Reducing codend mesh size and changing configurations to improve selectivity in Australian whiting (Sillago spp.) trawls

Abstract

In response to individual transferable quotas (ITQ) being implemented for whiting (Sillago flindersi and S. robusta) off New South Wales (NSW), Australia, two experiments were done to investigate if fish-trawl target inefficiencies and absolute bycatches could be reduced by replacing the legislated codend (handicapped by a minimum legal stretched mesh opening—SMO twice the optimum) with smaller-meshed designs. In experiment 1, compared to a conventional codend comprising 95 mm SMO (with a stretched circumference of ∼20 m, but restricted to 3.4 m by a rope), an unrestricted codend made from 46 mm SMO at an appropriate circumference (∼11 m stretched) and surrounded by a 94-mm SMO lifting bag caught ∼3–8 × more whiting h^–1 trawled. Total bycatch weight h^–1 trawled also increased, but only by ∼2 × and so, in terms of absolute amounts, fishing the smaller-meshed codend could evoke at least a 37 % reduction in discarded bycatch (or ∼1800 t) and ∼5500 h less trawling to harvest an average of ∼425 t of whiting quota each year by fish trawlers in NSW. In experiment 2, replacing 40 % of the 46-mm codend with a cylinder of smaller 41 mm SMO turned 90° (‘T90’) and with the same lifting bag further improved efficiency for whiting, but not size selectivity, and without affecting bycatch. The results illustrate the need to consider concomitant fishing-gear modifications in fisheries transferring to ITQ management to minimise unwanted environmental impacts.