372 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



SrZE CLASS 

 MEDIUM 



ESTIMflTEO 

 OVEHALC SEASON 



LARGE AVERAGE RECRUITED 



0.28 FALL 



1 1 — I — I r 



40 40 30 24 le I 



NUMBER PER POUND 

 (HEADS- ONI 



0.26 SPRING 



0.28 FALL 



26 SPRING 



031 FALL 



023 SPRING 



0-25 FALL 



34 SPRING 



Daro IncDmplatt 



Figure 21. — Yield per 24 hours' fishing bj' brood and size 

 class — Sanibel-Tortugas pink shrimp population, 1956-59. 



viding monthly landings at tlie low, intermodal 

 points on corresponding weight frequency curves. 

 Note that those broods recruited in the fall ordi- 

 narily yielded the greater portion of their virtual 

 biomass at juvenile (small) stages, whereas those 

 recruited in the spring j'ielded most heavily at 

 more advanced stages. A similar but not as pro- 

 nounced pattern is reflected in figure 21 which 

 depicts the average weight by size class contrib- 

 uted by each brood at successive life history 

 stages. The relative uniformity of average "small- 

 shrimp" catches yielded by broods considered in 

 this study suggests a correlation between availa- 

 bility of newly recruited broods (small shrimp) 

 and the amount of effort expended in their cap- 

 ture. Since peak fishing traditionally occiu-s 

 shortly after its appearance, the fall brood is sub- 

 jected to comparatively disproportionate exposure 

 at such times. Evidence suggests that the degree 

 of exploitation suffered by broods at and for a 

 short period following recruitment not only gov- 

 erns the magnitude of fluctuation in annual yields 

 but controls their composition as well. 



Growth in fished (or recruited) portions of the 

 Sanibel-Tortugas population was fairly uniform 

 during the 4-year study period. Assuming delin- 

 eation of successive generations in figiu-e 19C is 

 reasonable, members of broods recruited in the 



spring, on the average, grew slightly faster and 

 attained maturity earlier than members of broods 

 recruited in the fall. The former required an 

 average of 13 months contrasted to the latter 's 

 15 months to grow from a size equivalent to 

 31-40, to 9-12 shrimp to the pound. A growth 

 differential between sexes is recognized for com- 

 mercial Penaeidae (e.g., Lindner and Anderson, 

 1956), but its manifestations in the present 

 analysis are obscured. Sex composition data 

 would have permitted an evaluation of the effects 

 of a variable sex ratio on growth patterns in the 

 total population. 



Summary of Ji.-year status. — The Sanibel-Tortu- 

 gas pink shrimp stock evidenced more sensitivity 

 to the mechanics of exploitation than any stock 

 thus far examined. Causes of widely fluctuating 

 and declining annual yields despite a relatively 

 static effort expenditure can be traced to differ- 

 ential fishing on broods immediately following 

 their recruitment. This is reflected in what 

 appears to have been a progressively greater 

 demand for and utilization of small shrimp during 

 the 4-year study period (fig. 22). The net result 

 has been a corresponding downward trend in 

 stock biomass (fig. 18B), the dependent fishery 

 at the same time being subject to varying avail- 

 ability and inconsistent quality in terms of size 

 of shrimp harvested. In striving to obtain higher 

 and higher production, operations and general 

 economy have had to depend more and more on 

 harvests of small, non-premium shrimp. Unfor- 

 tunately, information on the value of landings 

 taken annually from the Sanibel-Tortugas stock is 

 not available for trend study. But Idyll (1957) 

 stated that the fishery's annual net profits report- 

 edly showed a gradual decline for the years 1953 

 through 1956, even though production in the 

 latter year represented an alltime high. Indica- 

 tions are that the trend has not changed. 



Examples of how recent patterns of exploitation 

 have contributed to the present status of the 

 Sanibel-Tortugas fishery may be readily provided. 

 As previously stated, relatively heavy fishing on 

 small post recruits visibly curtailed the expected 

 potential of certain broods. For instance, note 

 in figure 19C that brood "H" was fished compara- 

 tively hard immediately following recruitment (cf . 

 fig. 19A), and at a period when the total pink 

 shrimp biomass was at a reduced level (fig. 19B). 

 To reiterate, peak biomass ordinarily occurs 



