white shrimp are similar during a given period 

 over broad geographic areas. No single factor 

 has been shown to cause the yearly variations 

 in white shrimp abundance, but some evidence 

 suggests that changes in the salinity of shallow 

 coastal waters may play an important role. 

 A negative relation between the August to 

 December catch of white shrimp from 

 Louisiana and the velocity of spring water 

 flows in the Mississippi River suggests that 

 large harvests of white shrimp occur in those 

 years when the spring runoff is below normal. 

 Salinity itself may not have a direct effect 

 on the survival of the larval white shrimp, 

 but it appears that some associated factor 

 is involved. 



Even though the specific causes of fluctua- 

 tions in shrimp abundance have not been 

 identified, it is becoming apparent that they 

 result from environmental changes that occur 

 over large parts of the Gulf . Otherwise, meas- 

 ures of abundance of brown or white shrimp 

 would not be similar over wide geographic 

 areas nor would a correspondence be found 

 between population levels of Texas brown and 

 Florida pink shrimp. Similar densities of 

 shrimp over broad and widely separated areas 

 are also interpreted to mean that other factors 

 affecting shrimp abundance, such as differ- 

 ences in fishing intensities and in laws govern- 

 ing harvesting practices, have slight injfluence 

 on population levels. 



Richard J. Berry, Project Leader 



ABUNDANCE OF POSTLARVAL AND 

 JUVENILE SHRIMP 



Past success in predicting abundance of 

 adult brown shrimp from densities of post- 

 larvae and juveniles in estuarine nursery 

 areas has led us to expand our sampling 

 program. We are collecting postlarval shrimp 

 routinely from Aransas Pass, Galveston, 

 Gilchrist, and Sabine Pass, Tex, Additionally, 

 our study of juvenile shrimp collected from 

 the commercial bait-shrimp fishery of Gal- 

 veston Bay has been supplemented by our 

 investigations of bait- shrimp fisheries in 

 Aransas Bay and in lower Laguna Madre. A 

 study of the growth of juvenile brown and 

 white shrimp in Galveston Bay has also been 

 started to determine how year-to-year dif- 

 ferences in shrimp growth may be associated 

 with changes in abundance. 



Postlarval Shrimp 



The abundance of postlarval brown shrimp 

 in collections from the entrance to Galveston 

 Bay during March and April 1965 was similar 

 to that for the corresponding period in 1964; 



however, other factors suggest that the off- 

 shore harvest of brown shrimp may be some- 

 what greater in 1965. Postlarval brown shrimp 

 were caught at all sampling stations during 

 the winter of 1964-65, in contrast to their 

 near absence in samples of past winters. The 

 presence of postlarvae may have been related 

 to above-average water temperatures. We do 

 not know whether these postlarvae survived 

 short intervals of cold temperatures during 

 February and March. Also, measures of abun- 

 dance of juvenile brown shrimp, in terms of 

 pounds caught per hour of fishing, were slightly 

 higher this year than in 1964. All factors con- 

 sidered, the 1965 catch of brown shrimp should 

 exceed that of 1964. 



We made special studies this spring of fac- 

 tors influencing the movement of postlarvae 

 into nursery areas. Two 72-hr. studies were 

 performed at Rollover Pass, Tex., on March 

 22 to 25 and April 26 to 29. Sannples were 

 collected hourly over three complete tide 

 cycles with a l/2-m. (19.7-in.) plankton net 

 fitted with a flowmeter. A V-Series Ice Meter, 

 which records current velocity in feet per 

 second, was also used. Current velocities 

 during flood tides at Rollover Pass were found 

 to be double those recorded during ebb tides. 

 Apparently, a major peak of postlarval brown 

 shrimp was moving into the bay at the time 

 of the first study when 32,940 were caught. 

 Only 3,310 brown and 840 white shrimp post- 

 larvae were taken during the second survey. 

 To some degree, postlarvae moved in both 

 directions with the current, but greater num- 

 bers were caught on flood tides. We learned 

 of no relation between time of day and numbers 

 of postlarvae caught. 



A similar 72-hr. study was made on Galves- 

 ton Island beach during April 5 to 7. Hourly 

 tows were made with a 5-ft,, hand-drawn 

 beam trawl. The net was carried out 100 ft. 

 into the surf, set, and pulled back to shore. 

 Dye was released in the sample area to deter- 

 mine surface current movement. Fluctuations 

 in numbers of postlarvae caught at this loca- 

 tion may be related to time of day, because 

 68 percent of the 13,200 brown shrimp post- 

 larvae were taken at night. Additional studies 

 are needed, however, before we can draw 

 definite conclusions. 



Juvenile Shrimp 



We began a study of the growth of juvenile 

 shrimp in Swan Lake, a shallow nursery area 

 adjoining the lower part of Galveston Bay, 

 Collections were made with a 10-ft, otter trawl 

 fitted with a fine-mesh cover over the cod end. 

 The size distributions of shrimp caught from 

 April 1 through June 17 are shown in figure 22. 

 Postlarval brown shrimp were present incon- 

 siderable numbers by April 1, but little growth 

 was evident before April 15. Two groups of 



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