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



Table 3. — Estimates of yield per recruit (kg) when M = 0.8. initial F = 0.2, and growth curve of LeGuen and 



Sakagawa ( 1973) is used. — Continued. 



are shown in Figure 11. Table 3 and Figures 8, 

 9, and 11 indicate that if size at recruitment 

 remains constant at 32.5 cm, very little increase 

 in yield per recruit ('^5%) can be expected if 

 effort is increased, and if effort remains constant, 

 very little (~10% ) increase in yield per recruit 

 can be expected by increasing size at reciiiit- 

 ment. However, if fishing effort is doubled (i.e., 

 multiplier = 2.0) and size at recruitment in- 

 creased to 55 cm (3.2 kg), yield per recruit 

 would increase 15% , or if size at recruitment 

 is increased to 77.5 cm (~10 kg), yield per re- 

 cruit would increase about 30% (Table 3). Since 

 the line of eumetric fishing shows that optimum 

 size at recruitment changes with fishing effort, 

 any "minimum size" regulation must be geared 

 to fishing effort. 



If fishermen are unable to distinguish the size 

 of yellowfin before capturing them and a mini- 

 mum size regulation prevents their landing, then 

 the discarding of dead yellowfin will occur. 

 Table 4 presents landings per recruit by gear 

 and Figure 12 the landings per recruit for the 

 total fishery when killing and discarding 

 ("dumping") of all yellowfin smaller than the 

 size limit occurs. If the minimum size limit is 

 55 cm and effort remains the same, then a 2.7% 

 decrease in landings per recruit would occur; 

 and a 13% decrease in landings per recruit would 

 occur if the minimum size is set at 77.5 cm. If 

 effort is doubled and the minimum size is 55 

 cm, then a 1% increase in landings per recniit 

 would occur; with a minimum size of 77.5 cm, 

 a 16% decline in landings per recruit would 



30 4.0 4 5 50 5 5 



1,0 1.5 2.0 2.5 



MULTIPLIER OF EFFORT 



3.0 



3.5 



Figure 11. — Yield-per-recruit (kg) isopleths for the entire 

 Atlantic yellowfin tuna fishery. Dotted curve is the line 

 of eumetric fishing. 



00 



0.5 1.0 1.5 2.0 25 3.0 



MULTIPLIER OF FISHING EFFORT 



Figure 12. — Landings-per-recruit (kg) isopleths for Atlan- 

 tic yellowfin tuna when all fish less than the minimum size 

 that are caught are discarded dead. 



49 



