By comparison, changes in the level of fishing 

 may be expected to produce only relatively minor 

 differences in both the position and height of yield 

 curve maximums (fig. 13B). The revelation that 

 the magnitude of fishing (F) is actually of little 

 concern when deciding at what age or size to begin 

 harvesting a resource in order to achieve maximum 

 yields should not create the impression that fishing 

 mortality has no significance at all. Although 

 maximum utilization of a domestic shrimp resource 

 is the issue under discussion here, the fact remains 

 that resource maintenance is still the overriding 

 objective of shrimp research. As pointed out 

 earlier, the matter of recruitment age or size is at 

 this point largely one of economics. Yet to be 

 answered is the question of how much (in contrast 

 to what size) shrimp may be harvested and still 

 have sufficient residual to maintain the resource at 

 the highest level consistent with projected environ- 

 mental conditions. In effect, rather than inquir- 

 ing as to the optimum t p >, we should perhaps be 

 asking: What is the optimum F? 



Employing commercial fishery statistics, a prior 

 analysis of the Tortugas pink shrimp stock gave 

 rise to speculation that a decline in production over 

 the period 1956-59 was attributable more to poor 

 utilization of supplies than to too intensive fishing 

 and hence improper maintenance of the stock 

 (Kutkuhn, 1962). The observation was made that 

 increasingly heavy exploitation of new recruits, as 

 they enter the fishing grounds and before their 

 average growth rate reaches a maximum, appeared 

 to have systematically reduced annual biomass. 

 Although this conclusion seems to be in conflict 

 with the findings of the present study, it must be 

 stressed that the earlier analysis was necessarily 

 cufsory due to certain data inadequacies and that 

 it attempted to generalize over a long period <>f 

 time and a variety of conditions. In contrast, the 

 results reported here represent a well-organized 

 effort to acquire the information needed to answer 

 specifically the question posed. They are there- 

 fore quite explicit where the others were not, and, 

 accordingly, merit much greater attention and 

 could even be put to interim use. 



The present findings do not, however, constitute 

 the final solution to the stated question. As did 

 those of the first analysis, they only reflect the 

 accumulation of more and better data, and merely 

 serve as one of several anticipated stepping stones 

 toward an unassailable objective. 



M = 0.05 

 ■M = 0.55 



F = 0.96 



AGE AT RECRUITMENT (4») 

 ( Weeks ) 



Figure 13. — Yield in weight per recruit against age at 

 recruitment for observed and hypothetical levels of 

 growth (K), fishing mortality (F), and natural mortality 

 (A/) in a population of pink shrimp. [In plate A, 

 F = 0.96, W m =42.0 g., and the ordinate through t f = 15 

 weeks refers, in terms of corresponding shrimp size, 

 only to the curves for A' = 0.1)7; in plate B, A'=0.07 and 

 We =42.0 g.; * indicates an observed value.) 



YIELD IN VALUE AS A FUNCTION OF AGE 

 (SIZE) AT RECRUITMENT 



Perhaps more meaningful to the entrepreneur is 

 an indication of the size at which harvesting should 

 commence so that total shrimp production will 

 enjoy highest possible value. Necessitated by 

 consumer demands, a well-known characteristic of 

 the Gulf coast shrimp industry is the gradation of 

 ex-vessel price according to the size of shrimp 

 landed and sold. Knowledge of the price-size 

 relationship may be used to advantage in de- 

 termining where, in different growth and mortality 



334 



U.S. FISH AND WILDLIFE SERVICE 



