Sun et al.: Age and growth of Thunnus obesus 
505 
Combined sexes (n= 856) 
l/V=0. 00003 FL 2 9278 
r 2 = 0.97 
0 20 40 60 80 100 120 140 160 180 200 
Fork length (cm) 
Figure 4 
Relationships between weight and fork length of the west- 
ern Pacific bigeye tuna sampled at Tungkang fish market. 
calculate the fork lengths of previous ages. The mean 
back-calculated fork lengths for the first 10 years of life for 
the western Pacific bigeye tuna are given in Table 2. 
Parameters of the VBGF estimated by the Ford-Wal- 
ford method for males, females, and sexes combined are 
shown in Table 3. Growth was not significantly different 
between sexes (ARSS, F=1.98; df=3, 452; P> 0.05); the 
pooled growth curve is shown in Figure 7. VBGF param- 
eters computed by nonlinear regression are also shown in 
Table 3 and Figure 7. Length-at-age of bigeye tuna esti- 
mated by nonlinear regression is larger (up to age 6 years) 
than that estimated by the Ford-Walford method. 
Discussion 
Available genetic information supports the hypothesis of a 
single bigeye stock in the Pacific Ocean (Hampton et ah, 
Figure 6 
Monthly variation in percentage of the western Pacific big- 
eye tuna with a terminal translucent zone in dorsal spine 
sections, February 1997 to January 1998. 
Age (year) 
Figure 7 
Comparison of the growth curve obtained by the Ford- 
Walford plot with the growth curve obtained by nonlinear 
regression method for the western Pacific bigeye tuna. 
1998; Grewe and Hampton 3 ). Although the fishing area 
of the Taiwan fleet and thus the sampling area of bigeye 
tuna used in our study was limited to a small area of 
the western Pacific, our results may be representative of 
bigeye tuna throughout the Pacific Ocean. 
Monthly variation in percent terminal translucent edges 
in our study suggested the formation of growth rings once 
a year. Ehrhardt et al. ( 1996) attributed the narrow, trans- 
3 Grewe, P. M., and J. Hampton. 1998. An assessment of 
bigeye ( Thunnus obesus ) population structure in the Pacific 
Ocean, based on mitochondrial DNA and DNA microsatellite 
analysis. University of Hawaii, Joint Institute for Marine and 
Atmosphere Research Contribution 98-320, 29 p. Pelagic Fish- 
eries Research Program, University of Hawaii at Manoa, 1000 
Pope Road, Honolulu, HI 96822. 
