Growth increments were highly variable in each 

 size group, but no blue crab molted without some 

 increase in size. The smallest increase in width 

 was 7.8 percent; the largest, 50 percent. Mean 

 ]3ercentage growth ])er molt by 10-mm. -width 

 groups was 20.9 to 34.2 (table 2), and the groups 

 averaged 26.7 for females, 23.9 for males, and 25.3 

 for females and males combined. Among larger 

 crabs, females usually had greater width incre- 

 ments (at greater ages, they are relatively wider 

 than male crabs). On the assumption of homo- 

 geneous variance about tlie regression of postmolt 

 width on premolt width, an analysis of co variance 

 indicated significant interaction of sex and salinity 

 on growth (P <0.01). Because of this interaction 

 the separate effects of salinity on males and females 

 were examined. Each sex showed significant differ- 

 ences in growth between fresh and salt water 

 (P <0.05). All crabs less than 80 mm. wide and 

 many larger ones had greater average growth in 

 salt water (fig. 2) . 



Some factor other than salinity appears to 

 account for larger crabs in certain low-salinity 

 waters. In the ])resent experiments and in earlier 

 ones on the terminal molt of female blue crabs 

 (Haefner and Shuster, 1964), decrease in salinity 

 did not ])roduce increase in size. 



Female blue crabs undergo a final molt at which 

 they become sexually mature. This stage is easily 

 recognizable because the abdomen changes shape 

 from triangular to semicircular (the last stage of 

 males cannot be determined from external aj)- 

 pearance). In the St. Johns River, as in Chesa- 

 peake Bay (Tyler and Cargo, 1963), the sizes at 

 the last two instars overla]) widely. The smallest 

 mature female measured was 99 mm. wide, and 

 the largest immature female was 177 mm. 



I obtained data on the terminal molt of 170 

 females (83-139 mm. wide) — 85 at each site. 

 Most growth generally took jjlace at this molt. 

 The mean increase in width was 34.4 jiercent 

 (range, 19.6-50) in salt water and 30.2 percent 

 (range, 20.5-47.4) in fresh water. 



The variability of growth of juvenile crabs 

 causes irregularity in the recruitment of harvest- 

 able crabs (larger than about 120 mm. wide) and 

 in the time of crab maturity. For example, of two 

 21-mm.-wide males that molted seven times in 

 salt water from May to August (in 101 days and 

 103 days), one attained 139 mm. and the other 

 only 115 mm. The smaller crab would be more 



T.\BLE 2. — Relative growth of blue crabs, by lO-mm.-width 

 groups and by sex, in salt (S) and fresh (F) water, St. 

 Johns River, Fla., April 1 to November 15, 1964 and 1965 



likely to escape the fishery to complete its life 

 history (crabs smaller than 120 mm. made up less 

 than 5 ])ercent of the commercial catch). Another 

 example is the difference in growth of two females 

 in salt water that were 34 mm. wide in spring. One 

 matured after four molts and 78 days (114 mm.) ; 

 the other after seven molts and 179 days (175 

 mm.). The individual maturing at 114 mm. did 

 not attain harvestable size and probably would 

 have sjiawned in summer. The 175-inm. female 

 was acceptable for market during its last two 

 instars and had it escaped the fishery it jirobably 

 would have spawned in fall. 



WINTER 



Blue crabs grow during the winter in the St. 

 Johns River but more slowly than in summer. 

 I observed 287 molts among crabs in salt water 

 (November 20, 1964, to March 31, 1965) and 258 

 in fresh water (November 22, 1965, to March 31, 

 1966). 



Water temperatures in winter averaged 11.8° C. 

 lower in salt water and 13.5° C. lower in fresh 



284 



U.S. FISH AND WILDLIFE SERVICE 



