Observations of distribution, size, and sex ratio of mature blue crabs, Callinectes sapidus, from a Chesapeake Bay tributary in relation to oyster habitat and environmental factors

Blue crabs Callinectes sapidus (Rathbun, 1896) > 100 mm carapace width were sampled from a constructed oyster reef (1996 and 1997), a sand bar (1997) and a natural oyster bar (1997) in the piankatank River, Chesapeake Bay, USA to describe habitat use, sex ratios, and demographics across a gradient of habitat types. patterns of blue crab catch-per-unit-effort (CPUE), and demographics were similar on the oyster reef in 1996 and 1997. Average annual CPUE on the reef was 6–8 crabs pot⁻¹ with maximum CPUE of 15 crabs pot⁻¹. Daylength and water temperature significantly affected reef CPUE with more crabs observed in late August and early September. In 1997, average annual CPUE at the natural oyster bar was higher (9 crabs pot⁻¹) than on the reef or the sand bar (both 6–7 crabs pot⁻¹). Observed differences in habitat use may relate to site-specific differences in depth and tidal current as well as the presence of living oyster (biogenic) substrate. A transition in the sex ratio of crabs was observed as daylength declined seasonally. In May, males were 3–5 times more abundant than females at all sites but by early September, as daylength and water temperatures declined, female crabs were 3–4 times more abundant than males at all sites. The median size of males and females increased from spring into summer and female crabs were typically larger than males from the same habitats across all habitat types. The largest female crabs were observed in habitats with oysters. Biogenic oyster habitats are important estuarine habitats for blue crabs as well as oysters.