GULF OF MEXICO COMMERCIAL SHRIMP POPULATIONS 



373 



diirincr early and midwinter (fig. 19B) witii the 

 major portion of eacli calendar year's catch being 

 made during and for a short time following this 

 period (fig. 19A). But although brood "H" 

 sustained early 1959 production, it did so at an 

 appreciably reduced level, the consequence being 

 that the catch in 1959 was the lowest in recent 

 years. Moreover, its quality was lessened because 

 the fishery had to rely upon very small shrimp 

 belonging to the subsequent brood, "I". 



This gross analysis is clarified somewhat in 

 figures 20 and 21, which rouglily delineate absolute 

 and average yields by size classes for successive 

 broods. In both figures, the serial relationships 



30 



20 - 



10 - 



1959 



-JJ 



il±J 



30 



20 



10 - 



1958 



 lllll 



30 



20 



10 







1957 



30r 



20- 



10- 



30 40 >40 



NUMBER PER POUND 

 (HEADS-ON) 



FiGrnE 22. — Relative size composition of commercial pink 

 shrimp landings from the Sanibel-Tortiigas area, 1956-59. 



inferred above are reasonably clear. Harvest 

 of small siiriiiip [35 count (whole) and smaller] 

 in excess of that amount presumably compen- 

 sating for natural mortality markedly reduces 

 the expected yield of premium shrimp. Unfor- 

 tunately, measurement of each brood's biomass 

 at recruitment was impossible, thereby precluding 

 provision of a basis for comparing broods and 

 for studying the effects that varying fishing 

 intensities at different developmental stages had 

 on total brood yields. With controlled fishing, 

 however, a jdeld curve such as described by 

 brood "F" (figs. 20 and 21) would appear to 

 approach that which is most desirable from the 

 standpoint of achieving maximum utilization of 

 the resource. Note that its mode occurs at the 

 25-30 (41-50 heads-off) count-size range, the 

 point on the population growth curve where the 

 rate seemingly readies a maximum. 



The foregoing explanation of the Tortugas 

 fishery's 4-year decline was founded on the 

 premise that within- and bctween-year recruit- 

 ment remained fairly constant. This assumption 

 is not refuted upon gross examination of available 

 data. The problem then was one of deciding at 

 what stage of population growth the resource 

 should be cropped so as to obtain the maximum 

 virtual j-ield. No mention was made of the 

 possibility that the Tortugas stock on the whole 

 was being overfished although a continuation of 

 present trends might justify its speculation. Up 

 to the present stage of development in the 

 Tortugas fishery, the pink shrimp population 

 supporting it has displaj-ed great resiliency in 

 overcoming any adversities tliat might have 

 been associated with exploitation. Presumably, 

 the species liigh reproductive potential and the 

 relatively undisturbed state of its inshore nursery 

 grounds have thus far offset any incursions due 

 to fishing on the mature stock. On the one hand, 

 this suggests the likehhood that environment 

 control could enhance the carrying capacity of 

 estuarine waters in wliich immature shrimp 

 undergo early development, with annual recruit- 

 ment and yields being supplemented accordingly. 

 On the other, it stresses the importance of pro- 

 tecting existing nursery areas from ill-advised 

 modifications, and carefully regulating the take 

 of juvenile shrimp there. However, the pos- 

 sibility of excessive fishing on the mature stock 

 resulting in levels of reproduction below those 



