GULF OF MEXICO COMMERCIAL SHRIMP POPULATIONS 



379 



mutely 5 months after brown shrimp production 

 reiu'hod its highest level (cf. figs. 12A and 26A). 

 Larger sizes of shrimp (40 or less whole shrimp 

 to the pound) nearly always predominated during 

 periods of maximum offshore production (Octo- 

 ber-December). 



Fishahle biomass. — Population densitj^ curves 

 exhibited the same pattern of seasonal fluctuation 

 already described for other exploited shrimps (fig. 

 26B). Greatest white shrimp biomass, with 

 which peak commercial production coincided, 

 occurred in both areas toward the end of each 

 calendar j-ear. Although white shrimp biomass 

 in the Apalachicola area usually exceeded that 

 in the Pensacola-Mississippi River area during 

 corresponding periods of maximum density, greater 

 expenditures of effort for larger and more available 

 shrimp resulted in greater yields from the latter 

 area. Annual levels of fishahle biomass were 

 comparable for both areas, as were the 4-year 

 trends which indicated a slight overall population 

 rise. 



Population characteristics. — Despite rather 

 sketchj' data, semiannual spawning in the north- 

 eastern Gulf's white shrimp stock was suggested 

 by modal-size distributions derived from monthly 

 landings (fig. 26C). Spawning is evidently pro- 

 tracted tliroughout much of the j-ear but, using 

 time of recruitment as a point of reference, 

 heightened activity appears to take place during 

 late spring and early summer (shaded arrows), and 

 again in late fall and early winter (light arrows). 

 This contrasts with a similar phenomenon noted 

 for the coexistent brown shrimp stock in which 

 corresponding periods of peak spawning occur 

 somewhat later, respectively, in late summer and 

 late winter. 



Relative strengths of age classes generated 

 through intensified spawaiing at the beginning and 

 close of the annual reproductive season (late 

 spring-early winter) varied considerably. Broods 

 forthcoming at the season's beginning (shaded 

 arrows) were consistently larger and obviously 

 played the dominant role in sustaining the stock 

 and thus the fisheries dependent upon it. Rem- 

 nants of broods produced during later stages of 

 the annual spawning season, while apparently 

 contributing little to either offshore or inshore 

 fisheries, probably aided in population mainte- 

 nance by complementing spawning popidations. 

 The comparatively greater strength of early- 



season broods is emphasized on the corresponding 

 4-}'ear biomass curves (cf. figs. 26B and 26C). 

 Seasonal modes reflected the occurrence of these 

 broods in the form of (1) recruits-of-the-year and 

 (2) 1-year-old adults, the proportion of each group 

 varying from year to year. Modes largeh' repre- 

 senting late-season broods are barely noticeable. 



Early-season reproductive classes themselves 

 experienced appreciable year-to-year variation 

 both as to time of recruitment and size at recruit- 

 ment. The former is attributable only to varjnng 

 environmental conditions, wliereas the latter could 

 be due to the cumulative effects of fishing and 

 undue environmental changes. Any deleterious 

 effects of fishing, if operative, were so vague as 

 to be undetectable. But the possible effects of 

 large-scale environmental changes warrant some 

 comment. 



The question arises, for example, as to whether 

 the intense storm systems which lashed the Gvilf 

 coast west of the Delta in June and August, 1957, 

 could have caused substantial environmental 

 changes in areas as far east as the Pensacola- 

 Mississippi River area. Landings of white shrimp 

 from offshore and inshore waters, as well 

 as the species' overall population level, were 

 down markedly in this area during the last half 

 of that year (figs. 26-27, A and B). The 

 decline becomes more dramatic when it is 

 noted that effort expended in offshore and in- 

 shore waters during the period July-December 

 was down only 17 and 5 percent, respec- 

 tively, from that expended during the same 

 period in 1956, while corresponding white shrimp 



£ COMMERCIAL tiELO (million of poundi. "•Odi-onl 

 • - APALACMlCOL* 

 0- PENSACOLA-MISSISSIPPI RIVER 



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20-1 g. YIELD PER OAT'S TRiWLINO 



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Figure 27. — Total and nveragc yield of white shrimp 

 populations in inshore waters along the northeastern 

 Gulf coast, 1956-59. 



