Resilience of coastal ecosystems to green crab introduction and removal

Principal Investigators: Sophie St-Hilaire & Maya Groner

In the north Atlantic, oysters and eelgrasses provide essential habitat for many ecologically and commercially important fisheries, such as halibut, soft-shelled clams, Rock crab, salmon, oysters, mussels. The non-native green crab may threaten these habitats by consuming molluscs, including reef-forming oysters, and by digging and degrading the quality of eelgrass beds. The resilience of these ecosystems to green crab disturbances is unknown.

The Department of Fisheries and Oceans recently classified eelgrass habitats in Atlantic Canada as a conservation priority, as they provide critical habitat for economically important fisheries, stabilization of sediment, carbon trapping, and buffering of seawater pH. A number of stressors are associated with declines of eelgrass, including invasive green crabs, wasting disease, and over-fertilization by excess nutrients.

We conducted field surveys to quantify the effects of invasive green crabs and wasting disease on eelgrass health (shoot growth, density, and quality). We surveyed 13 sites in PEI and New Brunswick in early and late summer to determine the status of and evaluate the effects of green crabs on eelgrass health and density.

We found that the prevalence of wasting disease varied between 5 and 90% in the early summer, but increased, across all sites, to more than 80% by late summer. Shoot density was correlated with disease prevalence in the early summer, but not during late summer, suggesting that eelgrass density, which affects transmission and competition, may facilitate disease progression. Surprisingly, disease prevalence and population density were not associated with green crabs; however, 2014 had unusually low green crab populations, so an effect may not have been detectable.