Blue crab, Callinectes sapidus, response to the invasive common reed, Phragmites australis:: Abundance, size, sex ratio, and molting frequency

Much effort has been directed recently at restoring marshes, by the removal of the invasive common reed, Phragmites australis, yet it is not clear how fish and invertebrates have responded either to the invasion of Phragmites or to marsh restoration. The blue crab, Callinectes sapidus, uses marsh habitats during much of its benthic life. We investigated the response of blue crabs to Phragmites invasion and restoration efforts by comparing crab abundance (catch per unit effort), mean size and size frequency distribution, sex ratio, and molting of crabs in three physically similar areas differing in marsh vegetation; Spartina-dominated, Phragmites-dominated, and a treated area (Phragmites removed and now dominated by Spartina) in one marsh in the upper portion of Delaware Bay. Field sampling occurred monthly (April to November) from 1999 to 2001 using replicate daytime otter trawls in large marsh creeks. Crabs were categorized by carapace width into recruits (< 30 mm), juveniles (30-1.15 mm), and adults (> 115 mm). juveniles dominated the system, representing 69.4% of all crabs. Similar monthly increases in mean size and molting patterns during the growing season (May-August) occurred in Spartina (natural and treated sites) and Phragmites sites suggesting that, subtidal habitats, used for molting, in these areas do not differ. More juveniles in the feeding molt stage (i.e., intermolt) than in other molt stages and more recruits predominantly in the feeding molt stage than adults were in Spartina, suggesting differences in the marsh surfaces used as feeding habitats with Spartina being preferred. Sex ratios of each life history stage were skewed towards males, but this was related to the low salinity of Alloway Creek, rather than marsh surface vegetation. Our results suggest that marsh surface vegetation influences the way blue crabs use marsh surface habitats, thus restoration efforts focusing on changing vegetation type may have a positive influence on blue crabs.