Estimates of natural mortality for blue shark in the Indian Ocean

Biological parameters such as growth and natural mortality form the basis for the inputs for stock assessment models (Siegfried and Sanso 2013) and can be used to assess the vulnerability of the stock to fishing (Hilborn and Walters 1992). Some attempts have been made to combine gross estimates of biological change such as length or age over time, with catch data to get a rough estimate of trends in stock status (e.g. Clarke 2011). In the absence of good biological data, estimates derived from closely related populations or species can be used. However, these estimates may differ from reality and in order to encapsulate the uncertainty appropriately for use in a stock assessment, additional work is required (Cortes 2002).
This work was motivated by a desire for age and sex specific natural mortality estimates for a length based integrated stock assessment model. The IOTC Working Party on Ecosystems and Bycatch requested that age and sex-specific natural mortality estimates be developed for blue shark (Prionace glauca) in Indian Ocean based on published literature. This paper documents the methods and results of calculating sex specific natural mortality-at-age for Indian Ocean blue shark are estimated based on length-at-age from Arande et al 2019.