Dynamics of indigenous rock crab (Cancer irroratus) and invasive Green Crab (Carcinus maenas) populations during invasion of a southern Newfoundland estuary

Little literature exists on green crab (Carcinus maenas) and particularly rock crab (Cancer irroratus) in Newfoundland (Canada) waters. In this study, we document demographic composition for the two species upon invasion of green crab into the North Harbour, St. Mary’s Bay, estuary, an area previously only occupied by rock crab and undergoing progressive warming. Using data collected via a citizens science approach, we address objectives of documenting the rate of change in speciation upon green crab invasion in the estuary and providing novel data on basic life history processes for rock crab and green crab in Newfoundland waters. The study shows a rapid proportional switch in species composition in the estuary upon green crab invasion, with green crab increasing from a proportion of 0 to 0.75 of collected samples within three years of being detected. Novel data on rock crab suggest overall consistencies with life history processes as described in the broader literature, including presence of molting periods centred near May and August, a shallow water mating migration in fall, and a similar size-at-instar structure as crab along the eastern United States. Novel data on green crab biology suggests the St. Mary’s Bay population has similar life history attributes as the original invading population in Newfoundland, including spring-summer spawning and an inferred paucity of molting in fall. Allometric carapace relationships of the North Harbour green crab are the same as those described in the Pacific and northeast Atlantic oceans. The size structure of green crab in the North Harbour estuary has broadened in the three years since initial detection with increased presence of large crab beyond the supposed size-at-maturity in recent years.