FISHERY BULLETIN, VOL. 72. NO. 1 



several values of natural mortality. The results 

 (Figures 1-3) indicated that yield per recruit 

 would increase from to 20% if effective mini- 

 mum size is increased and effort remains con- 

 stant. Again, our most reasonable estimate of 

 the increase is only 8% . The results also indicate 

 that little if any increase in yield i)er recruit 

 would occur if fishing effort is doubled and 

 effective minimum size is unchanged. However, 

 if the effective minimum size is increased and 

 effort is doubled, a modest (20 to 40% ) increase 

 in yield per recruit could occur. All these results 

 again assume that there would be no dumping 

 of small yellowfin tuna. 



We finally assumed that the available data 

 are accurate enough to also make reasonably 

 accurate estimates of size-specific F. When 

 using our most reasonable parameter estimates 

 and holding effort constant, an increase in size 

 at recruitment to 55 cm (3.2 kg) would obtain a 

 3.9% increase in yield per recruit and to 77.5 cm 

 (8.9 kg) would cause less than a 10% increase in 

 yield per recruit. Increasing the size at 

 recruitment to 55 cm with M = 0.6 would 

 cause a 7% increase in yield per recruit, but with 

 M = 1.0 only a 1% increase would occur. In- 

 creasing the size at recruitment to 77.5 cm with 

 M = 0.6 would increase yield per recruit by 22% , 

 but with M — 1.0 no increase would occur. 

 When size at recruitment is held constant and 

 fishing effort is doubled, our best estimate of the 

 change in yield per recruit is a 6% increase. 

 Our estimates ranged from a 14% decrease to a 

 29% increase. It seems safe to agree with the 

 report of the Abidjan meeting that if conditions 

 remain constant, there is little to be gained on 

 a yield-per-recruit basis from increases in fish- 

 ing effort. With a doubling of fishing effort and 

 an increase in size of recruitment to 55 cm, our 

 most reasonable estimate is a 15% increase in 

 yield per recruit, with a range of a 1% decrease 

 to a 35% increase. When size at recruitment is 

 increased to 77.5 cm and fishing effort is doubled, 

 our most reasonable estimate of the change 

 in yield per recruit is a 30% gain; however, the 

 estimates range from 27 to 44% . Thus it appears 

 that if it is possible to increase the size at 

 recruitment, a doubling of effort would i)roduce 

 a modest increase in yield per recruit. These 

 results, it must be noted, assume that small 

 yellowfin tuna are not dumped. 



It is interesting to note that the same general 

 conclusions would be made using either the 



knife-edged recruitment or size-specific F 

 approaches. The size-specific F approach, in 

 addition, allows us to examine more precisely 

 the effects of an absolute minimum size regula- 

 tion and the effects on each gear. The general 

 conclusions from both aspects of this study also 

 agree fairly well with those of Joseph and Tom- 

 linson (1972, see footnote 4). It is not surprising, 

 however, that results from the size-specific F 

 approach agree with theirs because they used 

 similar methodology and data. Both estimates 

 suggest that under present conditions the fisheiy 

 is near the point of maximum yield per recruit. 

 Specifically addressing the recommendations 

 outlined in the introduction section of this paper 

 for considering a minimum size between 3.2 

 and 10 kg, we offer the following results based 

 on our most reasonable parameter estimates: 



1. Minimum size limit 55 cm (3.2 kg): 



a) Current levels of fishing mortality: 



i) No dumping results in a 4% increase 

 in landed yield per recruit 



ii) 100% dumping results in a 3% de- 

 crease in landed yield per recruit 



b) Doubling fishing mortality: 



i) No dumping results in a 15% in- 

 crease in landed yield per recruit 



ii) 100% dumping results in a 1% in- 

 crease in landed yield per recruit 



2. Minimum size limit 77.5 cm (8.9 kg): 



a) Current levels of fishing mortality: 



i) No dumping results in a 9% in- 

 crease in landed yield per recruit 

 ii) 100% dumping results in a 13% de- 

 crease in landed yield per recruit 

 b. Doubling fishing mortality: 



i) No dumping results in a 31% in- 

 crease in landed yield per recruit 

 ii) 100% dumping results in a 16% de- 

 crease in landed yield jier recruit. 



The 55-cm (3.2 kg) minimum size limit would 

 likely be of more benefit to the tuna fishery than 

 the larger minimum size limit of 77.5 cm (8.9 

 kg) since less dumping would occur. Therefore, 

 there would likely be, on the average, an increase 

 in landed yield per recruit at the current or 

 greater levels of fishing mortality; whereas, 

 if a larger size limit were adopted, there would 

 likely be, on the average, a decrease in landed 

 yield per recruit at current levels of fishing 



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