An invasive population of the Asian shore crab Hemigrapsus sanguineus was discovered in 1988 near the mouth of Delaware Bay, and populations now occur from North Carolina to Maine. The shore crab H. sanguineus competes with indigenous species and has displaced resident crabs throughout its invasive range. However, there have been few studies that document changes in populations of H. sanguineus after the species has become established.We compare sympatric populations of the Asian shore crab and a native mud crab (Panopeus herbstii) that were monitored initially in 2001 and again in 2011 and 2012. The historical study was conducted in a rocky habitat near Cape Henlopen at the southern terminus of Delaware Bay (38.793° N, 75.158° W). Results showed large differences in the relative abundance of the two species throughout the duration of the study. The Asian shore crab H. sanguineus accounted for 75% of total crab abundance in 2001, but abundance had decreased to less than 25% in both 2011 and 2012. Similar results were obtained when we compared the two species in terms of biomass. Additional sampling in 2012 showed comparable low values for H. sanguineus when compared with P. herbstii at two stations about 25 km and 50 km farther south along the coast. In contrast, H. sanguineus was strongly dominant at a station 50 km north of the historical sampling site. Percentage rock cover and size of rocks varied little among sampling locations, and all sites were proximal to the coastal ocean. However, basal sediment at the northern station was coarser than sediments at the other sites, which may have restricted the occurrence of mud crabs. Overall results of the study indicate a resurgence of native mud crabs at sites where sedimentary characteristics provide adequate habitat.
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Vol. 32 • No. 3