Improved size selection and estimated at-vessel mortalities for an Australian whiting (Sillago spp.) boat seine

Abstract

In response towards more species in New South Wales (NSW), Australia being managed via quotas, efforts have recently focused on developing more efficient fishing methods with concomitantly low unaccounted fishing mortality and/or other environmental impacts. The present study contributes information towards this initiative by investigating the relative selectivity of a boat seine with a traditional vs modified codend when targeting eastern school whiting, Sillago flindersi and stout whiting, S. robusta—species historically caught in NSW using otter trawls. The traditional boat seine represented those used in other Australian jurisdictions, tapering from an 85-mm stretched mesh opening in the body to a codend comprising 38.5-mm diamond-shaped mesh ('T0') throughout, and was alternately fished with a modified codend made entirely from 40.9-mm mesh to match the grith of mature individuals of both whiting species and orientated 90° ('T90') with shortened lastridge ropes. Regardless of codend, the boat seine was very efficient at quickly harvesting large quantities of whiting (i.e. often >3 tonnes 60 min deployment–1), with a total discarded-to-retained catch weight ratio (0.44:1) much less than for conventional otter trawls used across similar areas (e.g. >2:1). At-vessel mortalities were high (>95%) for both whiting species, but all were retained. Mortalities were lower for two key discards, bluespotted, Platycephalus caeruleopunctatus (61%) and longspine flathead, P. grandispinis (82%) and both were negatively affected by the total catch weight. The mortality of bluespotted flathead was also negatively correlated to their total length. The T90 codend allowed many immature eastern school whiting to escape, however there were issues with fish being entangled in meshes. We conclude boat seining could represent an effective and relatively selective method for harvesting whiting in NSW, but future work is required to progress an appropriate codend configuration (possibly with fewer T90 meshes) and also address any impacts of rapid, regional depletions on the overall stock structures of both whiting species.