There is a need for a simple, practical, and accurate aging method to document the population structure, age, and growth of bigeye tuna(). We collected samples of the first dorsal spine of bigeye tuna that were collected in the Southern Indian Ocean between September 2008 and May 2009. We evaluated whether the location at which the spine was sectioned affected the estimation of age. The spine sections were divided into three groups based on the cutting position. There was a linear relationship between spine radius and fork length. There was no significant difference in estimated age between the three groups. However, AIC values suggested that the best cutting location was at 10% of the total length from the condyle base. The fork length of bigeye tuna ranged from 570mm to 1 820 mm. The majority of fish (83.9%) were between 910–1 500 mm in fork length(FL) and 3, 4, and 5 years in age. The mean value and was