The shortfin mako shark, Isurus oxyrinchus, despite being a known highly vulnerable species and the second most common shark bycaught, has no current quantitative stock assessment in the Indian Ocean. The first stock assessment plan by the IOTC in 2020 will need accurate age and growth information. In our study, growth band deposition is assumed to be annual, with smaller individual having “shadow” bands which fuse together in larger organisms. A total of 704 vertebrae were sampled (399 females, 304 males and 1 undetermined sex) all caught in the southern Indian Ocean (between 22.68 °S - 34.85 °S (latitude) and 37.06°E - 95.55 °E (longitude)). Females ranged from 69 to 350 cm FL and males from 74 to 268 cm FL. The age rang was from 0-30 for females and 1-24 for males. The two-parameter von Bertalanffy growth function with a fixed L0 = 63 cm, was chosen as the best fit to the adjusted length-at-age data for females and male independently, with growth parameter estimates as L∞ = 392.4, 296.9 cm FL, k = 0.043, 0.07/ year, and L0 = 63 cm FL, respectively. To confirm the preliminary parameters now obtained, it is recommended that the age count would be done by other more experienced readers.
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