GULF OF MEXICO COMMERCIAL SHRIMP POPULATIONS 



371 



COMMCACIAL TIElO ( 





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Figure 19. — Yield and structure of the pink shrimp popu- 

 lation off the southwest Florida coast (Sanibel- 

 Tortugas area), 1956-59. 



(fig. 19 A). Tliis seasonal pattern contrasts with 

 those of upper Gulf coast fisheries wherein pro- 

 duction normally reaches a maxiniuiu diu-ing late 

 summer and fall months. 



Fishahle hiomafis. — A plot of monthly catch-per- 

 unit-intensit\- values suggests that the Tortugas 

 population's fishahle biomass maintained itself 

 within fairly narrow hmits during the better part 

 of the 4-year study period (fig. 19B). As would 

 have been expected, seasonal deviations did occur, 

 their analysis providing, perhaps, the principal 

 clues in defining causes for the fishery's attrition. 

 Meriting special attention are the reduced popu- 

 lation levels sustained through the early months of 

 1957 and 1959, together with a perceptibly down- 

 ward population trend during the 4-year period. 



A comparison of figures 19A and 19B indicates 

 little correspondence between jaeld and fishahle 

 biomass other than a suggestion that low annual 

 yields seemed to occur in years whon the popu- 

 lation maintained itself at minimum levels. Yields 

 considerably lower than average paralleled low 

 biomass levels during the early portions of 1957 

 and 1959, the relationship in the latter year con- 

 stituting the most noteworthy example. Although 

 the population during 1956 and 1958 remained at 

 levels appreciably higher than those in 1957 and 



1959, it is not unreasonable to conjecture that ex- 

 cessive fishing in each preceding year could have 

 contributed to the low population levels and hence 

 jields in the years respectively succeeding them. 



Population characteristics. — Semiannual periods 

 of peak spawning activity, already indicated for 

 stocks of other Gulf species, seem to be typical of 

 Sanibel-Tortugas pink shrimp as well. These are 

 revealed by serial plots of (catch) size composition 

 data which are believed reasonably descriptive of 

 the population's actual size or age structure (fig. 

 19C). Production of broods indicative of peak 

 spawning activity corresponded rather well with 

 that noted for brown shrimp off the northeastern 

 Gulf coast, spawning evidently taking place in 

 late winter or early spring (light arrows) and in 

 late summer or early fall (shaded arrows). 



On the other hand, intrayear variation in 

 relative brood strengths appeared negligible, con- 

 trasting sharply with the great seasonal variation 

 typical of brown shrimp broods produced along 

 much of the upper Gulf coast. The degree of ex- 

 posure to exploitation at different stages of brood 

 development did vary seasonally, however. This 

 is indicated in figure 20 where crude estimates of 

 the absolute contributions of each brood at succes- 

 sive developmental stages are grapliically shown. 

 These estimates were obtained by merely subdi- 



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ESTIMATEO 



TOTAL SEASON 



riELD RECRUITED 



8.9 FALL 



»40 40 50 ZA te I 



NUMBER PER POUND 

 (HEAOS-ON) 



7.2 SPRING 



10.3 FALL 



9.S SPRING 



12.0 FALL 



10 7 SPRING 



5.7 SPRING 



OofO Incotnplal* 



Figure 20. — Relative yields per brood by size class — Saui- 

 bel-Tortugas pink shrimp population, 1056 51>. 



