Non Technical Summary

The most common way to reduce bycatches from prawn-trawl fisheries is to install physical devices (collectively termed "Bycatch Reduction Devices" - BRDs) in the trawls. All BRDs can be classified under two categories according to the methods used to allow bycatch to escape. The first category are designs that function by mechanically partitioning the catch according to size.

These BRDs usually include a rigid grid with an escape exit in front of the codend, and are designed to exclude individuals that are larger than the spaces between the bars in the grid. The most well-known example of these sorts of BRDs is the Nordmøre-grid that was developed in Norway. The second category includes much simpler designs of BRDs, like the NSW Fisheries' composite square-mesh panel that exploit behavioural differences between prawns and similar-sized fish. Depending on the operational characteristics of a particular fishery and the species to be excluded, either category of BRD can be used on their own in trawls or together to exclude a range of different species.

In the Shark Bay prawn-trawl fishery in Western Australia, the main bycatch concerns are sea turtles and fish that are smaller than the targeted prawns. To address these bycatches, our main objectives in this study were to (i) quantify the effectiveness of a Nordmøre-grid in maintaining catches of prawns and (ii) examine the bycatch-reduction capability of the NSW Fisheries' composite square-mesh panel.

The results from this work showed that, compared to a conventional trawl with no BRD, a trawl containing a Nordmøre-grid only was effective in reducing the bycatches of some large organisms. However, installing a NSW Fisheries composite square-mesh panel as a secondary BRD further reduced the total bycatch by 47% and the numbers of several species of fish by up to 75% with no loss of targeted prawns. These results are comparable to those recorded for the composite square-mesh panel in NSW and further illustrate the effectiveness of this simple design in reducing unwanted bycatches from prawn trawls. This effectiveness and simplicity has led to numerous prawn-trawl fishers in Western Australia using the NSW Fisheries' composite square-mesh panel on a voluntary basis.