Sun et al.: Age and growth of Thunnus obesus 
503 
(model no. 11-1280) and diamond wafering 
blades. Sections ranging from 0.8 to 1.0 mm 
thick (Fig. 2B) were examined with a dis- 
secting microscope (model: Olympus SZH- 
ILLD) with transmitted light. Images of the 
dorsal spine sections were captured by using 
an image analysis software package, a CCD 
(charged coupled device) camera, and a high- 
resolution computer monitor. Translucent 
rings on the section images were counted 
by two readers independently. When ring 
counts disagreed, images were read again by 
both readers simultaneously, and any ques- 
tionable spines were discarded. 
Spine sections as the structure to estimate 
age have the advantage of requiring easy 
sampling and easy reading (the growth rings 
stand out clearly), and samples are easily 
stored for future reexamination (Compean- 
Jimenez and Bard, 1983). However, early 
growth rings may be lost in larger specimens 
because of increased size of the vascularized 
core in the spine. Accordingly, we estimated 
the number of lost (obscured) rings from ob- 
servations of their position and number in 
spines from young specimens as has been done for 
little tunny ( Euthynnus alletteratus ) (Cayre and Di- 
ouf, 1983), eastern Atlantic bluefin tuna (Thunnus 
thynnus) (Compean-Jimenez and Bard, 1983), and 
Pacific blue marlin ( Makaira nigricans) (Hill et al., 
1989). 
Age was determined from the translucent rings, 
assuming that two rings are formed each year — a 
translucent (light colored) ring formed during the 
slower growth period and an opaque (dark colored) 
ring formed during the fast growth period. This as- 
sumption was validated by observing a translucent 
or opaque edge on the dorsal-spine sections and a 
monthly variation in the number of translucent edg- 
es (Antoine et al., 1983). 
Distance between the center of the dorsal spine 
and the outer edge of each annual ring was mea- 
sured in microns with the software package after 
calibration against an optical micrometer. The cen- 
ter of the spine was estimated by following Cayre 
and Diouf (1983) (Fig. 2B). Distances (rf.) were then 
converted into radii (i? t -) by following Gonzalez-Gar- 
ces and Farina-Perez (1983). 
The relationship between fork length (FL) and 
dorsal spine radius (R) was modeled by a linear 
equation (Zar, 1999). Fork length was then back-cal- 
culated for each ring with the formula (Lee, 1920) 
100 
105 
110 
115 
120 
125 
130 
135 
140 
Figure 1 
Fishing areas of the Taiwanese offshore tuna longline fishery in the western 
Pacific Ocean (Yang et al. 2 ). 
FL, — a + 
(FL - a)R t 
R 
where FL t = predicted fork length of the fish corre- 
sponding to age or ring i in cm; 
a = ordinate in the origin of the equation 
FL = a + bR-, 
RING 
Figure 2 
First dorsal spine and the site of cross section (A) and the cross 
section showing annual rings and measurements taken (B) for 
age determination of the western Pacific bigeye tuna (c =width 
of condyle base; L 1DS =length of the first dorsal spine; R=radius 
of spine; Rj=radius of ring i; d=diameter of spine; dpdiameter of 
ring i). 
