Figure 9. — Variations in the apparent daily survival rate between 1st protozoeal and 1-spine postlarval 

 stages of P. duorarum on the Tortugas Shelf, August 1962 to October 1964. 



The most striking feature of figure 10 was the 

 capture of large numbers of P. duorarum larvae, 

 mainly protozoeae, in the Florida Straits. These 

 larvae were taken in the surface and midwaters 

 of the Straits in water between 50 and 180 m. 

 (28 and 98 fathoms) deep and at all the stations 

 occupied. In contrast, few larvae or postlarvae 

 were taken north of the Florida Keys. 



We can, therefore, postulate that larvae are 

 dispersed by transport out of Rebecca Channel 

 and into the Florida Straits. The larvae would 

 be carried eastward by the Florida Current and 

 eddied back into the Florida Bay region by tidal 

 currents moving through the inlets between the 

 Florida Keys. If this transport does occur, the 

 larvae would arrive at a point only about 37 to 

 56 km. from the Everglades nursery grounds, 

 about 6 days after their exit from Rebecca 

 Channel (calculated according to the velocity of 

 the margins of the Florida Current given by 

 Sverdrup, Johnson, and Fleming, 1942). The main 

 part of the journey from the spawning grounds 

 to the Everglades might take only a short time, 

 leaving the bulk of the larval and postlarval life 

 for the journey across Florida Bay, a distance 

 of 37 to 56 km. 



We have learned that postlarvae appear to 

 react to tidal currents and ascend into the moving 

 tide mainly during the nighttime flood tide. If this 

 is so, movement across Florida Bay, where the 



tides flow roughly north and south, might be 

 accomplished within a week. If this migration 

 takes place, the estimates of survival given in the 

 preceding section must be reconsidered. We have 

 shown that the apparent survival rate increases 

 steadily toward the eastern sector of the sampling 

 grounds (fig. 9). If larvae are removed from 

 Tortugas grounds via Rebecca Channel, then 

 their greatest loss through the channel would 

 occur at stations nearest to the channel (viz, the 

 western stations), and the apparently low rates 

 of survival in this area are caused by loss of older 

 larvae and not by movement of larvae in an 

 easterly direction. This loss might explain the 

 anomalous catches at station 32.70 (directly north 

 of the Marquesas) which yield estimates of 

 survival that are not of the same order as those 

 calculated for station 30.58 (fig. 7a), although the 

 stations are only 18.5 km. apart and of similar 

 depths (14.5 and 11 m. or 8 and 6.1 fathoms, 

 respectively). If larvae have traveled from the 

 spawning grounds via the Florida Current, and 

 then have been eddied back into the vicinity of 

 station 30.58 via tidal currents in the Northwest 

 Channel, the high proportion of older larvae and 

 postlarvae at this station could be explained. The 

 position of station 32.70, directly north of the 

 Marquesas and away from any tidal channels, 

 would preclude it from receiving older larvae. 



178 



U.S. FISH AND WILDLIFE SERVICE 



