brown shrimp at the postlarval and juvenile 

 stages and has demonstrated that predictions 

 of commercial catches are possible. To date, 

 predictions made on the basis of the abundance 

 of postlarvae have been somewhat less reliable 

 than those based on catches of juveniles, but 

 the postlarval method has greater potential 

 value because predictions can be made almost 

 2 months earlier. Emphasis in the past year 

 has been placed on the improvement of the 

 postlarval index of abundance. Continuous 

 sampling schemes are being investigated as 

 a means for reducing the influence of local 

 environmental variations on catches of post- 

 larvae. We are studying minimum temperature 

 limits of postlarvae in the laboratory and under 

 field conditions to determine whether periods 

 of low temperature during the spring are lethal 

 to postlarvae that arrive sooner than usual. 

 To discover the fate of postlarvae that arrive 

 later than usual, we are sampling juvenile 

 shrimp to determine which groups actually 

 contribute to subsequent offshore harvests. 



The University of Southwestern Louisiana, 

 which has studied seasonal variations in the 

 abundance of postlarval shrimp since 1963, 

 expanded its research during the past year. 

 Laboratory investigations of several types are 

 now underway to seek solutions to problems 

 confronting the three projects within the Shrimp 

 Dynamics Program. 



Richard J. Berry, Program Leader 



POPULATION STUDIES 



Detailed tabulations of shrimp landings from 

 the Gulf of Mexico have been published each 

 month since January 1956, These data are not 

 only a source of information on the shrimp 

 stocks and the shrimp fishery, but they can be 

 used to learn the effects of fishing on shrimp 

 populations. Before the tabulations can be 

 used, however, we must know (1) the reliability 

 of information about the size composition of 

 the shrimp landed ajid (2) the accuracy of 

 estimates of the amount of effort expended in 

 making the catches. 



To evaluate the published information, we 

 have collected similar records and compared 

 them with the published information. This 

 evaluation is based on landings from the 

 Texas ports of Galveston, Freeport, and 

 Aransas Pass during the season of heaviest 

 catches. Shrimp landed at these ports represent 

 nearly 50 percent of the total catchfrom Texas 

 waters. 



From July through November 1965, biolo- 

 gists at these three ports collected detailed 

 information on shrimp sizes and fishing effort 

 on a strict schedule. This schedule was de- 

 signed to provide an equal chance for each 

 landing to be sampled and for a crew member 

 from each vessel to be interviewed. Random 

 samples of shrimp were obtained as they 



moved along conveyor belts from the vessels 

 to the shrimp houses. At least 200 shrimp were 

 measured from each landing sampled. 



To estimate the size composition of shrimp 

 in a particular fishing area, we compared 

 the size of shrimp in samples obtained from 

 vessels that had fished together, even though 

 they may have landed their catches at any of 

 the three ports. A comparison of the size 

 of brown shrimp sampled during July at 

 Aransas Pass and at Freeport is shown in 

 figure 9. Similar tests were made each month 

 on data collected at the three ports; all 

 differences were minor. 



The size composition from published data 

 of all shrimp landed during July from the 

 same area sampled by our personnel is given 

 in figure 10. The two figures illustrate an 

 effect of the grading practices at the shrimp 

 houses. For the published data, shrinnp were 

 collected or lumped into fewer size categories. 

 Most of the landings which contributed to 

 figure 10 were box graded. Box grading 

 usually places the shrimp landed by a vessel 

 into one or two size categories by an averaging 

 process. The other technique, machine grading, 

 mechanically sorts shrimp into several size 

 categories. The latter is used primarily in 

 Louisiana atnd in Brownsville, Tex. Few shrimp 

 graded by machines were used in the above 

 comparison. Samples taken in areas where 

 machine grading is used indicate that this 

 method shows more accurately the sizes of 

 the shrimp lajnded than does box grading. 



Another discrepancy noted between the pub- 

 lished information and our sampling data 

 concerned the average size of shrimp landed 

 each month. The size group which included 

 the greatest weight of shrimp according to 

 the published data usually differed from that 

 indicated by our measurements. Early in the 

 season, when shrinnp were small, the published 

 data placed them in a larger size category 

 than indicated by our measurements. Agree- 

 ment between the reported landings amd our 

 measurements was close in midseason. Near 

 the end of the season, when shrimp were 

 large, the published data indicated a smaller 

 average size than did our measurements. 



Accurate information concerning the fishing 

 time involved in making catches is as im- 

 portant as the size composition of landings. 

 The ratio of catch to fishing effort is com- 

 monly used in commercial fisheries as an 

 estimate of relative abundance. This infor- 

 mation is obtained from interviews with the 

 vessel crew at the end of a trip. 



Estimates of the catch per hour of fishing 

 for vessels that had fished together and 

 landed at Freeport or Aransas Pass and the 

 catch rate for vessels fishing in the same 

 location, as obtained from published infornna- 

 tion, agreed well (table 3). Similar tests 

 made for other areas showed equally close 

 estimates. 



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