Pressure to reduce fishery-related dolphin mortality in the tuna purse-seine fishery of the eastern tropical Pacific Ocean (ETP) may alter fishing patterns from the current situation, in which most tuna is caught in association with dolphins, to a situation in which these "dolphin sets" are replaced by sets on unassociated schools of tuna (school sets) or on floating objects (log sets). Although not likely to effect any substantial change in dolphin population dynamics, owing to current low levels of dolphin mortality, such a change in fishing pattern could damage the commercial stock of yellowfin tuna and perhaps stocks of other tuna species because, unlike dolphin sets which generate almost no unwanted catch, school and log sets capture moderate to large amounts of undersized or otherwise undesirable tuna that are subsequently discarded at sea. In this study, we examined the differences between set types (in short tons of tuna discarded per set by U.S. vessels fishing in the ETP during 1989-92) and then predicted the increase in short tons of tuna discarded per set (all tuna species combined) that would likely result from replacing the dolphin sets with an equivalent number of school or log sets (or both). Expected discard weight of tuna was 100 times higher for log sets and 10 times higher for school sets than for dolphin sets. Average expected tuna discard per set was 0.06 short tons for dolphin sets, 1.15 short tons for school sets, and 10.5 short tons for log sets. Hypothetically, redistributing all dolphin sets to log sets increased estimated average discard of tuna by the international fleet in the ETP by about 337%. A hypothetical replacement of all dolphin sets with log sets would lead to estimated discards of 10-25% of the estimated average number of yellowfin recruits to the fishery each year. If this discard is combined with an estimated 25% reduction in tons of yellowfin tuna caught that would result from concommitant changes in size structure of the landed fish (Punsley et al., 1994), the fishery could lose 30-50% of the approximately 98 million individual yellowfin tuna estimated to recruit to the fishery each year. Sustained removals of this magnitude, combined with environmental variability, could pose problems for long-term sustainability of the ETP yellowfin tuna stock; moreover, the change in fishing-effort patterns leading to this effect on yellowfin tuna would likely contribute little to recovery or sustainability of ETP dolphin stocks.
