Preliminary age and growth of blue shark (Prionace glauca) in the southwest Indian Ocean

Citation
Farley J, Robertson S, Norman S, et al (2021) Preliminary age and growth of blue shark (Prionace glauca) in the southwest Indian Ocean. In: IOTC- 17th Working Party on Ecosystems & Bycatch (Assessment). IOTC-2021-WPEB17(AS)-12, Online, p 20
Abstract

This paper describes preliminary work to assess the age and growth of blue shark in the southwest Indian Ocean as part of the ‘GERUNDIO’ project1. A total of 262 vertebrae samples were available for analysis and 98 were selected for ageing as part of this initial phase of the project. The vertebrae were collected from sharks ranging in size from 96 to 276 cm straight fork length and were caught in the southwest Indian Ocean (close to the coast of South Africa). The maximum age (paired band count) was 17 years for males and 12 years for females. The youngest fish was aged two years.
Direct validation of the accuracy of the ageing methods used was not possible in the current project. However, our preliminary length at age data is consistent with the results of Andrade et al. (2019) for blue shark in the southern Indian Ocean. Limited age validation has been done using bomb radiocarbon dating (14C) methods and we think further consideration should be given to this method to continue efforts to validate the annual periodicity of the bands being counted. Without direct age validation, it is impossible to determine if the age estimates are accurate. We also recommend that additional vertebrae are collected from blue shark in the western and other areas of the Indian Ocean to provide further age information. Given the difficulty of reading blue shark vertebrae, an exchange of vertebrae sections (or images) among reading laboratories and an ageing workshop may help to standardise the approaches used for counting growth increments.