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. Blue crabs, Callinectes sapidus (Rathbun, 1896), are mobile opportunistic predators and scavengers that occupy estuarine and marine habitats in the western Atlantic ranging from Cape Cod, USA to Brazil (hines et al., 1995). habitat use by blue crabs within an estuary changes ontogenetically (e.g., hines et al., 1987, 1995). In Chesapeake Bay, USA, blue crabs < 100 mm carapace width (CW, maximum dimension from point to point across the carapace) remain in their natal estuaries until the end of their first year and then move to adjacent deeper water to overwinter (hines et al., 1995). In their second year, Chesapeake blue crabs 120–200 mm CW are sexually mature and seasonally move into and out of smaller estuaries to forage and mate (van Engel, 1958). Males typically remain in the Chesapeake Bay and its tributaries year round but may move upriver in spring and fall with females following in midsummer (Churchill, 1919; van Engel, 1958; Milliken and Williams, 1984; hines et al., 1987). Mating occurs during the summer months in the bay and its tributaries (Churchill, 1919; van Engel, 1958). Then, as water temperatures decrease during the fall, most females migrate to overwintering grounds in the mud of the southern Chesapeake Bay (Churchill, 1919; van Engel, 1958). historically, Chesapeake Bay estuaries offered a gradient of nursery and foraging habitats for blue crabs. Shallow salt marshes (e.g., Orth and von Montfrans, 1990; Thomas et al., 1990; Fitz and Wiegert, 1991; Minello, 1999) and seagrass beds (e.g., Orth, 1977; Orth and von Montfrans, 1987; Wilson et al., 1987; Sogard and Able, BULLETIN OF MARINE SCIENCE, vOL. 86, NO. 1, 2010 76 1994; Minello, 1999) are important nursery and forage habitat for post-settlement and juvenile crabs (< 100 CW mm) as well as adults (> 100 mm CW). Biogenic structures created and maintained by Eastern oysters [Crassostrea virginica (Gmelin, 1791)]; three dimensional structures are termed “reefs”, two-dimensional structures are termed “bars”) are also used as habitat by blue crabs (e.g., Wells, 1961; Coen et al., 1999; harding and Mann, 1999; posey et al., 1999; Meyer and Townsend, 2000; Lenhert and Allen, 2002). Infaunal soft sediment habitats between marshes, seagrass beds, and oyster reefs provide additional crab foraging habitat (virnstein, 1977; Seitz et al., 2005). prior to the 20th century, seagrass beds and oyster reefs spatially dominated mesohaline shallow (< 3 m) Chesapeake Bay habitats forming a mosaic of suitable habitats for crabs. The late 20th century decline of Chesapeake Bay seagrass beds (Orth and Moore, 1983) and oyster reefs (haven et al., 1981; Rothschild et al., 1994; hargis and haven, 1999) in terms of absolute abundance and spatial coverage has drastically reduced the availability and heterogeneity of these traditional crab habitats within Chesapeake Bay estuaries. Recent management efforts focused on enhancement of natural oyster populations and construction of oyster reefs to encourage oyster recruitment, may indirectly provide habitat enhancement for blue crabs (e.g., peterson et al., 2003). The piankatank River is a small, relatively pristine Chesapeake Bay tributary. during the 1990s, it was the location of focused oyster restoration efforts (e.g., Mann et al., 1996; Bartol and Mann, 1997, 1999) due to its history as a small, trap-type estuary (Andrews, 1979) with regular oyster recruitment, the absence of commercial oyster harvesting activity, and a relatively undeveloped watershed. A limited number of commercial watermen fish for blue crabs in the river. during 1996–1997, a multi-species monitoring program for fishes (harding and Mann, 1999, 2000, 2001a,b, 2003) and decapod crustaceans (Mann and harding, 1997, 1998) provided the opportunity to examine blue crab habitat use patterns around a constructed oyster reef in relation to a natural oyster bar and a sand bar. Our objectives were to quantitatively describe blue crab CpUE, sex, and size (1) around a constructed oyster reef between years (1996 vs 1997) and (2) across habitat types (sites) within 1 yr (1997) and (3) relate observed habitat use patterns to ambient environmental (water temperature, salinity, daylength) conditions. Materials and Methods