growth indices, K, agree surprisingly well (com- 

 parative K from the Sanibel experiment = 0.339/ 

 4 = 0.085). Weekly growth during the Tortugas 

 experiment averaged roughly 1.5 g. in terms of 

 weight and about 3.4 mm. in terms of total length. 

 From a commercial fisherman's viewpoint, the 

 experimental data (table 4) reveal that it took 12 

 weeks for the average shrimp to increase in size 

 from 123 to 37 count (headless). Observe also 

 that the minimum commercial size (70 headless- 

 count) in the marked populations was attained 

 about 9 weeks following the start of the Sanibel 

 experiment, and 4K weeks after the Tortugas 

 experiment began. 



AGE AT RECRUITMENT— MAXIMUM AGE 



If, as Beverton and Holt (1957) argue, biological 

 rather than mere empirical significance be attached 

 to von Bertalanffy's equation and, further, if the 

 growth pattern as fixed by the constants computed 

 earlier be assumed reasonably typical of pre- as 

 well as postrecruit development, then extrapolation 

 of the curves in figure 9 to the left of t^ should 

 provide a rough index to the actual ("average") 

 age of shrimp making up each experimental popu- 

 lation at the time it was established. Adding 

 this to the time lapsing between an experiment's 

 initiation and the attainment of minimum com- 

 mercial size by its elements gives a measure of 

 age at recruitment. Fear that the von Berta- 

 lanffy equation may not adequately describe 

 growth during the shrimp's earliest develop- 

 mental stages should not deter pursuit of such an 

 index. No better approach to solving the vexa- 

 tious problem of age determination in commercial 

 Penaeidae has yet been developed. 



•In treating accordingly the results of the Tor- 

 tugas experiment (upon which all subsequent 

 analyses and discussions will be based), a value 

 between 10 and 11 weeks was indicated as the 

 probable age of shrimp released at the experi- 

 ment's start. When this value is extended by the 

 4}i weeks the marked shrimp required to reach 

 minimum commercial size, an approximation of 

 15 weeks for their age at recruitment is obtained. 

 Despite the likelihood that this value may not be 

 too precise, its order of magnitude is quite reason- 

 able in view of what has been observed in a species 

 closely related to (and often occurring with) the 

 pink shrimp, viz., the brown shrimp. 



Along the Texas-Louisiana Gulf coast and with 



little year-to-year variation in chronology, height- 

 ened spawning activity in offshore brown shrimp 

 populations during February and March nor- 

 mally results in large masses of postlarvae enter- 

 ing adjacent estuaries during mid-March to 

 mid-April. Present studies by personnel at the 

 Bureau of Commercial Fisheries Biological Lab- 

 oratory in Galveston, Tex., are yielding good 

 evidence that brown shrimp postlarvae, 8 to 12 

 mm. long when they reach the barrier island 

 passes, are on the order of 3 to 4 weeks old. 6 

 Once in inshore waters they grow very rapidly 

 and begin their seaward migration about mid- 

 June as subadults of a size just below the 70- 

 count minimum established above. Thus, the 

 total lapse in time between hatching and attain- 

 ment of 70-count size (about 10.5 g.) is roughly 

 15 weeks, which agrees very well with the value 

 obtained for pink shrimp from the Tortugas 

 experiment. 



One other growth-associated feature of pink 

 shrimp hfe history about which the yield equa- 

 tion employed in a later section requires in- 

 formation is the maximum age, on the average, 

 that this species attains. For the common Pen- 

 aeidae, it has been generally assumed on empirical 

 grounds that 18 months (78 weeks) is a good 

 approximation thereto (Kutkuhn, 1962). Re- 

 cent analyses of weight frequency distributions 

 in commercial pink shrimp landings indicate, 

 however, that the lapse between an age group's 

 recruitment and its disappearance from the 

 fishery (i.e., its fishable life span) averages about 

 68 weeks (Kutkuhn, 1962). Hence, upon com- 

 bining this value with that of the species' average 

 age at recruitment calculated above, a value of 83 

 weeks, believed to be a better estimate of maximum 

 age, is obtained. 



ESTIMATION OF MORTALITY 



BASIC ASSUMPTIONS 



Computationally as well as conceptually, meas- 

 urement of mortality — especially natural mor- 

 tality — is without question more intractable 

 than that of growth and therefore makes greater 

 demands on experimental data. Moreover, since 

 the investigator cannot guarantee that the results 



• Lindner and Anderson (1956) offer supporting documentation in their 

 study of the contemporary white shrimp. /* wtifetus, whose postlarvae 

 Iso estimated to be about the same age when they enter estuaries at 

 comparable sizes 



826 



U.S. FISH AND WILDLIFE SERVICE 



