EUROPEAN GREEN CRABS IN MAINE: TRAPPING, PEAT STABILITY MEASUREMENTS, AND COMPUTER AIDED TOMOGRAPHY OF CORES REVEAL MARSH IMPACTS

Recent studies indicate that intertidal crabs contribute to substantial changes in marsh morphology, including plant dieback, erosion, and marsh loss through foraging and burrowing activities. The European green crab, Carcinus maenas, is a highly successful invader established in southern Maine since the 1890's. In the mid-coast region, green crabs are implicated in the widespread destruction of softshell clam flats, eelgrass beds, and salt marshes since 2012. This study quantifies green crab abundance at salt marshes in Damariscotta, Yarmouth, and Wells using fyke nets and baited, modified eel traps. Concurrent sampling in June and August revealed much greater green crab activity in the tidal channel compared to the marsh surface and/or greater trap efficiency. Crab densities ranged from 0-0.5 crabs/m²with greater densities observed during the day for most sites and the lowest densities at Yarmouth, until September which may be linked to cooling water temperatures. Trapping data from June, July, and August revealed that Yarmouth had many fewer crabs (0.2-3.0 crabs/hour) compared to Damariscotta (24.4-30.0 crabs/hour) or Wells (26.3-46.3 crabs/hour). The greatest sampling event to-date was at Wells in August, where 1,110 crabs were captured. Shallow (10 cm) and deep (30-50 cm) measurements of peat shear vane strength in healthy, reference versus crab-burrowed, impacted creek banks revealed that eroding banks had lower measured torque values compared to healthy marsh and that in eroding banks, no difference in torque with depth was observed, suggesting little to no live root zone. Preliminary analyses of computer aided tomography of geologic cores reveal clearly imaged crab burrows and the absence of live coarse roots and rhizomes in impacted versus reference cores in at least one site (Wells). Understanding spatial differences in population trends and direct impacts to marshes, may help inform management options for the species in the State of Maine.