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.

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.