Larval Distribution and Abundance 



Robert F. Temple 



The examination of the plankton samples from 17 cruises nnade 

 during 1961 in the offshore areas between Galveston, Tex. , and Cameron, La., 

 is completed. At each of the 11 stations in this area a 20-nriinute, "step- 

 oblique" plankton tow was made with a Gulf V plankton sampler. These sam- 

 ples are used in studying the early life history of shrimp, particularly the dis- 

 tribution, abundance, and survival rate of larvae. Eggs are not found in the 

 samples, but the occurrence of the nauplius, the first stage after hatching, 

 can be used as an indication of spawning. 



Commercially important shrimp, genus Penaeus, connprised 18 per- 

 cent of the total catch of 14, 218 larvae of the family Penaeidae, while non- 

 exploited shrinap of the genera Trachypeneus, Sicyonia , Solenocera , Para- 

 penaeus, and Xiphopeneus constituted the remaining 82 percent. All larvae 

 have been identified, and their seasonal and areal distribution and abundance 

 tabulated and graphed. 



The majority of larvae, genus Penaeus, were caught at 7^-fathom 

 stations between May and October. Larval abundance was greatest in July and 

 August. Spawning, based on the occurrence of nauplii, began in May and con- 

 tinued until October. The seasonal distribution of larvae coincides closely 

 with changes in bottom temperatures. Indications are that Penaeus in this 

 depth zone spawns at temperatures over 75' F. 



Average monthly bottom temperatures (° F. ) for 7|^-fathom stations 



Penaeus larvae of one stage or another were taken every month of the 

 year at the 15-fathom stations. Two general peaks of abundance were noted in 

 April and September. Three distinct periods of spawning, indicated by the oc- 

 currence of nauplii, were evident at 15-fathom stations: the first in April , the 

 second the last of June and first of July, and the third in September. The high- 

 est level of abundance, observed in September, was approximately three times 

 greater than any peak noted at the 7^ -fathom stations. 



Penaeus larvae were more abundant at 25 fathoms than at any other 

 depth. In the first 8 months, January to August, the abundance of larvae in- 

 creased, with the greatest level of abundance occurring from September through 

 November. Spawning at this depth first occurred in May. Additional spawning, 

 although in varying degrees of intensity, was evident from July through Decem- 

 ber. 



At the 45-fathom stations, larvae were taken sporadically throughout 

 the sampling period. The highest level of abundance was observed in Septem- 

 ber. Nauplii, although not abundant, occurred in June and September. 



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