Refining size selection in an Australian whiting (Sillago spp.) boat seine

Abstract

As part of an initiative to assess boat seining for harvesting eastern school (Sillago flindersi) and stout whiting (S. robusta) off New South Wales, Australia, two experiments were done to determine if changing the orientation and sizes of codend meshes reduced catches of immature (<17 cm total length; TL) individuals. The control codend comprised 37-mm diamond-shaped (T0) mesh throughout, and was alternately deployed against two codends with the same twine diameter and length, but comprising: (1) a 2-m cylinder of 40-mm mesh turned 90° (T90) attached to 46-mm T0 mesh (termed the ‘40/46-mm T90/T0 codend’) in experiment 1; and (2) 46-mm T0 mesh throughout (‘46-mm T0’) in experiment 2. During 90 deployments, 201 t was caught: 145 t (72 %) of whiting, 2 t of other retained catches, and 54 t (27 %) of discards. Both larger-meshed codends caught fewer small and mature eastern school whiting, but with no overall significant differences in size selection to the control codend. Conversely, there was a difference for stout whiting, with fewer small fish, but not mature individuals, escaping the 46-mm T0 codend. Retained octopus (Octopus spp.) catches were lower in the larger-meshed codends, but there were no significant effects on discarded species. Few whiting were impinged in T0 meshes, whereas up to 17 % (mature fish) in the 40/46-mm T90/T0 codend were secured in the T90 meshes. The data reiterate the utility of boat seining for targeting whiting with minimal bycatch—within which size selection might be controlled via a T0 codend comprising ∼40–47 mm throughout.