Edwards and Perkins: Estimated tuna discard from dolphin, school, and log sets 
21 1 
fish, and generally have been more valuable per pound 
than the smaller school- or log-associated tuna. Dol- 
phin sets traditionally have been opposed by conser- 
vationists because large numbers of dolphins have 
died annually as a result of net entanglement dur- 
ing these sets. Fishery-related dolphin mortalities 
began occurring in the ETP with the advent in the 
late 1950’s and early 1960’s of the tuna purse-seine 
fishery. At that time, new net-hauling equipment and 
net materials made large-scale purse seining practi- 
cal for the first time. Within a few years, the earlier 
practice of catching tuna by pole and line was re- 
placed almost entirely by purse seining. Estimates 
of dolphin mortality during the 1960’s and early 
1970’s were on the order of 300,000-400,000 dolphins 
annually. Mortality estimates exceeded 100,000 per 
year until 1977, when the effects of dolphin mortal- 
ity limits and improvements in fishing practices fi- 
nally brought mortality below 50,000 per year. Mor- 
tality estimates decreased through 1983 to about 
12,000 per year but increased significantly again to 
over 130,000 in 1986 when U.S. vessels left the ETP 
to move to the western Pacific and were replaced by 
vessels from countries with less experience in purse- 
seining techniques . Following a number of new restric- 
tions on fishing methods and limits on allowable spe- 
cies composition of dolphin mortality, enacted by the 
U.S. congress in the late 1980’s and early 1990’s, mor- 
tality decreased again to about 25,000 per year in 1991. 
Obviously, eliminating dolphin sets would virtu- 
ally eliminate dolphin captures and associated dol- 
phin mortality in the fishery. Recognizing this simple 
fact, the U.S. congress in 1992 passed legislation 
designed to eliminate fishing on dolphins by elimi- 
nating the U.S. market for tuna caught in associa- 
tion with dolphins while leaving the market open to 
tuna caught by other methods in the ETP, i.e. by 
school and log fishing. It was assumed that all effort 
previously directed to sets involving dolphins would 
simply be transferred to sets on schools and logs. 
It is probably fortunate for the commercial stock 
of yellowfin tuna in the ETP, as well as for a host of 
other species, that the legislation did not affect the 
fishery in the manner expected. More or less concur- 
rent with the changes in U.S. laws, the majority of 
the countries practicing purse-seining in the ETP, 
with the help and leadership of the Inter-American 
Tropical Tuna Commission, formed in 1992 a volun- 
tary agreement (the Panama Declaration) to limit 
dolphin mortality dramatically in the tuna fishery. 
A major factor influencing the remarkable success 
of this program was the introduction of dolphin mor- 
tality limits on a per-vessel basis. Under this sys- 
tem, the total mortality quota for a given year is di- 
vided equally among all the vessels fishing that year. 
In addition, annual quotas were adopted that de- 
creased annually to zero mortality by the year 2005. 
Results during the first year that this system was 
fully operational (1993) far exceeded the stated lim- 
its of the Panama Declaration (annual mortality 
quota of 15,000 dolphins). Annual dolphin mortality 
during 1993 dropped to about 3,600 animals. Annual 
dolphin mortality has continued to decrease since 
then, to less than 2,600 animals during 1996. This 
decrease in mortality was not accomplished by de- 
creasing the number of sets on dolphins, but rather 
by significantly decreasing dolphin mortality per set, 
from about 5 animals per set in 1986 to about 0.3 
per set in 1996. The number of dolphin sets remains 
around 8,000 per year, down only about 20% from 
the approximately 10,000 sets per year that were 
practiced from the early 1970’s through the early 
1990’s. At the current time, annual dolphin mortal- 
ity is between 0.1% and 0.2% of estimated abun- 
dances of all dolphin stocks affected by the fishery, 
compared with annual net reproductive rates of 2- 
4% per year. Thus, fishery-related dolphin mortality 
at present is negligible compared with estimated 
current abundances and replacement rates. 
This decrease in dolphin mortality, achieved by 
decreasing kill-per-set rather than decreasing the 
number of dolphin sets, has probably been fortunate 
for the commercial stock of yellowfin tuna and other 
biota in the ETP because it has apparently forestalled 
any major shift of fishing effort from dolphin sets to 
school or log sets. 
The remainder of this paper focuses on the poten- 
tial changes in tuna by catch (discard) that could have 
been expected had dolphin set effort actually been 
transferred to school and log sets. We focus on tuna 
discard because bycatch data for tuna were the only 
available data to us when this study began. 4 Pro- 
vided that the currently low levels of dolphin mor- 
tality are maintained, so that maintaining effort on 
dolphin sets does not appear to endanger dolphin 
population status or dynamics, the results presented 
here imply that encouraging such a change in effort 
patterns would be ecologically unsound. 
This study presents the first report characteriz- 
ing patterns and extent of tuna discard from the U.S. 
tuna purse-seine fishery in the ETP. We describe here 
general characteristics of the discard data available 
to us and apply statistical methods described else- 
where (Perkins and Edwards, 1996) to estimate av- 
erage discard per set for each set type. We use these 
estimates of discard per set to estimate average an- 
4 The Inter-American Tropical Tuna Commission initiated in 1993 
a large-scale bycatch monitoring program in the ETP, but those 
data are still being collected and analyzed by that agency. 
