NUMBER -per-POUND EQUIVALENTS (HEADLESS 



WEIGHT of WHOLE SHRIMP 



(g.) 



< Commercial Size 



Figure 14. — Ex-vessel price (p) against weight (w) of pink 

 shrimp landed at Key West, Fla., during September- 

 December 1961. Also shown are the expression for con- 

 verting price to value (v) on an individual-shrimp basis, 

 and the resulting plot of value as a function of shrimp 

 weight. In the upper left-hand corner, the value per 

 shrimp in cents is graphed as a function of price per pound 

 in the same units. 



-CORRESPONDING NUMBER -PER-POUND EQUIVALENTS 

 (Heodless) 



AGE AT RECRUITMENT U/l 

 (Weehs) 



Figure 15. — Yield in value per recruit against age at 

 recruitment for different levels of natural mortality in a 

 population of pink shrimp. [F=0.96; A% = 0.08; V a 

 = 4.8 cents (per shrimp); a, indicates an observed as 

 contrasted to a hypothetical value.] 



curves (fig. 15). Upon comparing these with the 

 yield-in-weight curves shown in figure 12, the 

 most noticeable (though not necessarily significant) 

 difference is seen in the relative positions of their 

 maximum points. Unless natural mortality is 

 extremely low, highest economic yield from a 

 given age group during its fishable life span 

 clearly occurs when exploitation begins as soon as 

 the average shrimp attains commercial acceptance. 

 In all but those circumstances where extraordi- 

 narily low natural mortality operates, no advance- 

 ment of the recruitment age is indicated. This 

 implies, of course, that the relationship between 

 price and size holds relatively static. 



Of interest is the general observation that, in 

 situations where postponement of exploitation 

 may be in order, maximum economic yield can be 

 expected at a recruitment age (or size) slightly 

 lower than that suggested for maximum biological 

 yield. Figures 12 and 15 for M=0.05 illustrate 

 this very nicely. Note that the yield-in-weight 

 curve (fig. 12) reaches its highest point at a 

 recruitment size of about 31-35 count (headless), 

 whereas the yield-in-value maximum occurs 

 roughly at a headless-count somewhere between 

 36 and 40 (fig. 15). This difference is of appreci- 

 able biological magnitude and can be largely attrib- 

 uted to the somewhat higher rate of growth in terms 

 of value (k„=0.08 versus #=0.07). To reiterate, 

 at levels of natural mortality known to be well 

 within the range of expectation, the total yield of 

 a given age group during its fishable life span will 

 be maximal only when fishing begins at the mini- 

 mum marketable size. Postponement to a larger 

 initial size would engender economic loss. 



SUMMARY 



Measures of growth and mortality in a popula- 

 tion of pink shrimp obtained simultaneously by 

 the mark-recapture technique permitted critical 

 examination of the interrelationship of these 

 parameters in assessing present utilization of the 

 resource. 



Of 2,090 carefully graded, precommercial-size 

 shrimp injected with blue dye and released in late 

 September (1961) at a point on the periphery of the 

 well-known Tortugas (Florida) fishing grounds, 

 443 (21 percent) were returned during the ensuing 

 3 months. Recapture (commercial fishing) effort 

 was closely surveyed throughout this experiment 



336 



U.S. FISH AND WILDLIFE SERVICE 



