Population Studies of Blue Crabs in Soft-bottom, Unvegetated Habitats of a Subestuary in the Northern Gulf of Mexico

Abstract

Studies were conducted to assess seasonal and interannual changes in population abundance, size frequency, recruitment, sex ratio, molt condition, and the reproductive cycle of the blue crab Callinectes sapidus in Weeks Bay, Alabama, a subestuary of Mobile Bay. Populations sampled from soft-bottom, unvegetated sites from May 1989 to August 1991 varied significantly in density, but with little predictable annual pattern. Brief periods of peak abundance occurred sporadically in summer and spring months, followed by relatively long periods of relative stable abundance. Interannual variations in abundance occurred, with numbers of crabs much higher during 1989-1990 than 1990-1991. A cohort of subjuvenile crabs (<20-mm CW) was detected in late summer of 1989. New recruits attained a size of 20-40-mm CW by fall 1989 and exhibited little growth until spring of 1990, when they grew rapidly to the 40-80-mm CW size classes. A second cohort of recently settled subjuveniles was observed in March 1991. At least 2 year classes of crabs coexisted within the bay during the study. Most crabs collected were juveniles (20-80-mm CW), indicating that the bay serves as an important nursery area. Adult blue crabs (>80-mm CW) occurred mostly at the mouth of the bay. Sex ratios were skewed towards males when examined over the entire course of the study period. This was due to an excess of males in 1989. Crabs which had heavily fouled or soft carapaces (near or recent molt) were collected more frequently in the warm season (April through October) than the cool season (November through March). An annual reproductive cycle was evident, with gonadal maturation occurring in the summer and fall. No berried female crabs were collected, indicating that sexually mature females migrate out of the subestuary to hatch their eggs at higher salinities.