Natural mortality of Trachurus novaezelandiae and its size selection by purse seines off south‐eastern Australia

Abstract

The natural mortality (M) and purse‐seine catchability and selectivity were estimated for yellowtail scad, Trachurus novaezelandiae, Richardson, 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 2,289 to 642 boat days between 1999/2000 and 2015/2016) and interdispersed (over 9 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 × 10−7 boat/day), M (0.22/year) 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 very low 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 additional fishing effort and, in doing so, alleviate pressure on other regional small pelagics.