69 days to reach 20 niin. The mean percentage 

 growth per nioU was the same as in hirger crabs. 

 To estimate the time needed to attain harvestable 

 size, I useil 25.:^ percent to calcidate rehitive 

 increases from tlie 1st crab stage (2.5 mm.) to 

 a 19th crab stage (142 nun.), applied mean molt 

 intervals, and allowed V/i months for larval 

 development (Costlow- and Bookhout, 1959). 

 Tliese calculations indicate that most blue crabs 

 in the St. Johns River reach harvestable size 

 within 1 year after hatching (table 6). 



T,\BLf; 6. — Estimated growth by month of St. Johns River, 

 Fla., blue crabs hatched in April, July, and October 



Month 



April 



May 



June 



July 



August 



September- - 



October 



November.. 

 December. . 



January 



February... 



March 



.\pril 



May 



June 



July 



August 



September.. 



Maximum width 



Mm. 



11 



5 



12 



23 



46 

 58 

 90 

 113 

 113 

 113 

 113 

 142 



n 



5 

 12 

 23 

 46 

 46 

 58 

 58 

 72 

 90 

 113 

 142 



Mm. 



 1 



5 



8 



10 



12 



15 



29 



46 



72 



90 



113 



142 



' Larvae. 



MORTALITY OF EXPERIMENTAL 

 ANIMALS 



I recorded deaths of 94 blue crabs in salt water 

 and 212 in fresh water during the summer. Mor- 

 tality was 8 percent for crabs 20 to 79 mm. wide 

 and 19 percent for those 80 to 139 mm. Most 

 crabs that died were well i)ast the average molt 

 interval of their size group. 



Only eight animals died in salt water (two after 

 molting once) and seven in fresh water (three 

 after molting once) during the winter — a mortality 

 of 3 percent. 



Year-round mortality at ecdysis was 36 crabs 

 (salt water, 11; fresh water, 25) out of a total of 

 2,782 molts. The few deaths at ecdysis indicate 

 that molting in itself may not be a critical source 

 of mortality. In nature, however, crabs are easy 

 prey immediately' after they emerge from their 

 shells. 



SUMMARY 



From April 1964 through March 1966, juvenile 

 blue crabs (20-139 mm. wide) were held in floats 

 in St. Johns River to determine growth incre- 

 ments, molt intervals, and the effects of salinity 

 and temperature on gi'owth. 



Measurements of 2,000 blue crabs, 10 to 149 

 mm. wide, collected in the river indicated that the 

 length- width ratio decreased as the animals grew. 

 This decrease was less among males than females. 



In salt water (salinity greater than 5 p.p.t.), 

 data were obtained on 1,085 molts during the 

 summer (April 1 to November 15) and on 287 

 during the winter (November 20 to March 31); 

 in fresh water — on 1,152 molts in summer and 258 

 in winter. 



Growth increments per molt varied from 7.8 

 to 50 percent in summer, and mean increment, by 

 lO-mm.-width groups, was 20.9 to 34.2 percent. 

 Among larger crabs, females usually had greater 

 width increments than males. Percentage growth 

 in the terminal molt of females averaged 34.4 

 in salt water and 30.2 in fresh water. Males and 

 females showed significant difference in growth 

 between fresh and salt water. Each sex generally 

 averaged the most growth per molt in salt water. 



Relative growth per molt in winter was similar 

 to that in summer. 



Molt intervals in the summer did not appear 

 to be affected by the sex of the crab and the 

 salinity of the water, but were affected by crab 

 size and water temperature. The smallest crabs 

 (20-29 mm. wide) molted after an average of 11 

 days and animals over 120 mm. wide after an 

 average of 42 days. Molt intervals usually were 

 sliortest during the warmest months. The interval 

 of the terminal molt of females averaged 38 days 

 in salt water (mean width, 106 mm.) and 40 

 days in fresh water (mean width, 117 mm.). 



The time between molts was three to four times 

 as long in winter as in summer. Most juveniles 

 20-59 mm. wide molted two or three times, but 

 many of those larger than 99 mm. did not molt. 



Calculations based on the data presented 

 indicate that most blue crabs in the St. Johns 

 River reach harvestable size within 1 year after 

 hatching. 



Year-round mortality at ecdysis at both sites 

 was only 36 crabs out of a total of 2,782 molts; 

 other deaths totaled 285. 



GROWTH OF JUVENILE BLUE CRABS IN ST. JOHNS RIVER, FLA. 



287 



