Edwards and Perkins: Estimated tuna discard from dolphin, school, and log sets 
219 
tal fish currently in the ecosystem, or a few percent 
of annual fishery recruitment (i.e. new young fish 
becoming vulnerable to purse-seine fishing each 
year), or individuals from a stock with very short life 
span and high annual ability to restock. Potential 
significance of discard calculations can be determined 
by combining estimates of recruitment with infor- 
mation about the species and size composition of fish 
discarded. These calculations are a rough estimate 
of the number of fish discarded (i.e. lost from the fu- 
ture commercial population) in relation to fish avail- 
able for commercial catch. Estimates of discards of 
young, small fish are especially important in terms 
of future resources because these young fish have 
the most biomass to gain per individual, as well as 
the greatest reproductive potential. 
Data recently collected by the IATTC as part of 
their bycatch investigations indicate that both the 
fraction and the size composition of yellowfin tuna 
discards change dramatically with set type ( Hall and 
Deriso 13 ). These data can be used to estimate the 
numbers of yellowfin tuna discarded under various 
fishing regimes. 
IATTC data show that yellowfin tuna composed 
92% (by weight) of tuna discard from dolphin sets, 
50% of discard from school sets, and 22% of discard 
from log sets during the years 1993 and 1994 (Table 2). 
13 Hall, M., and R. Deriso. 1995. Inter- American Tropical Tuna 
Commission, c/o Scripps Institute of Oceanography, La Jolla, 
CA, 92038. Personal commun. 
Of this yellowfin tuna discard, small yellowfin tuna 
(defined as less than 2.5 kg, or 5 lb) discarded from 
log sets were by far the most serious type of loss, 
particularly in the scenario where all dolphin-set ef- 
fort was redistributed to log sets. Although small 
yellowfin tuna composed only 3% and 17% (by weight) 
of yellowfin tuna discards from dolphin and school 
sets, they composed about 76% of the discard from 
log sets (Table 2). 
These data can be used to predict yellowfin tuna 
discard under different fishing regimes, given the per- 
centage of total effort devoted to each of the set types. 
Assuming, as before, that effort patterns of the in- 
ternational fleet were similar to effort patterns of 
the U.S. fleet during the study period, we can use the 
percentages of total observed discard resulting from 
each set type in the U.S. fleet to estimate tonnages of 
discard from each set type for the international fleet 
during the same period. 
Of the total annual discard of 6,080 short tons from 
the U.S. fleet under observed patterns of effort, 88 
short tons (1.44%) resulted from dolphin sets, 595 
short tons resulted from school sets (9.78%) and 5,400 
short tons resulted from log sets (88.8%). Applica- 
tion of these percentages to the estimated total an- 
nual discard of 31,500 short tons of tuna from the 
international fleet produces estimates of 454 (31,500 
x 1.44%), 3,079 (31,500 x 9.78%) and 28,000 (31,500 
x 88.8%) short tons of tuna discarded annually from 
dolphin, school, and log sets by the international fleet 
(Table 2). The percentages of discard composed of 
Table 2 
Steps in estimating number of individual yellowfin tuna of different sizes discarded from each set type. “% yellowfin” is the sum 
of all yellowfin bycatch divided by the sum of all tuna bycatch for the years 1993-94 in the IATTC bycatch data base. “% small 
yellowfin” is the average of percent values for 1993 and 1994; “% medium yellowfin” and “% large yellowfin” were calculated 
similarly. “Small” yellowfin are less than 2.5 kg. “Medium” yellowfin are 5-15 kg. “Large” yellowfin are greater than 15 kg. 
Weights are expressed in short tons. 
Total tuna 
Yellowfin 
% 
% 
% 
Small 
Medium 
Large 
discard 
% 
discard 
small 
medium 
large 
yellowfin 
yellowfin 
yellowfin 
Set type 
(tons) 
yellowfin 
(tons) 
yellowfin 
yellowfin 
yellowfin 
(tons) 
(tons) 
(tons) 
dolphin 
475 
92 
437 
3 
31 
66 
13 
135 
288 
school 
2,976 
50 
1,488 
17 
31 
53 
253 
461 
789 
log 
28,177 
22 
6,199 
76 
24 
1 
4,711 
1,488 
62 
Ranges of percent values: 
(left = 1993, right =1994) 
% 
% 
% 
% 
small 
medium 
large 
Set type 
yellowfin 
yellowfin 
yellowfin 
yellowfin 
dolphin 
(92-99) 
(1-5) 
(20-42) 
(57-74) 
school 
(49-50) 
(16-17) 
(25-36) 
(48-57) 
log 
(19-29) 
(73-78) 
(20-27) 
(0.2) 
