Abstract
In response to a shift towards directed targeting of whiting, Sillago spp. by some penaeid trawlers in coastal waters off New South Wales, Australia, 3 experiments were done to examine the effects on the size and species selectivity of trawls associated with (i) a 40% increase in mesh size used in the body, (ii) altering the configuration of mesh in the codend, and (iii) a separating grid in the extension section. Each experiment involved paired comparisons between conventional and modified gears during normal commercial fishing operations. The results showed that increasing mesh size in the trawl body from 45 to 63 mm had no effect on any catches and, while the grid has the potential to improve species selection by excluding macrofauna, further refinement and testing are required to minimize the escape of commercial sized whiting. Changing the orientation of mesh in the codend from diamond- (40 mm mesh opening) to square-shaped (41 mm mesh, hung on the bar) had a highly significant effect on the size selection of red spot whiting, Sillago flindersi: reducing the catches of small, unwanted individuals (< 17 cm total length) by up to 99%, with minimal impact on the retention of larger, commercial sized individuals. We conclude that the use of square-mesh codends (with a bar length approaching 20 mm) by trawlers targeting whiting is an appropriate management tool for minimizing the fishing mortality of juvenile and maturing conspecifics.
Highlights
In oceanic waters off northern New South Wales (NSW), Australia, up to 280 vessels tow smallmeshed (40-50 mm stretched mesh opening) triplerigged otter trawls at night, mostly targeting eastern king prawns, Penaeus plebejus
Like most prawn-trawl fisheries throughout the world, in addition to the targeted species, trawlers in NSW catch a diverse assemblage of non-target organisms
Of the three modifications examined, changing the orientation of meshes in the codend had the most impact on the selectivity of the trawl
Summary
In oceanic waters off northern New South Wales (NSW), Australia, up to 280 vessels tow smallmeshed (40-50 mm stretched mesh opening) triplerigged otter trawls at night, mostly targeting eastern king prawns, Penaeus plebejus. Like most prawn-trawl fisheries throughout the world, in addition to the targeted species, trawlers in NSW catch a diverse assemblage of non-target organisms (collectively termed ‘bycatch’ - for reviews see Andrew and Pepperell, 1992; Alverson et al, 1994). The byproduct from oceanic trawlers in NSW is economically important, contributing more than. 65% of the total landed catch (by weight) and up to. Of the byproduct species landed and sold, red spot Sillago flindersi and stout whiting, S. robusta (typically longer than cm total length - TL and collectively sold as ‘school’ whiting) have traditionally dominated catches. In recent years, owing to improved local and international markets, the retained catches of whiting have substantially increased
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