Predator-prey interactions between the European green crab (Carcinus maenas) and bivalves native to Prince Edward Island

The commercial harvesting and culture of American oysters (Crassostrea virginica), blue mussels (Mytilus edulis ), and soft-shell clams (Mya arenaria) has long been an important source of economic income for rural areas of Prince Edward Island (PEI). However, as seen in other bivalve industries elsewhere (Dare et al. 1983), the sustainability of PEI's bivalve industries are threatened by the establishment of the non-indigenous European green crab (Carcinus maenas). The green crab is a predator of most bivalves and has the ability to greatly reduce the population size of its prey in virtually every coastal habitat (Cohen et al. 1995, Mascaro & Seed 2000).

To document their potential population growth and impact, trapping surveys were carried out during 2008, 2009 and 2010 in two prime bivalve harvesting estuaries located in southern PEI: North River and Bedeque Bay. The trapping surveys confirmed that there is an ongoing spread of green crabs into new shellfish habitats while rapidly increasing in previously established areas.

To help prioritize future research, this study explored size-dependent green crab prey preference on C. virginica, M edulis, and M. arenaria in a coastal system of PEI. Using two sizes of prey (15-25 and 25-35 mm shell length; SL) and three different experimental manipulations, small (35-45 mm carapace width; CW), medium (45-55 mm), and large (55-75 mm) green crabs were given a choice among these three bivalves and their daily feeding rates were monitored over the course of three days. For both prey sizes, green crabs showed an early feeding preference for soft-shell clams and, only as they declined in numbers, a switch towards mussels and subsequently towards oysters. Such changes in order and timing of prey preference were directly related to differences in prey shell thickness, a fairly reliable indicator of prey shell-strength.

As the C. virginica industry is believed to be the most threatened by the green crab invasion, three types of experiments in the laboratory and the field were utilized to assess predation rates exhibited by small (35-45 mm CW), medium (45-55 mm), and large (55-75 mm) green crabs feeding on small (5-15 mm SL), medium (15-25 mm), large (25-35 mm), and extra-large (35-50 mm) oysters. All experiments lasted three to five days but oyster mortality was assessed on a daily basis. Overall, the outcome of green crab-oyster predator-prey interactions was heavily dependent on both oyster size and crab size. While almost no predation occurred on extra-large oysters, large and medium green crabs preyed heavily on all other sizes, usually in the first three days of manipulation. Small and medium oysters were the most vulnerable to predation but reached a partial size refuge at ∼35 mm SL.

Under that size threshold, oysters remain highly vulnerable and require management measures in order to secure their sustainability. As green crabs continue to spread in PEI and the Gulf of St. Lawrence, further mitigations strategies are likely to be needed. The results presented in this thesis provide the foundation for future developments of these mitigation strategies.