A comparison of the distribution and abundance of European green crabs and American lobsters in the Great Bay Estuary, New Hampshire, USA

- Distribution and abundance of invasive green crabs to determine an impact on lobster populations.
- Green crabs were found in warm, low salinity regions while lobsters were in colder, more saline areas.
- In lab trials, lobsters were more aggressive than green crabs, except adult green crabs paired with small lobsters.
- Data suggest that green crabs could have a moderate impact on lobster populations in estuaries.

Green crabs (Carcinus maenas) are an invasive species documented as having negative impacts on the biota of marine and estuarine communities. However, their impact on the American lobster (Homarus americanus) is not well understood. During a two-year trap study (2013-2014) in The Great Bay Estuary, NH, we captured 1229 green crabs and 144 lobsters in 248 individual trap hauls (average catch per unit effort = 10.98 ± 1.51 for green crabs and 0.49 ± 0.08 for lobsters), or ∼8.5 times more green crabs than lobsters. In general, green crabs were more abundant in areas furthest from the coast (up-estuary), which also tended to be warmer, while lobsters were more abundant in areas closer to the coast (down-estuary). Nevertheless, there was still considerable overlap between the two species. We evaluated the competitive interactions between green crabs and lobsters in the laboratory using a behavioral assay and found that in 31% of the trials, large lobsters (>80 mm in carapace length) killed (and consumed) green crabs of varying sizes that failed to escape or move to safe areas of the enclosure. These results suggest that adult lobsters are not likely vulnerable to green crabs. While there may be reasons why lobsters did not select specific sizes of green crabs to prey on, some crabs may have an impact on juvenile lobsters. These data provide some insight into the distribution and abundance of green crabs and their impact on the lobster population in a large New England estuary that supports a commercial lobster fishery.