ASPECTS OF BROWN SHRIMP, PENAEUS AZTECUS, GROWTH 

 IN THE NORTHERN GULF OF MEXICO 



Michael L. Parrack' 



ABSTRACT 



The growth of brown shrimp, Penaeus aztecus, was studied by utilizing forms of growth models 

 compatible with mark- recapture data. The analysis of 5,100 individuals marked and later recaptured 

 in the northern Gulf of Mexico indicates that the von BertalanfTy model is slightly superior to the 

 logistic in reflecting growth in length and the monomolecular model is superior to the Gompertz in 

 expressing growth in weight. Linear functions are apparently inadequate growth models for brown 

 shrimp. Estimated size-age relationships are appreciably different for each sex in that females are 

 much larger than males of the same age. The pattern of growth shown in this analysis for populations in 

 the northern Gulf is different from that reported in the southern Gulf off the Mexican coast and that 

 reported in U.S. Atlantic coastal waters. 



The commercial importance of brown shrimp, 

 Penaeus aztecus (Ives 1891), has precipitated sev- 

 eral studies of the growth rate for individuals of 

 that species. Definition of the growth rate is neces- 

 sary in order to develop an understanding of the 

 population dynamics of the resource. Growth 

 models have been reported for wild populations in 

 the northwest Atlantic off North Carolina (McCoy 

 1972) and in the southern Gulf of Mexico off Tam- 

 pico, Mexico (Chavez 1973). Several workers have 

 described the growth rate of small brown shrimp 

 in the northern gulf ( George 1962; St. Amant et al. 

 1963, 1966; Loesch 1965; Ringo 1965; Jacob 1971; 

 Wengert 1972; Gaidry and White 1973; Rose et al. 

 1975; Welker et al. 1975; Knudsen et al. 1977). 

 The growth rate of larger brown shrimp, however, 

 has not been documented for populations in the 

 northern gulf. 



Generally, growth equations define the relation 

 between the size and age of individual animals. 

 Three such equations descriptive of growth are the 

 logistic (Pearl and Reed 1920), von Bertalanffy 

 (Bertalanffy 1938), and Gompertz (Gompertz 

 1825; Silliman 1967) functions. The logistic 

 and von Bertalanffy models are employed to 

 reflect growth in length whereas the Gompertz is 

 usually used to model growth in weight. The von 

 Bertalanffy function may be directly fit to weight 

 data to model growth in weight. If it is so used, it is 

 then correctly referred to as the monomolecular 



'Southeast Fisheries Center Galveston Laboratory, National 

 Marine Fisheries Service, NOAA, 4700 Avenue U, Galveston, 

 TX 77550. 



growth model (Medawar 1945; Fabens 1965). This 

 study appraised the abilities of these functions to 

 model brown shrimp growth. Additionally the 

 linear relation between size and age was also con- 

 sidered. 



The absolute age of shrimp cannot be deter- 

 mined directly by counting annuli on hard parts. 

 Shrimp molt many times during their life cycle; all 

 hard parts are lost, then reformed with each molt. 

 Therefore, age-size data of individuals cannot be 

 obtained for growth modeling; another technique 

 must be employed. Although the age of brown 

 shrimp at mean size has been discerned from large 

 volume size-frequency samples ( Chavez 1973), age 

 was not directly observable and therefore was in- 

 ferred. Mark-recapture data affords a direct mea- 

 sure of the changes in size per change in time. 

 Forms of the growth functions were employed to 

 utilize mark-recapture data so that error resulting 

 from incorrect age determination was avoided. 



METHODS 



Brown shrimp spawn in offshore Gulf of Mexico 

 waters ( 14-100 m deep) throughout the year ( Cook 

 and Lindner 1970). Eggs hatch within 14-18 h 

 (Cook and Murphy 1966) and larvae undergo 

 metamorphosis within 12-15 days (Cook and 

 Lindner 1970). Shrimp then migrate into es- 

 tuaries to undergo their juvenile period. Large 

 juvenile shrimp, usually 75-90 mm total length, 

 migrate to offshore waters as. they become sexu- 

 ally mature, thus completing the life cycle. Ap- 



Manuscript accepted June 1978. 



FISHERY BULLETIN: VOL. 76, NO. 4. 1979. 



827 



