502 
Age and growth of the bigeye tuna, 
Thunnus obesus, in the western Pacific Ocean 
Abstract-Bigeye tuna (Thunnus obe- 
sus) age and growth studies were last 
conducted in the western Pacific in 
1967 and no study has ever attempted 
to age bigeye tuna from this area by 
using dorsal spines. The objective of 
our study was to estimate bigeye tuna 
age and growth rate in the western 
Pacific based on counts of growth rings 
on sections of the first dorsal spine. 
Length and weight data, and the first 
dorsal spine from bigeye tuna in the 
Tungkang (southwest of Taiwan) fish 
market were collected monthly from 
February 1997 to January 1998. In total, 
1149 specimens were collected. The 
fork lengths of individuals ranged from 
45.6 to 189.2 cm. Cross sections from 
dorsal spines were taken and examined 
under a dissecting microscope equipped 
with an image analysis system. The 
monthly percentage of specimens having 
a terminal translucent zone indicated 
that growth rings formed once a year; 
therefore, the age of each fish was 
determined from the number of visible 
growth rings. Von Bertalanffy growth 
parameters were estimated for males, 
females, and both sexes combined. There 
was no significant difference between 
males and females. The parameter 
estimates for the combined sexes were 
asymptotic length (L M ) = 208.7 cm, 
growth coefficient ( K) = 0.201/yr, and 
age at zero length (t 0 ) = -0.9906 yr. 
Manuscript accepted 12 December 2000. 
Fish. Bull. 99:502-509 (2001). 
Chi-Lu Sun 
Chien-Lung Huang 
Su-Zan Yeh 
Institute of Oceanography 
National Taiwan University 
Taipei, Taiwan 
E-mail address (for C.L. Sun): chilu@ccms.ntu.edu. tw 
Bigeye tuna (Thunnus obesus Lowe, 
1839) are a commercially important 
species of tuna inhabiting the warm 
waters of the Atlantic, Indian, and 
Pacific oceans. They are found across 
the entire Pacific between northern 
Japan and North Island of New Zea- 
land in the west and from 40°N to 
30°S in the east (Calkins, 1980; Mat- 
sumoto, 1998). Adult bigeye tuna are 
caught mainly by longlines, but sub- 
stantial numbers of juveniles are taken 
by purse seines. 
Taiwanese distant water tuna long- 
line fleets have operated throughout 
these three oceans since the late 1960s 
targeting albacore. In the early 1980s, 
the Taiwanese began equipping their 
longliners with very cold (below -55°C) 
freezers and deep longlines in the Indi- 
an and Atlantic oceans, which allowed 
them to target bigeye tuna for the lu- 
crative sashimi market in Japan. In 
the western Pacific, the Taiwanese off- 
shore longline fleets, based in domestic 
(Tungkang mainly) and foreign fishing 
ports, have landed more bigeye tuna 
than in the past. 
Growth studies of Pacific bigeye tu- 
na conducted in the 1950s and 1960s 
were based either on increments be- 
tween modal points in size-composition 
data (Iversen, 1955; Shomura and Ke- 
ala, 1963; Yukinawa and Yabuta, 1963; 
Kume and Joseph, 1966; Suda and 
Kume, 1967) or on the number of an- 
nual markings (annuli) on scales (Nose 
et al., 1957; Yukinawa and Yabuta, 
1963). Recently, Hampton and Leroy 1 
and Matsumoto (1998) presented pre- 
liminary results from growth studies 
based on otolith increment counts. No 
previous study had aged Pacific and In- 
dian bigeye tuna from dorsal spines, al- 
though a few age determination studies 
existed for Atlantic bigeye tuna (Gaikov 
et al., 1980; Draganik and Pelczarski, 
1984; Delgado de Molina and Santana, 
1986; Alves et al., 1998). Accurate age 
structure of stocks is essential for stock 
assessment and fishery management. 
Our study provides estimates of the age 
and growth rate of bigeye tuna in the 
western Pacific from growth rings on 
sections of the first dorsal spine. 
Materials and methods 
Fork length (in cm), weight (in kg), and 
sex were determined for bigeye tuna 
caught by Taiwanese offshore longlin- 
ers in the fishing area from 23°N to 0°N 
and 110°E to 140°E (Fig. I 2 ) and sold 
at the Tungkang fish market between 
February 1997 and January 1998. In 
addition, a total of 1149 first dorsal 
spines were collected. Three cross sec- 
tions were taken along the length of 
each spine above the condyle base (Fig. 
2A) with a low-speed “ISOMET” saw 
1 Hampton, J., and B. Leroy. 1998. Note 
on preliminary estimates of bigeye growth 
from presumed daily increments on oto- 
liths and tagging data. Working paper 
18, eleventh meeting of the standing com- 
mittee on tuna and billfish, Honolulu, 
Hawaii, USA, 30 May-6 June 1998, 3 p. 
Oceanic Fisheries Programme, Secretar- 
iate of the Pacific Community, B.P.D5, 
98848 Noumea, New Caledonia. 
2 Yang, R. T., R. F. Chung, and C. L. Chang. 
1982. Taiwanese offshore tuna longline 
fishery. Part I: fishing ground, fishing 
season, and fishing condition. Spec. Rep. 
36, 6 p. [In Chinese with English abstract.] 
Insitute of Oceanography, National Taiwan 
University, no. 1, sec 4, Roosevelt Rd, 
Taipei, 106 Taiwan. 
