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

Abstract

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.