Abstract. — The eastern Pa- 

 cific purse-seine tuna fishery has 

 historically been very productive, 

 yielding up to 400,000 metric tons 

 (t) per year of primarily yellowfin, 

 Thunnus albacares, and skipjack, 

 Katsuwonus pelamis. However, ef- 

 forts to minimize dolphin (prima- 

 rily spotted dolphin, Stenella 

 attenuata, spinner dolphin, S. 

 longirostris, and common dolphin, 

 Delphinus delphis) mortality inci- 

 dental to tuna seining in the east- 

 ern Pacific ocean have been in- 

 creasing. Therefore, predictions of 

 what the tuna catches will be in 

 the future, if there is a ban or 

 moratorium on catching dolphin- 

 associated tuna, are useful. Based 

 on recruitment levels, age-specific 

 catchability coefficients for yellow- 

 fin tuna caught without dolphins, 

 and average fishing effort ob- 

 served during 1980-88, we pre- 

 dicted that yellowfin catches 

 would be reduced by an average of 

 about 25%. These results were 

 verified by Monte Carlo simula- 

 tions, by using average effort and 

 randomly selected yellowfin re- 

 cruitment and catchability coeffi- 

 cients from 1980 to 1988, which 

 predicted a mean annual decrease 

 of 55,563 t or 24.7% of yellowfin 

 catch. The actual reduction in yel- 

 lowfin catch might be greater be- 

 cause 1) fishing effort will prob- 

 ably decline, 2) the range of the 

 fishery might be reduced to the 

 traditional inshore non-dolphin 

 regions, and 3) yellowfin recruit- 

 ment could be reduced by the 

 change in age structure and popu- 

 lation size likely to result from a 

 moratorium. Because skipjack sel- 

 dom associate with dolphins, redi- 

 rection of fishing effort to schools 

 of tuna not associated with dol- 

 phins would probably result in in- 

 creased skipjack catch rates. How- 

 ever, the magnitude of the in- 

 crease is difficult to estimate, be- 

 cause the population dynamics of 

 skipjack are poorly understood. 

 Finally, this study predicted that 

 the catches in the first years after 

 a moratorium on dolphin sets 

 would not necessarily reflect long- 

 term catches. 



Potential tuna catches in the eastern 

 Pacific Ocean from schools not 

 associated with dolphins 



Richard G. Punsly 

 Patrick K. Tomlinson 

 Ashley J. Mullen 



Inter-American Tropical Tuna Commission 

 8604 La Jolla Shores Dr. La Jolla. CA 92037 



Manuscript accepted 22 July 1993 

 Fishery Bulletin 92:132-143 (1994) 



Since the late 1950's, purse-seine 

 fishermen in the eastern Pacific 

 Ocean (EPO), knowing that schools 

 of yellowfin tuna (Thunnus alba- 

 cares) often associate with dolphins 

 (primarily spotted dolphins, Sten- 

 ella attenuata, spinner dolphins, S. 

 longirostris, and common dolphins, 

 Delphinus delphis), have used the 

 dolphins to help locate and capture 

 yellowfin. Dolphins are relatively 

 easy to detect, being larger and 

 closer to the surface than yellowfin. 

 In fact, the most efficient means of 

 catching the 2- and 3-year-old yel- 

 lowfin, which comprise the largest 

 component of the tuna catch in the 

 EPO, is purse-seine fishing for dol- 

 phin associated schools (Punsly 

 and Deriso, 1991). Yellowfin remain 

 associated with dolphins while the 

 net is being set around the dolphin 

 herds. The fishermen attempt to 

 release all of the dolphins from the 

 net; however, incidental mortality 

 sometimes occurs through entang- 

 lement. 



As a result of increasing public 

 pressure to prevent mortality of 

 dolphins incidental to tuna purse 

 seining, elimination of setting on 

 dolphin-associated tunas is being 

 considered. Therefore, fishermen, 

 biologists, and managers need to 

 know the extent to which tuna 

 catch in the EPO might be reduced 

 by the elimination of sets on dol- 

 phin-associated fish. The objective 

 of this study was to estimate this 

 potential reduction in the catch. No 



such estimates have been pub- 

 lished previously. 



Tuna catches could be affected by 

 a ban or moratorium on dolphin 

 sets in six ways: 



1 The overall catchability of yel- 

 lowfin by purse seiners could be 

 reduced. 



2 The yield per recruit of yellow- 

 fin could decline because non- 

 dolphin-associated yellowfin 

 caught by purse seiners are 

 mostly composed of fish younger 

 than the optimum age of entry 

 (Calkins, 1965; Allen, 1981). 



3 The average age of yellowfin and 

 mean biomass may be reduced 

 by fishing on younger age 

 groups. This might not only re- 

 duce the catch in weight, but 

 also reduce the spawning poten- 

 tial and possibly the resulting 

 recruitment. 



4 Since the offshore EPO purse- 

 seine fishery is directed prima- 

 rily at dolphin-associated fish 

 (Fig. 1, A and B), a moratorium 

 on setting on dolphin herds 

 could result in a contraction of 

 the range of the fishery into in- 

 shore regions. The number of 

 fish recruited to this new 

 smaller area might be lower 

 than the number recruited to the 

 entire area. Lower effective re- 

 cruitment would also result in 

 lower catches. 



5 If a moratorium on catching dol- 

 phin-associated tuna occurs, 



132 



