# Natural mortality of Trachurus novaezelandiae and their size selection by purse seines off south-eastern Australia

## TL;DR

This study estimates natural mortality and size selection of Trachurus novaezelandiae off southeastern Australia, revealing low catchability and implications for fishery management and conservation efforts.

## Contribution

It provides the first detailed survival analysis and size selectivity estimates for T. novaezelandiae using two decades of data and hazard functions.

## Key findings

- Estimated natural mortality rate of 0.22 year-1.
- Catchability is extremely low, less than 0.1 x 10-7 boat day-1.
- Size selection shows 50% retention at 5 years, larger than maturity.

## Abstract

The natural mortality (M) and purse-seine catchability and selectivity were estimated for Trachurus novaezelandiae, Richardson, 1843 (yellowtail scad)-a small inshore pelagic species harvested off south-eastern Australia. Hazard functions were applied to two decades of data describing catches (mostly stable at a mean +- SE of 315 +- 14 t p.a.) and effort (declining from a maximum of 2289 to 642 boat days between 1999/00 and 2015/16) and inter-dispersed (over nine years) annual estimates of size-at-age (0+ to 18 years) to enable survival analysis. The data were best described by a model with eight parameters, including catchability (estimated at < 0.1 x 10-7 boat day-1), M (0.22 year-1) and variable age-specific selection up to 6 years with a 50% retention among 5-year olds (larger than the estimated age at maturation). The low catchability implied minimal fishing mortality by the purse-seine fleet. Ongoing monitoring and applied gear-based studies are required to validate purse-seine catchability and selectivity, but the data nevertheless imply T. novaezelandiae could incur substantial additional fishing effort and, in doing, so alleviate pressure on other regional small pelagics.

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Source: https://tomesphere.com/paper/1705.10001