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This paper describes a study to estimate the age and growth of albacore tuna in the western Indian Ocean using otoliths. A total of 600 otoliths were selected for analysis. Females ranged in length from 74 to 108 cm fork length (FL) and males from 67 to 115 cm FL. Otolith morphometric data indicate sex and regional (ocean) differences in otolith growth.
Annual age was estimated following protocols developed and validated for South Pacific albacore. Decimal (fractional) age was estimated using count of opaque zones in the otolith, an assumed birth date, capture date and the state of completion of the marginal increment (otolith edge classification). A final age was estimated for 574 fish, ranging from 2.0 to 16.0 years for females and 2.4 to 14.0 years for males. Growth differed between the sexes with males growing faster than females after ~85 cm FL and reaching larger mean asymptotic length. Albacore in the western Indian Ocean appear to grow slightly faster than in the Pacific. However, this may be partly due to the absence of small fish in our samples resulting in higher estimated length-at-age for young fish. Additional age data for small fish (particularly <75 cm FL) and from the south and eastern Indian oceans would improve this study. Further work is required to examine the timing of increment formation and refine the age algorithm. Direct validation of the age estimation methods for the Indian Ocean are also recommended.
