Postlarvae occurred throughout the area in 

 January to March, but when abundance and 

 areal distribution of larvae were increasing in 

 April to August, postlarval catches were gen- 

 erally low and postlarvae were not as widely 

 distributed as larvae. During the ensuing 

 months, however, the distribution of post- 

 larvae increased, and in September to Decem- 

 ber it closely approximated that of the larval 

 stages. 



ABUNDANCK OF LARVAK AND POSTLARVAE 



IN RELATION TO SPAWNING PERIODS AND 



POSSIBLE OVERWINTERING 



Although species differentiation of the larval 

 stages of Penaeus spp. is impossible at this 

 time, we have hypothesized that trends in larval 

 abundance observed in different depths of water 

 are the result of differences in depth and season 

 of spawning of white and brown shrimp. The 

 hypothesis that greatest abundance of larvae 

 at the shallowest stations (14 m.) follows the 

 spawning of white shrimp is compatible with 

 the works of Pearson (19;^>9) and Lindner and 

 Anderson (1956). In addition, since larval 

 development requires 2 to 3 weeks (Pear.son, 

 1939), agreement is also close between larval 

 cccurrence at 14-m. stations and postlarval 

 movement of white shrimp into Galveston Bay 

 reported by Baxter and Renfro (1967). Spawn- 

 ing, maximum larval abundance, and postlarval 

 movement of white shrimp into the nursery 

 areas are apparently completed over a 7- to 

 8-month pericd within a calendar year (fig. 8). 



The chronology of spawning, larval abun- 

 dance, and postlarval movement of bi'own 

 shrimp into Galveston Bay is not as apparent as 

 for white shrimp. Increasing abundance of 

 larvae at station depths of 27 m. or deeper ap- 

 pears to follow closely the spawning of brown 

 shrimp reported by Renfro and Brasher. (See 

 footnote 4.) When compared with movement 

 into Galveston Bay reported by Baxter (1963), 

 however, a definite anomaly is apparent (fig. 

 9). Most postlarvae moved into the Bay in the 

 spring, but the largest catches of larvae in the 

 Gulf were made in the fall. Because .similar 

 trends in larval abundance (Fischer, 1966) and 

 postlarval movements (Baxter and Renfro, 

 1967) have been observed in recent years. 



the possibility that techniques of sampling bias 

 the results appears to be negligible. 



POSTLARVAE ENTERING , 

 GALVESTON eAY^ 



LARVAL ABUNDANCE TREND 

 27-B2 METER STATIONS 



TAHEN FROM BUTen,l96} 



Figure 'J. — Larval abundance trends in 27 to 82 m. of 

 water adjacent to Galveston and postlarval movement 

 of brown shrimp into Galveston Bay, 19G1. 



The difference in timing between offshore 

 larval and inshore postlarval peaks can possibly 

 be explained by examining seasonal abundance 

 trends and length-frequency distributions of 

 postlarval brown shrimp taken in plankton 

 hauls. Postlarvae were generally taken in 

 plankton hauls from January through April, 

 and Augu.st through December (fig. 4). As 

 with larvae, most postlarvae were taken in the 

 fall. Length-frequency distributions, based on 

 total lengths from the tip of the rostrum to the 

 tip of the telson, reveal two distinctly different 

 size groups (fig. 10) . In January to April, most 

 postlarvae averaged 11 to 12 mm., whereas in 

 August to December they averaged only 6 to 

 7 mm. 



The size difference between the two groups of 

 postlarvae has significance when compared with 

 length-frequency distribution of postlarvae 

 migrating into Galveston Bay (Baxter and 

 Renfro, 1967). Postlarval shrimp taken dur- 

 ing the peak inshore movement average about 

 12 to 13 mm. in total length, and were prob- 

 ably represented offshore by the group of 

 postlarvae averaging 11 to 12 mm. taken be- 

 tween January and April. The question still 

 remains, however: From what spawning and 

 peak of larval abundance did the postlarvae 

 originate? Kutkuhn (1966) stated that the 

 postlarvae probably originated from ". . . 

 heightened spawning activity in off.shore brown 

 .shrimp populations during February and 



332 



DISTRIBUTION, SHRIMP IN GULF OF MEXICO 



