Direct age estimation, interpretation and precision of bigeye tuna (Thunnus obesus) in the Southern Indian Ocean using vertebrae

The estimation on age is an essential part of the population assessment of bigeye tuna (Thunnus obesus). Based on 288 samples of vertebrae collected on board operating in the southern Indian Ocean (SIO) for special investigation from September 2008 to May 2009, age interpretation and precision are performed upon improving staining technique. The results indicate that a remarkably effect of degreasing was brought out owing to the repeating bleach of different concentration of alcohol solution. A risen error is brought about for age interpretation in high-aged and low-aged individuals. The three indices for age precision (TC, CV and APE) are 44%, 9.4% and 7.1% respectively. In the age determination of age-classes, a highest point is achieved at age 2 simultaneously for CV and APE. An extremely significant linear relationship can be expressed as CV = 1.3375 APE – 0.0011 (R2 = 0.9992, n = 8). Compared to CV and APE, limitation exists in age precision for TC, especially in the stage of high-age and low-age when an interpretation deviation appears. Contributions of APE for three readers are 33.34%, 32.22% and 34.44% respectively. The age-bias vary broadly at age 2, 6 and 7 for between ager 1 and 2, which also demonstrate the assumption that a large deviation emerges at high and low aged individual in the aspect of age estimation.