GULF OF MEXICO COMMERCIAL SHRIMP POPULATIONS 



363 



COMMEOCIAl. TIELO (mlllant Of eoundf 

 °PENSAC0LA-WIS9lSSlPPl RlWER 



"A 



.^2A. 



& FISHASLE eiOMASS INDEX 



FiGiRE 12. — Yield and structure of brown shrimp popu- 

 lations off the northeastern Gulf coast, 1956-59. 



coast area. Population cycles as exliibited by 

 bioinass curves derived for the Apalachicola and 

 Pensacola-Mississippi River areas showed ap- 

 preciable similarity except that brown shrimp 

 abundance dropped to a much lower level during 

 the winter months oft Apalachicola. This was 

 likelj^ due to the tact that specimens maturing and 

 renuiining in this area had already passed out of 

 the range of fishing operations at winter's onset. 

 Whereas the range of year-round exploitation 

 may extend to 45 fathoms in the Pensacola- 

 Mississippi area, it rarely goes beyond 20 fathoms 

 in tlie Apalachicola area. 



Population characteristics. — The brown shrimp 

 population inhabiting eastern Gulf waters evi- 

 dentl}' produces two broods per year (fig. 12C). 

 Roughly describing population age structure, 

 distribution of modal-sizes comprising monthly 

 yields suggested altcrnale roles for alternate 

 broods — one "reproductive", one "conunercial." 



643091 O— G2 4 



Those broods forthcoming in late summer and 

 fall (indicated by shaded arrows) are usually of 

 comparatively small size due, pcriiaps, to reduced 

 survival, increase in weight slowly, and apparently 

 contribute little to the annual jield (cf. figs. 12A, 

 I2B, and 12C). Their residuals, however, seem- 

 ingly constitute the greater proportion of spawn- 

 ing populations which produce the "commercial" 

 broods in succeeding years. These broods, in 

 contrast, are generated in late winter and early 

 spring (indicated by light arrows), are larger (hie 

 to better survival tluring larval and postlarval 

 stages, increase in weight more quickly, con- 

 tribute disproportionately more to the annual 

 yield (cf. figs. 12A, 12B, and 12C), and, accord- 

 ingly, make up a smaller proportion of the spawn- 

 ing population. Significantly, late winter-spring 

 broods are harvested at an earlj- age (fig. 12(') with 

 few individuals surviving to maturity. Those 

 that do mature supplement, by virtue of wliat 

 might be termed "compensatory" growth, tlie 

 spawning population represented primarily by 

 members of the preceding or fall-winter brood. 

 This relationship is shown in figure 12C where 

 coalescence of members of adjacent pairs of life 

 history curves is vaguely suggested. For the 

 most part, fall-winter broods impart little evidence 

 of existence or strength on the biomass curves 

 until the following fall and winter when their 

 mature elements contribute to small catciies. 

 (Note slight modes on curves in fig. 12B during 

 early 1956, 1958, and 1959 in the Pensacola- 

 Mississippi River area.) 



Comparative interpretation of figure 12C is 

 rendered somewhat difficult bj' the paucity of 

 data for the Apalachicola area. Despite this 

 shortcoming, the interrelationships of successive 

 broods still seem sufficiently dear. Causes of 

 variation about the fitted lines do pose a question, 

 however. Part of this variation could be attrib- 

 uted to dift'erential fishing with respect to bio- 

 mass distribution ("sampling error") and, perliaps, 

 to disproportionate vulnerability or availability of 

 sexes, but most is probably due to the fact tliat 

 spawning continues in varjing degree througliout 

 the year.' Even though two peaks of si)awniiig 

 mav nearlv alwavs be defined, tliis liofiiiition is 



' Ono year's (19fil) hloloBlcal sampling on tlio cont Inontal shelf by personnel 

 at the Bureau of Comnierical Fisheries BioloRical Laboratory. Oalveslon. 

 Texius. has revealed the presence in every month of the year of brown shrimp 

 ready to spawn. 



