Cumulative selectivity benefits of increasing codend and lifting-bag mesh sizes in an Australian whiting (Sillago spp.) boat seine

Abstract

Benthic boat seines (i.e. ‘Danish’ and ‘Scottish’ seining) are globally used to harvest inshore schooling species, however compared to other towed gears such as otter trawls, there have been few studies assessing factors affecting selectivity. Here the importance of ‘lifting-bag’ (a protective bag over the codend) mesh size was investigated as part of a broader initiative to determine suitability for harvesting adult (≥17 cm total length; TL) whiting (Sillago robusta and S. flindersi) under quota in New South Wales, Australia. Two codends made with either 37 or 45 mm stretched mesh openings (SMO) and representing the ranges within many existing Australian configurations were covered with either a conventional (91-mm SMO) or new (216 mm SMO) lifting bag and assessed during three experiments targeting 156 t of quota over 27 days (133 deployments). In experiment 1, alternately fishing the 37-mm codend with each lifting bag did not significantly affect seine selectivity. In experiment 2, the 37- and 45-mm codends with the 91-mm lifting bag were compared and there was a significant increase in the escape of small (<17 cm TL) S. robusta (but not S. flindersi which were caught across larger sizes) from the larger-meshed codend. Increasing the lifting-bag mesh size for the 45-mm codend in experiment 3 increased the escape of most sizes of S. robusta without affecting S. flindersi. The total discarded catch was 61 t and while some unwanted species also showed improved selectivity due to either the 45-mm codend (experiment 2) or increasing its lifting-bag mesh size (experiment 3), there were no significant effects. While the whiting catches (∼1.2 t deployment^–1) and low discarding (28 %) recorded here support assertions of boat-seine efficiency, divergent morphology and/or depth-dependant distributions between S. robusta and S. flindersi may preclude optimising selectivity for both in the same seine. Nevertheless, codend and lifting-bag SMOs approaching ∼43 and 216 mm, respectively should allow some juvenile conspecifics to escape while maintaining targeted catches.