7o 



Figure 7a, b, and c. — Variations in apparent gross sur- 

 vival of larval and postlarval P. duorarum at 10 sam- 

 pling stations on the Tortugas Shelf, August 1962 to 

 October 1964: (a) 1st protozoeal to 3d protozoeal 

 stage, (b) 3d protozoeal to 3d mysis stage, and (c) 

 3d mysis to 2-spine postlarval stages. 



in the catch curves and which have been discussed 

 above. Lowest apparent survival was consistently 

 in the deepwater stations in the northwestern 

 sector of the sampling area, and greatest apparent 

 siu"vival (in some cases exceeding 100 percent) 

 was in the southeastern sector. It appears highly 

 unlikely that the calculated values are real values 

 and that survival of larvae is greater in the south- 

 eastern sector. These consistent differences are 

 probably a result of the migration of larvae away 

 from the center of spawning. Stations 40.90 and 

 50.80 are believed to represent the centers of 

 spawning, and the larvae disperse from this 

 center. 



Calculated daily survival rates between succes- 

 sive larval stages at each of the 10 stations are 

 given in table 8. Total survival between first 



protozoeal and one-spined postlarval stages also 

 is given (right-hand column); the highest rate 

 of survival between these stages is usually at 

 the shallowest, more easterly stations (fig. 8). 



Because the dispersal of larvae from the center 

 of spawning appears to affect the survival esti- 

 mates pertaining to specific stations, it is difficult 

 to estimate the true survival rate. Calculations of 

 survival rate based upon the total catch of 

 successive stages in relation to the total catch 

 of first protozoeae will have validity, however, 

 provided it can be assumed that the main route 

 of dispersal is over the area covered by the sam- 

 pling stations (table 9). 



If estimates of survival rates between first pro- 

 tozoea and first and second mysis are discarded, 

 all remaining estimates are nearly constant and 

 range from 78.6 to 82.0 percent (mean, 80.4 per- 

 cent) per day. 



Postlarvae enter the Everglades nursery grounds 

 in the six-spine stage (Tabb, Dubrow, and Jones, 

 1962). On the basis of age data provided by Ewald 

 (1965), these larvae are about 35 days old. At a 

 survival rate of 80.4 percent per day, the fraction 

 of the original population which survives to enter 

 the nursery grounds can be estimated at 0.804^', 

 or 0.05 percent. The fraction of the original popu- 

 lation of first protozoeae (average age 5 days) 



100 



95 



90 



85 



80 



75 



15 



20 



25 



DEPTH (M.) 



30 



35 



40 



Figure 8. — The relation at 10 sampling stations on the 

 Tortugas Shelf, August 1962 to October 1964, between 

 depth and apparent survival rates of P. duorarum 

 between 1st protozoeal and 1-spine postlarval stages. 



176 



U.S. FISH AND WILDLIFE SERVICE 



