Population dynamics and habitat partitioning by size, sex, and molt stage of blue crabs Callinectes sapidus in a sub-estuary of central Chesapeake Bay

Abundances, size-frequency &stributions, sexual composition and molt-stage composition of blue crabs Callinectes sapjdus were measured during 1983 to 1985 in the Rhode Rver, a subestuary of central Chesapeake Bay, USA. Crabs at the mouth and head of the river basin were sampled with monthly triplicate otter trawls. Crabs in the principal tidal creek of the river were sampled 3 d a week with a fish weir, which caught crabs moving upstream and downstream separately. Crabs exhibited consistent, marked seasonal cycles in abundance as well as considerable annual variation in July peak abundances. New recruits entered the subestuary in late fall and spring and grew rapidly to 70 to 100+ mm in their first summer; by the second year they reached mature sizes of 120 to 170 mm. Sixty % of crabs in the river basin were males; and after maturation and copulation in late summer to early fall, mature females left the subestuary. Crabs partihoned habitats within the Rhode fiver subestuary by size, sex, and molt stage. Polymodal size structures were similar throughout the river basin, but increased percentages of males were found at the head of the river. Predominantly (90 %) medium-sized (80 to 120 mm) males uhhzed the tidal creek as a molting habitat. Most crabs moving upstream were in premolt, whereas most crabs moving downstream were significantly larger and in postrnolt, so that about 70 % of crabs in the creek were near ecdysis. Males and females exhibited significantly lower, but still appreciable (about 25 O h ) rnolting activit; Lhrsughout t i e river basin. These data provide one of the best documented cases of habitat partitioning by size, sex and molt stage in crustaceans.