Figure 1. — A molting blue crab emerging from its old shell. 



MATERIALS AND METHODS 



Juvenile blue crabs, 20 to 139 mm. wide, were 

 held in wooden floats in salt water (salinity greater 

 than 5 p.p.t. — parts per thousand) from April 

 1964 to March 1965 and in fresh water (salinity 

 less than 1 p.i).t.) from April 1965 to March 

 1966. Studies in salt water were in Cedar Point 

 Creek, a tidal tributary of Clapboard Creek, 

 which enters the St. Johns River 15 km. from the 

 Atlantic Ocean. Studies in fresh water were in 

 the St. Johns River near Green Cove Springs, 

 90 km. upstream from the mouth of the river. 

 Both sites were natural habitats for crabs in the 

 size range used. 



Tavo hundred crabs in fovn- floats were used at 

 each location. The floats had hinged covers and 



were about 3 m. long, XYi m. wide, and Yi m. 

 deep. A 15-cm.-wide rim allowed thi-ee-fourths 

 of the float to hang submerged. Each float had 

 50 compartments, 24 cm. by 24 cm. by 46 cm. 

 deep, covered with 6.4-mm.-mesh hardware cloth. 



Blue crabs were collected near the floats, and 

 only one animal was put in each compartment 

 (to prevent cannibalism). About 2,000 of those 

 captured were measured for length-width ratio. 



Tlu-ee times each week crabs were fed, those 

 that liad molted were measured, and the dead 

 ones were removed. The crabs normally were 

 sui^iilied witli more cut fish than they would eat. 

 I measured carapace width and length of each 

 animal tluit had molted since the last observation; 

 •sliell hardness indicated the day of ecdysis. Width 



2S2 



U.S. FISH AND WILDLIFE SERVICE 



