Age and Sex Specific Natural Mortality of the Blue Shark (Prionace glauca) in the North Pacific 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 ISC Shark working group (SWG) recognized the potential
to develop age and sex‐specific natural mortality estimates for blue shark (Prionace glauca) in the north
Pacific Ocean based on published literature and documents submitted to the SWG. This paper
documents the methods and results.
Sex specific mortality‐at‐age for north Pacific blue shark are estimated based on length‐at‐age from two
different sources in the Pacific. Hsu et al. (2011) and Nakano (1994) both aged blue shark vertebrae
from samples collected in the north Pacific.