142 



Fishery Bulletin 92(1). 1994 



In order to predict future recruitment, the IATTC 

 is currently studying the relationship between the 

 environment and yellowfin recruitment. If they are 

 successful the yield-per-recruit estimates in Figure 

 9 could be multiplied by the recruitment estimates 

 to better predict future yellowfin catches. 



Little is known about the rate of natural mortal- 

 ity of yellowfin. However, there is no reason to be- 

 lieve this rate will change. But, if it does change, a 

 reasonable assumption would be that if natural 

 mortality goes up, catch will go down and vice versa. 

 Little is known about skipjack population dynam- 

 ics. We assumed that local depletion is negligible for 

 skipjack. However, since skipjack are primarily 

 caught in association with floating objects, if the 

 amount of effort per floating object increases as a 

 result of effort being re-directed from dolphin-asso- 

 ciated tunas to floating objects, then the chances of 

 depletion is certainly possible. If this occurs, our 

 estimates of skipjack catch rates will be too high. 

 This effect could be compounded during years in 

 which floating objects are scarce, because the num- 

 ber of sets per floating object would increase. Since 

 the skipjack catches have been increasing in the west- 

 ern Pacific Ocean, their abundance and catch in the 

 eastern Pacific could be lower than our estimates. 



A moratorium on dolphin sets is likely to result 

 in reduced catchability, yield per recruit, average 

 age, and total biomass of yellowfin. The catch of 

 yellowfin, based on these factors only, was predicted 

 to decline by approximately 55,600 t (25%). On the 

 other hand, skipjack catches could increase, making 

 the reduction in total tuna catches much smaller 

 (4%). The effects of reductions in fishing effort, the 

 range of the fishery, and recruitment were not ana- 

 lyzed in this study because they are currently un- 

 predictable; however, all three would result in an 

 additional decrease in total tuna catches. If better 

 predictions of effort levels and yellowfin recruitment 

 are made, the yield-per-recruit estimates in Figure 

 9 could be used in conjunction with them to better 

 predict yellowfin catches. The results of our analy- 

 sis indicate that catches in the first years after a 

 moratorium begins may not be indicative of the long- 

 term catches. Fishermen, biologists, and managers 

 should not consider these first-year catches as indices 

 of future catches, because recruitment and catchability 

 vary annually. On the other hand, our estimates of 

 future average catches should be useful unless there 

 are long-term changes in catchability or recruitment. 



Acknowledgments 



We would like to thank James Joseph, director of 

 investigations of the Inter- American Tropical Tuna 



Commission for suggesting the need for this re- 

 search, Richard B. Deriso for his many methodologi- 

 cal suggestions, Alejandro Anganuzzi for his reviews 

 and for sharing his knowledge about about dolphins, 

 and William H. Bayliff for his extensive editorial re- 

 views of this manuscript. 



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