Punsly et al.: Potential non-dolphin-associated tuna catches in the eastern Pacific Ocean 



141 



*: 



8 



OC 1.5 



S 



a. 

 ■o 



O 1.0 



ated yellowfin to purse seiners could 

 result in significantly different catches 

 than we estimated. Allen and Punsly 

 (1984) showed that both environmental 

 and vessel efficiency factors affect the 

 catchability of yellowfin by purse sein- 

 ers in the EPO. Improvements in vessel 

 efficiency could increase future 

 catchability coefficients; whereas, envi- 

 ronmental factors could produce either 

 higher or lower catchability coefficients 

 than those observed during 1980-88. 

 Environmental factors affecting 

 catchability could conceivably mask the 

 effects of a moratorium on dolphin sets 

 for several years. For example, if a 

 moratorium on dolphin sets had been 

 imposed at the beginning of 1983, the 

 low catch in 1983 would have made it 

 appear that the decline resulted from 

 the moratorium. However, we predicted 

 that a moratorium would have had the 

 smallest effect in 1983 (Table 1). Fish- 

 ermen, biologists, and managers should 

 be aware that catches during the first year after a 

 moratorium starts may not be indicative of long- 

 term averages. However, since 9 years of data were 

 used, our long-term average estimates should only 

 be affected by long-term changes in catchability. 



An assumption that effort will be lower in the 

 future may be more realistic than our assumption 

 that effort will remain at 1980-88 levels. However, 

 we could not predict the extent to which effort might 

 be reduced because it is affected by ex-vessel tuna 

 prices at canneries all over the world, the prices of 

 other foods, and the cost of fuel. Nevertheless, if we 

 could estimate what the effort reductions would be 

 in the future, the effort multipliers in the the yield- 

 per-recruit estimates in Fig. 9 could still be used. 



If the fishery contracted into the traditional in- 

 shore school- and log-set areas after a moratorium 

 on dolphin sets, then catches may be lower than we 

 estimated them to be. For example, if the area fished 

 were smaller, and mixing between the fish inside 

 and outside the area were incomplete, then the new 

 fishing area would encompass fewer fish than the 

 total area. Therefore, all of the population sizes of 

 yellowfin used in the equations in the methods sec- 

 tion would be overestimated. Recruitment estimates, 

 which are estimates of the number of 30-cm yellow- 

 fin, would also be overestimated. In addition, if fish- 

 ing effort remained high, but the range contracted, 

 then a gear- competition effect might lower the catch 

 of both yellowfin and skipjack. However, since effort 

 levels are expected to decline after a moratorium, 



j I l I I I i 



Observed Mixture 

 of Fishing Modes 



No Dolphin Sets 



1 I I I L 



0.2 0.4 O.e 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 



Effort Multiplier 



Figure 9 



Yield per recruit of yellowfin tuna (Thunnus albacares) (kg) for 

 the observed 1980-88 fishery (solid line) and a fishery with all 

 effort directed at non-dolphin (Delphinidae) sets (dashed line). 

 An effort multiplier of 1.0 refers to the 1980-88 average effort. 



localized depletion of tuna due to a contracted fish- 

 ery is unlikely. 



We assumed that yellowfin recruitment in the 

 future would not be affected by the changes in popu- 

 lation size and age structure which might result 

 from re-directing effort toward smaller fish, because 

 a relationship between yellowfin spawning biomass 

 and recruitment has not yet been demonstrated. 

 However, a spawner-recruit relationship for yellow- 

 fin may be discovered in the future, because better 

 estimates of yellowfin fecundity by size offish, sea- 

 son, and area are currently being developed at 

 IATTC. When this work is completed we may be able 

 to predict recruitment levels and their resulting 

 catches more accurately in the future. If future re- 

 cruitment levels could be estimated, the future catches 

 could be derived by multiplying the recruitment esti- 

 mates by the yield per recruit shown in Figure 9. 



Environmental factors have long been suspected 

 of having significant effects on yellowfin recruit- 

 ment. For example, favorable conditions in the late 

 1980's may have contributed to the large number of 

 recruits (Bayliff, 1992). In 1987, the number of re- 

 cruits was so large that the effect of a moratorium 

 in 1988 would have been masked by a high catch of 

 1.5 year old yellowfin, first recruited during 1987. 

 In 1988, the high abundance of 1.5 year old fish (Fig. 

 4) coupled with their high catchability for non-dol- 

 phin sets (Fig. 5) caused the estimated yellowfin 

 catch if all effort were directed at non-dolphin sets 

 to be almost as high as the estimated actual catch. 



