Table 4. — Summary 0/ Torlugas lows: Cruise totals by stage. All values are the su7n of the computed numbers of each larval stage 

 of P. duorarum under 10 m.^ surface area at 10 stations on the Tortugas Shelf,^ August 1962 to October 1964 



' The totals for cruises 6201, 6206, 6308, and 6401 are the sums of 9, 8, 9, and 8 stations, respectively. 



the dififerent sampling characteristics of the two 

 nets, data for the first and second halves of 1962 

 cannot strictly be compared. The average catch 

 of the Gulf V net per unit volume of water strained 

 during the spawning seasons of 1963 and 1964 was 

 about 10 times greater than the catch of the 

 Discovery net during the spawning seasons of 

 1959-61. In the construction of the histograms 

 for 1962, the numbers per 1,000 m.' filtered by 

 the Discovery net have been equated with the 

 numbers per 100 m.' filtered by the Gulf V net. 



Spawning occurred throughout the annual tem- 

 perature range of 19° C. to 30° C, but most 

 spawning occurred when temperatures exceeded 

 25° C. and peaks were usually associated with the 

 highest temperatures reached in each year. High- 

 est temperatures were recorded as early as June 

 and as late as the end of September, resulting in 

 corresponding shifts from year to year in the 

 main time of spawning. 



SEASONAL RELATION BETWEEN DEPTH AND 

 SPAWNING 



In general, a seasonal shift was observed in the 

 depth at which most spawning (as measured by 

 the presence of first protozoea) occurred. The 

 center of abundance of first protozoea was usually 

 at the shallowest stations in early spring and then 

 moved toward deeper water as the season pro- 

 gressed until in the fall it was at depths greater 

 than 30 m. (16.7 fathoms). In 1962 the center of 

 abundance of first protozoea moved from about 

 25 m. (13.8 fathoms) in August and September to 

 about 32 m. (17.7 fathoms) by early November 

 (fig. 4). In 1963 a steady movement of the center 

 of spawning was again apparent from the shallow 

 waters (20 to 25 m. or 11.1 to 13.8 fathoms) in 

 early spring to a depth of 36 m. (20 fathoms) in 

 early November. This movement was not as 

 apparent in 1964, possibly because maximum 

 spawning occurred earlier in the year. 



170 



U.S. FISH AND WILDLIFE SERVICE 



