LENARZ ET AL.: YIELD PER RECRUIT OF ATLANTIC YELLOWFIN TUNA 



creases from 55 cm to 97.1, 107.9, and 123.5 cm, 

 respectively depending on L^ . The increase in 

 yield per recruit by increasing the effective 

 minimum size from 55 to 83.2 cm, our most 

 reasonable estimate, is only 7.9% . 



From the above analysis using a wide range 

 of parameter estimates, we can conclude with 

 reasonable assurance that virtually any increase 

 in the effective minimum size will cause an in- 

 crease in yield per recruit. Our most likely 

 estimate of this increase in yield per recruit is 

 only 7.9% which is bounded, with reasonable 

 parameter estimates, by 0% and 45% . 



Ricker Model 



Ricker model yield-per-recruit isopleths were 

 calculated using values of M of 0.6, 0.8. and 1.0 

 to illustrate our estimates of actual (rather than 

 relative) yield per recruit (Figures 1, 2, and 3). 

 As will be mentioned in the next section it is 

 difficult to estimate the location of the fishery 

 on the graphs, i.e., when fishing mortality is 

 size specific it is not a trivial matter to make 

 reasonable estimates of age at recruitment, 

 t^, and a constant total mortality coefficient, 

 Z. Our most reasonable estimates, taken from 

 Lenarz and Sakagawa (1972, see footnote 5), of 

 these parameters are: t '. is 1.41 yr and Z is 

 1.4. 



0.5 10 1.5 2.0 2.5 30 



INSTANTANEOUS FISHING MORTALITY (F) 



35 



5 10 15 2,0 2 5 



INSTANTANEOUS RATE OF FISHING MORTALITY (F) 



Figure 2. — Yield-per-recruit isopleths as functions of fish- 

 ing mortality and age (and weight) at recruitment when 

 M = 0.8. 



-60.5 



48.0 



229 



Figure 1. — Yield-per-recruit isopleths as functions of fish- 

 ing mortality and age (and weight) at recruitment when 

 M = 0.6. 



5 10 1.5 20 25 



INSTANTANEOUS RATE OF FISHING MORTALITY (F) 



Figure 3. — Yield-per-recruit isopleths as functions of fish- 

 ing mortality and age (and weight) at recruitment when 

 M = 1.0. 



The results (Figures 1, 2, and 3) show, for 

 example, that with M = 0.6 and Z remaining 

 constant (1.4), an increase in age at recruitment 

 from 1.41 to 1.83 yr (or 77.5 cm) raises the yield 

 per recruit about 20% ; if iV/ = 0.8, the same 

 change raises the yield per recruit on the order 

 of 10% ; and if M = 1.0, the same change does 



43 



