1961, 1964, 1966) ; Iversen and Idyll (1960), Iver- 

 sen and Jones (1961), and the sampling studies 

 in the area by Ingle et al. (1959), Iversen et al. 

 (1960), and Eldred et al. (1961). The investiga- 

 tion of Iversen et al. (1960) on the size distribu- 

 tion of shrimp suggested that they leave the 

 Tortugas grounds in a northwe-sterly direction and 

 move toward deeper water. 



Evidence suggests that hurricanes markedly in- 

 fluence the movement of shrimp. Massive offshore 

 migration talies place in response to abnormalities 

 brouglit about by tlie storms. In 1960 after luir- 

 ricane "Donna" struck southern Florida, the 

 catches on the Tortugas grounds included a much 

 higher percentage of small individuals, and the 

 average size of shrimp was smaller than in pre- 

 vious years (Eldred et al., 1961). These changes 

 indicated that liurricanes can cause the shrimp to 

 move offshore earlier and at a smaller size than 

 normal. Also, hurricanes in the nursery areas are 

 known to cause high mortality among the young 

 (Tabb, Dubrow, and Jones, 1962). 



In contrast to our rather clear understanding of 

 migrations of P. sefiferus. almost nothing is known 

 about coastal migrations of P. d. duorarum. 

 Neither seasonal migrations nor movements in 

 response to temperature changes in the latter have 

 been adequately investigated. Recent data acquired 

 by McCoy and Brown (1967) from release and re- 

 capture procedures indicated that pink shrimp 

 migrated southward from Beaufort Inlet, N.C., 

 from May tlirough August ; however, the greatest 

 distance traveled was only 193 km. in 5 weeks. 



EFFECTS OF TEMPERATURE 



P. d. duorarum is the only Penaeus that 

 overwinters in the estuaries of North Carolina. 

 There Williams (1955a) found them at 6° C. and 

 suggested that they may survive the severe winter 

 cold by burrowing deeply into the substrate. The 

 laboratory experiments of Williams (196(1) showed 

 that P. d. duorarum was better able to withstand 

 a combination of low salinity and temperature than 

 P. a. aztecus. This discovery may help to explain 

 the occurrence of pink shrimp in the northern 

 inshore waters of North Carolina during the win- 

 ter and even in those of Virginia, where, according 

 to Van Engel (1965), specimens are taken in 

 almost all months. On the basis of our pi-esent 

 knowledge, there is no explanation for the fact 

 that even though the j^ink shrimp is more tolerant 

 of low temperatures than are white and brown 



shrimps, the northern limit of its range is some 

 2°30' and 3°25', respectively, south of that of the 

 others; obviously, some factor other than tem- 

 perature has prevented their spreading farther 

 north. 



Local movements of young shrimp in response 

 to sudden changes in temperature have been ob- 

 served. In southwest Florida, Tabb, Dubrow, and 

 Jones (1962) noted that the shrimp moved out of 

 the shallows to deeper, warmer water during cold 

 periods and returned again as soon as the water 

 began to warm. They indicated that if the descent 

 in temperature occurred gi'adually, the shrimp 

 seemed to remain in the inshore waters. 



Temperature is one of the principal factors gov- 

 erning growth (see. above) and survival of pinlv 

 shrimp as it is known to be for the closely allied 

 Penaeus. The maximum temperature at which P. d. 

 duorarum has been recorded is 35.5° C, at which 

 Eldred et al. ( 1961 ) collected them in Tampa Bay. 

 In general, it seems that the lower temperature 

 limit for pink shrimp activity is about 14° to 16° C. 

 (Williams, 1955a; Fuss and Ogren, 1966). Com- 

 plete cessation of activity occurs below about 

 10° C. (Williams, 1955a; Eldred et al., 1961 ; Fuss 

 and Ogren, 1966). The lowest temperature at 

 which pink shrimp have been found is 3.5° C. ; a 

 single specimen was collected in Lower Chesa- 

 peake Bay (Thimble Slioal Light) in February 

 (W. A. Van Engel, personal communication). 

 EFFECTS OF SALINITY 



Larval and early postlarval stages, large sub- 

 adults, and adults abound in oceanic waters of high 

 salinity. On the Tortugas fishing grounds Iversen 

 etal. (1960) reported bottom salinities (10-15 fm.) 

 of 36.15 p.p.t. to 37.73 p.p.t. 



Postlarvae, juveniles, and early subadults live in 

 inshore water within a wide range of salinities. 

 Tabb, Dubrow, and Jones (1962) have recorded 

 salinities of zero to 47 p.p.t. in the nursery areas of 

 south Florida. Joyce (1965) found specimens 

 80.5 km. up the St. Johns River, Fla., in almost 

 fresh water. Gunter et al. (1964) indicated that 

 salinity seems to be a limiting factor in the abun- 

 dance of P. d. duoraimm as well as of P. setiferus 

 and P. a. aztecus. Their investigations in inshore 

 water adjacent to the Gulf of Mexico showed that 

 pink shrimjj were more abundant at salinities of 

 18 p.p.t. and above, whereas brown shrimp were 

 more abundant in water of 10 to 20 p.p.t. and white 

 shrimp in water of salinity lower than 10 p.p.t. 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



519 



