Science Advisory Report 2010/033

Ecological Assessment of the Invasive European Green Crab (Carcinus maenas) in Newfoundland 2007-2009

Summary

Green Crab Population Dynamics

• Following the first reports of the invasive green crab in North Harbour, Placentia Bay, Newfoundland in August 2007, the species is spreading rapidly throughout several areas of Placentia Bay and St. Georges Bay on the west coast of the island.
• The presence of several size classes, reflecting multiple year-classes, indicates that a reproductive population of green crab has established in northern Placentia Bay in abundances several orders of magnitude greater than populations previously documented in the Maritime provinces and the western United States and Canada.
• Genetic analyses of green crab in Newfoundland indicate a close relationship with a cold-tolerant green crab population introduced into the Maritimes in the late 1980’s. Anthropogenic activity through vessel traffic is the most likely vector of introduction into Newfoundland.
• Green crab population density information is a critical knowledge gap in population dynamics.

Biological Impacts of Green Crab on Native Species and Habitats

• In areas of high green crab abundance, the impact of green crabs is substantial on commercial and noncommercial mollusk and crustacean species and the natural habitat.
• A direct relationship between the invasive green crab and the Newfoundland native rock crab, Cancer irroratus, is indicated where the native species appears to be displaced or preyed upon by the invasive green crab.  Similar declines and impacts on native crabs in other regions of North America have been reported in areas of high or even moderate green crab invasion.
• Shellfish have been determined to be the predominant prey in the Newfoundland environment (as is the case in other regions). Soft-shell clam beds and scallop beds, particularly smaller scallops, are affected in areas where these beds are located in more shallow areas with high green crab concentrations.  Predation on American lobster is a significant concern as gut content analysis, laboratory trials and anecdotal reports indicate that green crab can and do prey on juvenile and trapped adult lobster.
• Eelgrass beds have been noticeably reduced in recent years in Newfoundland where green crabs are in high concentration. Green crab burrowing for shelter and digging for prey in eelgrass beds have been shown in many areas of North America to substantially reduce the amount of eelgrass and reduce coverage of this ecologically and biologically significant habitat.
• Although research has shown that green crab do impact biodiversity and habitat, the threshold levels for impact or critical number of green crab per area has not been determined and has been identified as a critical knowledge gap.

Mitigation Measures and Their Effectiveness to Control Green Crab Populations

• Direct removal of green crab by trapping has been determined to be an effective mitigation measure to evaluate. Trapping is the current experimental control method in California, Prince Edward Island and Nova Scotia as well as in Newfoundland.
• In all cases (across regions), analyses of trapping methods indicate that catch rates decrease with concentrated trapping. Continuous trapping gradually reduces the average size of green crab, eventually shifting the green crab from being primarily a predator to a more vulnerable size as prey for several native predators, e.g. shorebirds and some larger crustaceans.
• In areas where intense trapping takes place, the native rock crab species numbers often increase over time.
• Intense trapping appears to be an effective control method for mitigation of green crab invasions; however, threshold levels of green crab population densities, timelines for action relative to impact, and measures of success still need to be well-defined based on specific environments.
• Even with incomplete or inadequate information and with many uncertainties, control of this species by whatever means appropriate, is supported by the precautionary approach as green crab has been flagged as a high risk species both ecologically and economically through National risk assessment exercises.

This Science Advisory Report is from the Fisheries and Oceans Canada, Canadian Science Advisory Secretariat, Regional Advisory Meeting of March 17, 2010 on The Newfoundland Green Crab Populations and Mitigations.  Additional publications from this process will be posted as they become available on the DFO Science Advisory Schedule.

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