Table 8. — Calculated daily survival rales between successive larval and posllarval stages of P. duorarum at 10 sampling 



stations on the Tortugas Shelf 



' Pi, Pi, Pj = lst, 2d, and 3d protozoeal stages; Mi, Mj, M3 = lst, 2d, and 3d mysis stages; and Pli, Plz, Pl3+s = l-, 2-, 3- to 6-spine postlarval stages. 



T.iBLE 9. — Estimated daily survival rates of larval P. 

 duorarum over successively longer intervals on the Tortugas 

 Shelf, August 1962 to October 196J, 



Pl-P2... 



Pi-Ps.... 

 P,-M,.... 

 P,-M,.... 

 Pi-Ms.... 

 Pi-Pli... 

 Pi-Ph... 

 P1-PI3+1- 



Stages I 



' Pi, P2. P3 = lst, 2d, and 3d protozoeal stages; Mi, M2. and M! = lst, 2d, and 

 3d mysis stages; and Pli, Ph, Pljts = l-, 2-, and 3- to 5-spine postlarval stages. 

 2 Estimates affected by sampling error. 



would be 0.8043", or 0.14 percent. In 1963, about 

 60,500X10* first protozoeae were produced within 

 the sampling area; at the postulated rate of sur- 

 vival, about 85X10' six-spine postlarvae woidd 

 enter the nursery grounds. The total commercial 

 catch of P. duorarum adults in the Tortugas area 

 during 1963 was about 500 million individuals 

 (U.S. Fish and Wildlife Service, 1964), implying 

 that about 6 percent of the postlarvae may sur- 

 vive to be captured when they move to the 

 trawling grounds as juveniles and adults. 



Owing to the lunar periodicity of spawning, the 

 proportions of larval stages in the catch change 

 constantly, and we cannot estimate the survival 

 rate operating at a given time nor obtain any 

 information on the effect of temperature on 

 survival. 



DIRECTION OF MIGRATION 



The data on survival rate shown in figures 7 

 and 9 indicate that the migration pattern of the 

 larvae radiates away from the centers of spawning 

 and that the apparent drift is mainly in an easterly 

 direction. Jones et al. (footnote 6) showed that 



few larvae are present north and west of our 

 area of study. Koczy, Rinkel, and Niskin (1960) 

 investigated the current system of the region and 

 concluded that, although the water masses are 

 subjected to a tidal movement of 9 to 11 km. per 

 day, the resultant movement over a full tidal 

 cycle is slight. Also, the small resultant drift is 

 in a westerly direction and not in the required 

 easterly direction. Additional evidence regarding 

 the current system has been obtained recently 

 through the release and recovery of seabed 

 drifters. A vector analysis of the seabed drifter 

 returns has indicated the presence of a slow 

 southwest drift over the shrimp grounds. The 

 detailed results have been reported by Relirer, 

 Jones, and Roessler (1967). The conclusions drawn 

 from these results are that direct dispersal to the 

 east seems improbable and that postlarvae reach- 

 ing the nursery grounds of the Everglades are 

 carried there by currents via some indirect route. 



The most likely alternative means of larval 

 transportation is by the Gulf Stream, passing 

 through the Florida Straits. Biologists on cruises 

 to the Tortugas region have on many occasions 

 reported a strong southerly current flowing 

 through Rebecca Channel and entering the 

 Florida Straits. Current speed was measured on 

 one occasion at 2.6 km. per hour (1.4 knots). 

 Because Rebecca Channel is directly southwest 

 of the centers of spawning (stations 40.90 and 

 50.80), considerable numbers of larvae must be 

 swept through the channel and into the Florida 

 Straits. 



A survey cruise in October 1964 covered an 

 area of some 10,789 km.^ between Cape Sable on 

 the Florida mainland and the Tortugas slirimp 

 fishing grounds (fig. 10). Additional stations were 

 in the Florida Straits south of the Florida Keys. 



PINK SHRIMP ON FLORIDA TORTUGAS SHELF 



177 



