highly invasive species, and provide key insights into possible management solutions in our area. Since the prevalence of the shellfish industry in the Souris River is obvious, studies such as these may have important implications to our area.
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