average time spent in each larval stage have been 

 derived from the information given by Ewald 

 (1965) on the rate of development of larvae at 

 26° C: 



stage Days 



Egg 1.5 



Nauplius 2. 



1st protozoea 3. 



2d protozoea 2.4 



3d protozoea 2. 2 



1st mysis 2.0 



2d mysis 2.3 



3d mysis 3. 



1-spine postlarva 2. 



2-spine postlarva 2. 



3-spine postlarva 3.0 



4-spine postlarva 3.0 



5-spine postlarva 6.0 



Growth and development of larvae in the labo- 

 ratory may differ considerably from that in the 

 ocean; however, the relative duration of the in- 

 stars may be expected to be nearly constant, and 

 in the absence of better data the times given above 

 have been used to plot catch curves for the larvae. 



Catch curves, obtained by plotting the loga- 

 rithm of the total "catch" of each stage against 

 the estimated average age of the larval stage, are 

 shown in figure 6. A feature common to all of the 

 curves is that the catch of first and second myses 

 appears to have been depressed. At all stations, 

 the catch of second myses was less than the catch 

 of third myses. Several possible explanations for 

 this depression of the catch curve present 

 themselves. 



First, it is possible that the larvae are most 

 active in the first and second mysis stages, and 

 that the depressed catch of these stages reflects 

 active avoidance of the Gulf V net. Heldt (1938) 



Table 7. — Totals of the estimates of numbers of each 

 larval stage of P. duorarum under 10 m.^ surface area 

 for 37 cruises in the Tortugas Shelf area, August 1962 to 

 July 1964 



Note. The five highest values in each column are In italic. 



174 



has stated that the myses appear to be the most 

 active larval stages. 



Secondly, third myses and older postlarvae 

 spawned in another area (possibly to the north of 

 the Tortugas) may drift into the Tortugas area, 

 thus increasing the catch of the stages older than 

 second mysis. Jones et al. (footnote 6), however, 

 reported no evidence of any other center of 

 spawning. 



Finally, an error in the estimated time required 

 for the shrimp to develop through the mysis stages 

 may account for this discrepancy. Since mysis 

 stages are rather ill-defined, allocation to first, 

 second, or third mysis may be somewhat arbitrary, 

 depending upon the degree of development of the 

 pleopods and telson. More than three stages may 

 exist at temperatures lower than 26° C. (Ewald, 

 1965). If average duration of mysis stages is less 

 than the postulated 7.3 days, then the daily 

 survival rate must decrease considerably after the 

 third protozoeal stage and then increase again 

 during the postlarval stages. 



The first alternative — avoidance of the net — 

 suggested above is considered to be most likely. An 

 inspection of the catch curves shows that the slope 

 of the curves during the protozoeal stages and 

 during the stages subsequent to third mysis is fairly 

 uniform; in a number of catch curves, the points 

 describing the abundance of the third protozoeal 

 stage and of the third mysis stage can be joined to 

 produce an almost uniform slope throughout the 

 catch curve (fig. 6). This view is also supported by 

 the work of Jones et al. (footnote 6), who found 

 that the numbers of larvae decline steadily between 

 the third protozoeal stage and the third mysis 

 stage. Jones et al. (footnote 6) used a Discovery 

 net of 76-cm. mouth diameter and with larger mesh 

 sizes than those of the Gulf V sampler. Such a net 

 may capture a more representative proportion of 

 the highly motile stages than the faster moving but 

 smaller Gulf V sampler (Barkley, 1964). 



The slopes of the catch curves shown in figure 

 6 appear to vary. For comparing these apparent 

 variations in survival, we have calculated the 

 gross survival between selected stages for each 

 of the 10 stations. Distributions of gross survival 

 values between first and third protozoeae, third 

 protozoeae and third myses, and third myses and 

 two-spine postlarvae are plotted in figure 7a, b, 

 and c. Choice of these stages for the calculations 

 was dictated by the anomalies that appeared 



U.S. FISH AND WILDLIFE SERVICE 



