140 



Fishery Bulletin 92(1), 1994 



■j. 



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IX 



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250 



200 



150 



100 



50 



50 60 70 80 90 100 110 

 Yellowfin Catch Retained (%) 



Figure 8 



Frequency distribution of the percent of the yel- 

 lowfin tuna (Thunnus albacares) catch in weight 

 retained in the ninth year of a ban on dolphin 

 (Delphinidae) sets, from 1,000 Monte Carlo simu- 

 lated replicates. The percent of catch retained is 

 calculated as 100x (the catch if all effort were 

 directed at non-dolphin sets) / (the catch from the 

 observed mixture of set types). 



lations (Table 2) were heavily influenced by the re- 

 cruitment and fishing effort levels used. Recruit- 

 ment in the future may be different from that of 

 past, because of changes in population size, age 

 structure, and environmental factors. Therefore, 

 actual future catches could be different from what 

 we estimated. For these reasons, results in terms of 

 reduction in yield per recruit are of interest. We 

 estimated that the change to non-dolphin sets only 

 would result in the reduction of the yield per recruit 

 of yellowfin from the observed value of 2.8 kg per 

 recruit to 2.1 kg as shown in Figure 9. In addition, 

 effort levels could change in the future, perhaps as 

 a reaction to the moratorium. Therefore, estimates 

 of yield per recruit for various levels of effort might 

 be useful. If effort levels change in the future, the 

 multipliers on the X-axis in Fig. 9 could be used to 

 estimate the potential yellowfin catch. 



Discussion 



In order to predict what the tuna catches might be 

 in the future if there were a moratorium on dolphin 

 sets, we estimated what the tuna catches would 

 have been during 1980-88, had there been a mora- 

 torium on dolphin sets beginning in 1980. Using 

 these estimates to predict future catches required 

 the following assumptions: 



1 Age-specific, non-dolphin 

 catchability coefficients will 

 be the same in the future 

 as during 1980-88. 



2 Fishing effort will remain 

 at 1980-88 levels. 



3 The geographic distribution 

 of effort will be the same as 

 during 1980-1988 (Fig. 2, A 

 and B combined). 



4 Recruitment will be at 

 1980-88 levels. 



5 Natural mortality will not 

 change in the future. 



6 Skipjack abundance will 

 not significantly change. 



Significant deviations from 

 these assumptions could make 

 our estimates less valid. There- 

 fore, the potential ramifications 

 of deviations from the assump- 

 tions are discussed in detail below. 



Major changes in the vulner- 

 ability of non-dolphin-associ- 



