Summary

By catch reduction devices (BRDs) are physical modifications to trawls designed to reduce the catches of unwanted organisms, while maintaining catches of prawns. After several years' research by NSW Fisheries, BRDs were mandated for use in NSW's prawn-trawl fisheries in 2000. For the oceanic fishery, a variety of square-mesh panels are permitted, providing they conform to restrictions on area, mesh size and location in the codend. Prawn-trawl fishers have developed and tested a number of different designs of square mesh and whilst anecdotal information suggests that some of these work quite well, their performances appear to vary - particularly according to their position in the codend.

Our aims in this work were to determine the optimal position for these BRDs and then compare the relative performance of two industry-developed square-mesh panel designs against the composite square-mesh panel at this optimal position. The goal of this work was to provide recommendations on how to "tighten up" current regulations mandating BRDs in the NSW oceanic prawn-trawl fishery.

We did two experiments. In the first we tested a simple plastic square-mesh panel (area, 6 x 5 bars; bar length, 45 mm) at three locations in the codend (starting 0.7, 1.2 and 1.6 m in front of the last row of meshes in the codend). The results showed that the optimal position, in terms of maximising the reduction of unwanted bycatch and maintaining catches of target species, was 1.2 m. This was attributed to the behaviour and swimming ability of fish in the trawl and the hydrodynamics of the codend.

In the second experiment, we compared a composite square-mesh panel against two industry-developed square-mesh panels (made from plastic and steel). All 3 designs had the same mesh size and area in the main escape panel and were placed at the optimal position determined in Experiment 1. The results showed that the composite square-mesh panel significantly reduced more bycatch than either of the two industry designs. This was attributed to the flexible design of the composite square-mesh panel and a better maintenance of codend geometry.

These results have implications for the management of BRDs involving square-mesh panels in the NSW oceanic prawn-trawl fishery.