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



1.5 kg 2,0 kg 2 5 kg 3.0 kg 



40 kg 



10 1.5 2.0 2.5 



MULTIPLIER OF EFFORT 



35 



Figure 17. — Yield-per-recruit isopleths for the entire 

 Atlantic yellowtin tuna fishery with M = 1.0. 



5 7 8 



0.5 



1.0 15 2.0 2 5 



MULTIPLIER OF EFFORT 



3.0 



3.5 



Figure 18. — Yield-per-recruit isopleths for the entire 

 Atlantic yellowfin tuna fishery with A/ = 0.6. 



when M = 1.0 and more sensitive to changes in 

 size at recruitment when M = 0.6 than when 

 M = 0.8. When M= 1.0 and effort is constant 

 an increase in size at recruitment to 77.5 cm 

 does not change yield per recioiit. However, 

 when M = 0.6, the same change in size at re- 

 cruitment causes a 22% increase in yield per 

 recruit. When M = 1.0 and size at recruitment 

 is held constant, a doubling of effort causes a 

 29% increase in yield per recruit. When M = 0.6, 

 the same change causes a 14% decrease in yield 

 per recruit. When M = 1.0 and size at reci-uit- 

 ment is increased to 77.5 cm. a doubling of effort 

 causes a 39% increase in yield per reciTiit. When 

 M = 0.6, the same changes cause a 27% increase 

 in yield per recruit. 



DISCUSSION 



The use of results of our study must be based 

 on three further assumptions: (1) the composi- 



tion of the fleet will not change; (2) either the 

 gear is currently dispersed so that all qualitative 

 characteristics of the population are available to 

 capture by each gear, or that the dispersal of 

 gear as it now stands will not change; and (3) 

 recruitment is constant. 



Relation Between Composition of Fleet 

 and Optimum Size at Recruitment 



The preceding text has assumed that the 

 composition of the fleet remains constant. The 

 history of the fishery reveals that the composi- 

 tion has been a very dynamic process and there 

 is no reason to believe that it will not continue 

 to be. Since each fishing gear has a different 

 curve of size-specific F, changes in the fleet 

 composition will cause changes in size-specific 

 F for the entire fleet. These changes will cause 

 changes in the yield-per-recruit isopleths. 



To illustrate the influence of changes in fleet 

 composition on management strategy, the 

 optimum size at recruitment was estimated for 

 441 combinations of baitboat and longline effort. 

 For simplicity, effort of purse seiners is not 

 included, i.e., we excluded tw^o variables — 

 small and large purse seiners. Multipliers of 

 effort for each gear ranged from to 2.0 with 

 increments of 0.1. 



The results (Table 10) show a considerable 

 range in the estimates of optimum size at re- 

 cruitment and that minimum size regulations 

 must be adjusted to changes in the composition 

 of the fleet to maintain maximum yield per 

 recruit. As an example, with a 1.0 level of effort 

 by both gears, the minimum size should be 

 about 72.5 cm. If this were instituted as a mini- 

 mum size regulation, the bait boat effort might 

 decline to about 0.2 because of the extreme loss 

 of catch. The minimum size, therefore, should be 

 lowered to 67.5 cm. Now the longline effort 

 might increase by about 80% due to the decrease 

 in competition from bait boats — the minimum 

 size should be increased to 77.5 cm. Finally, 

 suppose an innovation occurs in bait fishing 

 such that non-nominal effort again increases 

 to about 0.7 — the minimum size should be raised 

 further to about 82.5 cm. These changes could 

 occur slowly allowing for a smooth transition 

 of the minimum size regulations. When 

 economics are involved, however, the changes 

 might be precipitous causing the confusion in 

 the above example. If the possible changes in 



57 



