Relative performance of two bycatch reduction devices for excluding juvenile mulloway, Argyrosomus japonicus and other fish from south-eastern Australian penaeid trawls

Abstract

Mulloway, Argyrosomus japonicus is an economically valuable species across southern Australia, but the south-eastern population is considered depleted owing to excessive historical commercial and recreational fishing mortality. To address this concern, stock-recovery initiatives have been implemented in New South Wales, which include minimising unaccounted fishing mortality by regional penaeid trawlers targeting school prawns, Metapenaeus macleayi. This study presents four experiments to evaluate the relative effectiveness of two mandated bycatch reduction devices (BRDs) to reduce incidental catches of mulloway across volunteer estuarine penaeid trawlers. Both BRDs were ‘mechanical separators’ designed to partition catches based on size. The first design (termed the ‘blubber-chute’) is the most commonly used in the assessed fisheries and comprised a funnel of 96-mm mesh inside the extension that directed larger organisms through a posterior, top opening. The second design (‘Nordmøre-grid’), comprised a 400- × 872-mm aluminium grid with 20-mm bar spacings and inserted at 33° in the extension. Organisms larger than the bar spaces were directed upwards to an escape exit similar in size to that in the blubber-chute. The BRDs were assessed using covers on single-rigged trawlers and simultaneously without covers on double-rigged trawlers. Trawls with the BRDs had the same quantities of school prawns but those with the blubber-chute retained significantly more total incidental catches (by up to 5.2 ×) and mulloway > 20 cm TL (up to 9 ×) than those with the Nordmøre-grid. Relatively greater numbers of unwanted juveniles of another economically important species, tailor, Pomatous saltatrix, and others were also excluded by the Nordmøre-grid. The results support preferential use of the conventional Nordmøre-grid for eliminating the bycatch of mulloway >20 cm TL. Secondary BRDs that mitigate the capture of smaller fish warrant ongoing assessment.