FISHERY BULLETIN, VOL. 72. NO. 1 

 Table 2. — Basic data on size (age) composition of catch of yellowfin tuna from the tropical Atlantic Ocean. 



by Lenarz ( 1971b). '^ We followed the modifica- 

 tion of Joseph and Tomlinson (1972, see foot- 

 note 4) by using the inverse of the von Bertalanffy 

 growth equation to convert size distributions to 

 age distributions. This method assumes that 

 there is a reasonably accurate relationship be- 

 tween length and age of yellowfin tuna. This 

 assumption has not been verified. Ageing by 

 modal progression would probably be more satis- 

 factory, if more complete length composition 

 data were available on a monthly or quarterly 

 basis. 



The reverse iterative i)rocedure with com- 

 puter program COHORT and M = 0.8 was used 

 to estimate size-specific values of fishing mortal- 

 ity (F) starting at the 180-cm interval. Four 

 initial values of F were tried: 0.2, 0.4, 0.6, and 

 0.8 (Figure 7). Estimates of F tend to converge 



as size of the yellowfin tuna decreases with the 

 range of initial values tried as is characteristic 



'2 Lenarz, W. 1971b. Yield per recruit of Atlantic 



yellowfin tuna for multigear fisheries. Southwest Fisheries 



Center, National Marine Fisheries Service, La Jolla. 

 Calif. Unpublished manuscript. 



>| ' I I I I I I Ill 



40 60 80 100 120 140 160 180 



FORK LENGTH (cm.) 



Figure 7. — Estimates of size-specific instantaneous fishing 

 mortality coefficients (F) with several initial F values. 



46 



