210 
Abstract .—Pressure to reduce fish- 
ery-related dolphin mortality in the 
tuna purse-seine fishery of the eastern 
tropical Pacific Ocean (ETP) may alter 
fishing patterns from the current situ- 
ation, in which most tuna is caught in 
association with dolphins, to a situa- 
tion in which these “dolphin sets” are 
replaced by sets on unassociated 
schools of tuna (school sets) or on float- 
ing objects (log sets). Although not 
likely to effect any substantial change 
in dolphin population dynamics, owing 
to current low levels of dolphin mortal- 
ity, such a change in fishing pattern 
could damage the commercial stock of 
yellowfin tuna and perhaps stocks of 
other tuna species because, unlike dol- 
phin sets which generate almost no un- 
wanted catch, school and log sets cap- 
ture moderate to large amounts of un- 
dersized or otherwise undesirable tuna 
that are subsequently discarded at sea. 
In this study, we examined the dif- 
ferences 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 dol- 
phin sets to log sets increased esti- 
mated average discard of tuna by the 
international fleet in the ETP by about 
337%. 
A hypothetical replacement of all dol- 
phin 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 concom- 
mitant changes in size structure of the 
landed fish (Punsley et al., 1994), the 
fishery could lose 30-50% of the ap- 
proximately 98 million individual yel- 
lowfin tuna estimated to recruit to the 
fishery each year. Sustained removals 
of this magnitude, combined with en- 
vironmental variability, could pose 
problems for long-term sustainability 
of the ETP yellowfin tuna stock; more- 
over, the change in fishing-effort pat- 
terns leading to this effect on yellowfin 
tuna would likely contribute little to re- 
covery or sustainability of ETP dolphin 
stocks. 
Manuscript accepted 8 July 1997. 
Fishery Bulletin 96: 210-222 (1998). 
Estimated tuna discard from dolphin, 
school, and log sets in the eastern 
tropical Pacific Ocean, 1 989-1 992 
ESizabeth F. Edwards 
Peter C. Perkins 
Southwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
RO. Box 271, La Jolla, California 92038 
E-mail address (For Elizabeth Edwards): liz@caliban.ucsd.edu 
Tuna purse-seiners fishing in the 
eastern tropical Pacific Ocean ( ETP) 
conduct three different types of sets 
on tuna schools: dolphin sets, school 
sets, and log sets (IATTC 1,2 ). Set 
types differ only in the technique 
used to capture the fish; the net 
used is the same in all cases. Dol- 
phin sets include all sets in which 
the tuna were captured in associa- 
tion with dolphins. School sets in- 
clude all sets on tuna schools not 
associated with either floating ob- 
jects or with dolphins. “Log” sets as 
defined in our study include all sets 
on any floating object encountered 
by the purse seiner, including natu- 
ral objects such as trees and tree 
limbs, dead whales, and other large 
animals, or other natural debris, as 
well as human-generated objects 
such as rafts and discarded trash. 
The overwhelming majority of 
dolphins caught during purse-sein- 
ing activities in the ETP are caught 
during dolphin sets, the only set 
type during which dolphins are cap- 
tured intentionally. A few dolphins 
(generally fewer than 25-50) are 
captured accidentally each year in 
a few school or log sets or both (gen- 
erally fewer than 5-10 dolphins), 
usually because the dolphins were 
not observed prior to making the 
set. School sets tend to catch free- 
swimming schools of moderately 
small yellowfin tuna, Thunnus 
albacares (about 7-8 kg, 60 cm to- 
tal length, and not associated with 
dolphins and with other, unwanted 
fish), or mixed schools of yellowfin 
and skipjack tuna ( Katsuwonus 
pelamis), and little else. Dolphin 
sets catch relatively large yellowfin 
tuna (15-25 kg and 75-125 cm) as- 
sociated with dolphins (Punsley 1 2 3 ), 
some or (rarely) all of the associated 
dolphins, and very little else. Log 
sets have tended historically to 
catch small, prereproductive yellow- 
fin tuna smaller than about 5 kg 
and less than about 50 cm fork length 
or skipjack tuna (or a mixture of both 
tuna), together with a wide variety 
and large quantity of other biota in- 
cluding sharks, billfish, other large 
and small sportfish, and a variety of 
other small noncommercial tunas. 
Dolphin sets traditionally have 
been preferred by tuna fishermen 
because the associated yellowfin 
tuna are abundant, large, relatively 
easy to locate and capture, not as- 
sociated with unwanted (nontarget) 
1 LATTC ( Inter-American Tropical Tuna Com- 
mission) Annual Report. 1988. Inter- 
American Tropical Tuna Commission, c/o 
Scripps Institute of Oceanography, La 
Jolla, CA 92038, 288 p. 
2 IATTC Annual Report. 1990. Inter- 
American Tropical Tuna Commission, c/o 
Scripps Institute of Oceanography, La 
Jolla, CA 92038, 261 p. 
3 More recently (1995 onward), some log sets 
have begun to capture some bigeye tuna 
and larger yellowfin although the major- 
ity of logs still capture mostly smaller 
fish. Punsley, R. 1995. Inter-American 
Tropical Tuna Commission, La Jolla, CA 
92038. Personal commun. 
