than 0.17 kn. Progressive vector analysis 

 revealed a westerly current at all depths; the 

 velocity decreased as depth increased. The 

 average resultant current at 10 ft, was flowing 

 285 at 0.85 kn., whereas the current at 23 ft. 

 averaged 277 at 0.55 kn. Although direction 

 and velocity differences between instruments 

 were slightly greater within 10 ft. of the 

 bottom, currents were also flowing westerly. 



David L.. Harrington, Project Leader 



Distribution and Abundance of Shrimp Larvae 



A thorough knowledge of the life history of 

 the planktonic stages of shrimp is essential to 

 understand and manage offshore shrimp popu- 

 lations. To attain this knowledge, we have, 

 since 1962, been extensively sampling offshore 

 in the northwestern Gulf of Mexico. W e collected 

 plankton samples nnonthly at predetermined 

 stations along transects extending from 4^ to 

 60 fath. At each station, a 20-nnin., "step- 

 oblique" plankton tow is nnade with a Gulf-V 

 plankton net. We sorted penaeid shrimp larvae 

 and postlarvae from the samples. The result- 

 ing data, reported in number of organisms 

 per 100 m.^ (3,534 ft. 3) of water strained, 

 provided information on seasonal and areal 

 distribution and abundance of planktonic- stage 

 penaeids. All samples collected through 

 December 1963 have been examined, providing 

 2 yr. of comparable data for the survey area. 



Catch Composition between 1962 and 1963 



Six genera of planktonic- stage penaeids 

 occurred in plankton samples fronn the sur- 

 vey area: the comn-iercially important shrimp, 

 Penaeus (white, brown, and pink shrimp), and 

 the nonconnmercially important shrinnp genera, 

 Trachypeneus , Xiphopeneus , Sicyonia , 

 Solenocera , and Parapenaeus . Catch data 

 grouped for all genera indicated that plank- 



tonic stages were more abundant in 1963 than 

 in 1962 (table 1). This difference was caused 

 by a twofold increase in numbers of Penaeus 

 species and "TX" species in the western 

 portion (Galveston to Brownsville) of the 

 study area. (Young shrimp of the genera 

 Trachypeneus and Xiphopeneus are connbined 

 and designated " TX " in this report because 

 it is impossible now to differentiate between 

 thenn.) 



Seasonal Distribution of Commercial 

 Species, 1963 



Spawning, as indicated by the scarcity of 

 early stages (naupliar and protozoeal) in plank- 

 ton samples, was low in January to March. 

 During this period, planktonic- stage Penaeus 

 species were present in most of the survey 

 area except for an area southeast of Corpus 

 Christi, Tex. Concentrations were greatest 

 inside the 15-fath. contour and extended along 

 the coast from central Texas to Louisiana. 

 About 55 percent of the catch was postlarval 

 shrimp. Postlarval brown shrimp were enter- 

 ing the bays during this time, and about 50 

 percent of the total postlarval catch was taken 

 at 4|-fath. stations (fig. 4). 



In April to June, the increase in numbers of 

 naupliar and protozoeal stages in plankton hauls 

 over those during the previous 3-mo. period 

 indicated increased spawning. Planktonic 

 stages were widely distributed over the sur- 

 vey area; the greatest concentrations were 

 off the Texas coast in the depth zone bounded 

 by the 15- and 25-fath. contour lines. 



The high catch of protozoeal stages indicated 

 continued spawning in July to September, 

 Planktonic-stage Penaeus species were more 

 widely distributed than in any previous period 

 and were concentrated most heavily along the 

 Texas coast in 15 fath, southeast of Galveston, 

 in 7| fath. northeast of Corpus Christi, and in 

 4| fath. northeast of Brownsville. These three 

 areas are in a zone of high concentration that 



Table 1. --Average catch per unit of effort of six genera of shrimp, 

 northwestern Gulf of Mexico^ 



Genus 



Penaeus 



"TX"2 



Sicyonia 



Solenocera . . 

 Parapenaeus . 



Total... 



1962 



East West Both areas 



15 

 7 

 5 

 1 



36 



13 



36 



21 



7 



2 



79 



10 

 25 



6 

 2 



58 



1963 



East 



32 



West Both areas 



160 



22 

 49 



10 



4 



< 1 



85 



Average number of planktonic stages per 100 m.^ (3,534 ft.^). 

 Trachypeneus and Xiphopeneus . 



10 



