WEEKLY AVERAGE 



120 



PUMP 



Figure 7. — Indices of abundance of postlarvae at Galveston Entrance, February-May 1969. 



abundance and abundance of white shrimp 

 postlarvae. To obtain more information on the 

 spa wner- recruit relations of white shrimp, we 

 began to sample landings from the Gulf waters 

 adjacent to Galveston in April 1969. Daily in- 

 terviews also were obtained from fishermen 

 in the Gulf. In addition, random samples ofthe 

 white shrimp landed were measured, and stages 

 of sexual development were recorded for fe- 

 males so that the spawning population could be 

 defined precisely. We tested several methods 

 of distinguishing stages of maturity. 



Kenneth N. Baxter, Project Leader 

 Population Dynamics 



As an approach to the recommendation of 

 methods for optimal utilization of existing 

 penaeid shrimp resources, we focused our 

 attention on two problem areas. The first was 

 that of defining the time to begin harvesting 

 shrimp if maximum value of landings is to be 

 realized from a population. The second was to 

 describe the long-term effects of different 

 levels of fishing intensity on shrimp stocks. 



Timing of harvest .-- We have made several 

 refinements in our methods of estimating 

 growth and mortality rates used to deternnine 

 optimum timing of harvest. One refinement 

 was in the marking methods used for shrimp. 

 We completed our con-iparison of two methods 

 used most successfully to mark shrimp indi- 

 vidually at the Galveston Laboratory. In Biloxi 

 Bay, Miss., small brown shrimp, marked with 

 either Niagara sky blue and a numbered internal 

 tag or a small Petersen disk tag, were re- 



leased into the fishery. The proportion of 

 shrimp marked with Petersen disks that was 

 recovered was higher than the proportion of 

 stained shrimp recovered. The difference in 

 recovery rates, however, was not apparent 

 until the fourth week (fig. 10). Growth of the 

 stained shrimp during weeks 4 through 8 was 

 more rapid than that of those marked with the 

 Petersen disk (fig. 11). Because few marked 

 shrinnp were recaptured after the eighth week, 

 we could not connpare growth rates for shrimp 

 at large for longer periods. 



An experiment was made to determine if 

 stable strontium could be used to mark white 

 shrimp permanently. Strontium has been used 

 successfully for marking scale and bony tissue 

 of fish by feeding diets that contain a strontium 

 supplennent. White shrimp held in aquaria were 

 used to compare the following treatments: 

 (1) shrimp fed prepared food with no strontium 

 supplement; (2) shrimp fed prepared food with 

 a supplement of strontium lactate (7.7 g. 

 strontium lactate per 100 g. of food); (3) shrimp 

 fed prepared food (no strontium supplement) in 

 sea water containing 177 p. p.m. (parts per 

 million) of strontium. 



Samples of shrimp and exuviae were re- 

 moved regularly and checked by flame-emis- 

 sion spectrophotometry for strontium content 

 during the 40-day experiment. Shrimp assimi- 

 lated significant amounts of strontium from the 

 food and the water but lost a large portion of it 

 when they molted. The strontium apparently 

 accumulated in the exoskeleton but not in other 

 parts of the body. Because most of the ac- 

 quired strontium is lost during the naolt, this 

 method of marking is not satisfactory for our 

 work. 



14 



