Table 11. — Food of the blue crab, by crab size and by season, in the St. Johns River as percentage frequency of occurrence 



and {in parentheses) as percentage of total volume per stomach 



[Number of stomachs with food in parentheses below designation of season] 



Food item 



Bryozoans 



Mussels 



Clams 



Oysters 



Snails- 



Nereis pdagica 



Other annelids 



Amphipods- 



Barnacles 



Ostracods 



Shrimplilje forms.. 



Crabs. 



Other crustaceans.. 



Insects 



Fish 



Algae 



Vascular plants — 

 Organic debris 



Width in mm., season, and number of stomachs with food 



5-50 



Dec.-Feb. 

 (61) 



26(13) 



■6(0) 

 '"6(3) 



2(1) 



6(6) 



3(2) 



18(14) 



■2(0) 



30(23) 



23(10) 



5(1) 



38(28) 



Mar.-May 

 (160) 



11(0) 

 4(2) 

 34(20) 



2(2) 

 17(4) 



11(6) 



11(0) 



1(1) 



1(1) 



7(4) 



13(6) 



6(3) 



20(11) 



17(5) 



8(1) 



66(36) 



June-Aug. 

 (85) 



7(2) 

 44(20) 



16(4) 

 18(4) 



16(7) 

 ■1(0) 

 12(0) 

 ■2(0) 



9(2) 

 18(12) 



6(2) 

 33(13) 

 11(0) 



7(2) 

 60(31) 



51-100 



Dec.-Feb. 

 (21) 



14(9) 

 24(15) 



■5(0) 

 28(14) 



10(2) 

 "6(1") 



57(44) 

 24(3) 

 ■5(0) 

 14(10) 



Mar.-May 

 (74) 



11(0) 



44(35) 



28(20) 



13(0) 



9(1) 



■1(0) 



24(11) 



■3(0) 

 22(8) 

 13(0) 

 ■1(0) 

 24(14) 

 1(1) 

 14(2) 

 15(6) 



June-Aug. 

 (44) 



27(15) 

 60(26) 

 4(3) 

 9(3) 

 16(2) 



32(13) 

 12(0) 



■4(0) 

 23(8) 

 9(3) 

 12(0) 

 32(16) 



14(1) 

 18(10) 



101-200 



Dec.-Feb. 



(25) 



20(14) 

 40(31) 



12(1) 



48(42) 

 24(3) 

 ■8(0) 

 12(9) 



Mar.-May 

 (143) 



■2(0) 

 52(42) 

 28(13) 

 6(2) 

 ■1(0) 

 ■3(0) 



■6(0) 



■1(0) 

 ■1(0) 

 11(2) 

 ■1(0) 

 ■1(0) 

 41(27) 

 ■1(0) 

 9(3) 

 17(9) 



June-Aug. 

 (55) 



■4(0) 

 42(29) 

 38(22) 

 4(2) 

 ■2(0) 



22(6) 

 7(2) 

 ■2(0) 

 ■2(0) 

 25(7) 



■2(0) 

 26(17) 



5(1) 

 24(12) 



1 Less than 0.6 percent. 



The year-round abundance of preferred food 

 items undoubtedly is a major cause for the large 

 concentrations of juvenile blue crabs near Picolata. 

 Random sampling with trawl and dredge below 

 Picolata (between buoys 18 and 22) in summer 

 indicated relatively less plant cover and less food, 

 particularly amphipods and mussels. Juvenile 

 crabs that fed on mollusks in the lower river con- 

 ceivably are attracted to an accustomed food, such 

 as mussels, at Picolata. Crabs also benefit from an 

 interesting food chain transition at buoy 24. Young 

 mussels occur most commonly within large clam 

 valves. When the mussels are chewed off, chirono- 

 mid larvae build algal tunnels on which amphi- 

 pods feed and provide a second feeding for crabs. 



In brief, blue crabs 5 to 200 mm. wide fed prin- 

 cipally on mollusks, fish, and crustaceans and to a 

 lesser degree on plants, annelids, insects, and biyo- 

 zoans. Crabs generally ate the same type of food 

 regardless of crab size, area, and season. They 

 usuallj' consumed the most available foods. 



SUMMARY 



Studies on the biology of the blue crab in the St. 

 Johns River were made to obtain information nec- 

 essary to determine factors which affect the har- 

 vestable population. 



Females made up the majority of adult crabs 

 during most of the year in the lower St. Johns 

 River, but males were dominant in the upper river. 



Blue crabs commonly mated from March to July 

 and from October to December. The proportion 

 of males and females that matured at a small size 

 was larger in salt water than in fresh water. Size 

 at maturity varied wudely as evidenced by a 

 177-mm. wide immature female and a 99-mm. 

 mature female; some of this variation can be at- 

 tributed to individual differences in growth rate. 



Spawning occurred in the first 30 km. of river 

 above the mouth, and the eggs hatched in the ocean 

 within 6 km. of shore. Spawning began in Febru- 

 ary 1962 and in March 1963 and continued to Oc- 

 tober in both years. An appreciable number of 

 ovigerous females from the St. Johns River ap- 

 parently entered the ocean by way of the Intra- 

 coastal Waterway and other rivers. Some females 

 entered the river from the ocean before the spawn- 

 ing season. Many others returned to the St. Johns 

 River or entered other rivers, often within 15 days 

 after their eggs hatched, to spawn a second time 

 in the same season. 



Of crabs tagged 40 to 195 km. above the mouth 

 and recovered at a distance from the tagging site, 

 89 percent of the males and 96 percent of the fe- 

 males were recaptured downstream from the i^oint 

 of release. In fall and early winter, females ft-om 

 the upper river congregated 30 to 55 Ian. above the 

 mouth. During the same period aggregations of 

 males appeared nearer the mouth and in the ocean 

 directly off the mouth. Crabs of both sexes mi- 



BIOLOGY OF BLUE ORAB IN ST. JOHNS RIVER. FLORIDA 



31 



