Breakthrough trawl net design helps SA prawn fisheries

by SARDI on 25 May 2011 Small fish and juvenile prawns are swimming free of prawn trawl nets following the breakthrough development of an alternative net design.

SA prawn fishers have been enthusiastic about the new design which is expected to bring major benefits both to the environment and industry, reducing catches of non-target species (by-catch) and improving prawn catches.

South Australian Research and Development Institute (SARDI) scientist, Dr Shane Roberts, who led the two-year project, tested the new design with prawn fishermen in the Gulf St Vincent Fishery earlier this year.

'The fishermen are enthusiastic about it because not only is the new net achieving reductions in catches of both small fish and small prawns by up to 50%, it retains larger prawns better, and allows greater water flow through the nets allowing them to be towed more efficiently, ultimately resulting in fuel savings.'

Dr Roberts said the new net design is a different configuration (called T90) of the traditional trawl net. In the new design, the mesh is orientated 90 degrees to the body of the net, and together with a different knot and thicker twine, it allows the holes of the mesh to remain fully open when trawled through the water, even during large catches. As a result, small fish and prawns escape more easily and towing efficiency improves. He added that with the traditional configuration, the diamond shaped holes of the mesh tended to close as the net was pulled through the water, making it difficult for small fish and prawns to escape, while reducing towing efficiency. This was clearly seen from underwater video footage set up by the researchers.

T90 Net
The new trawl net design allows the holes of the mesh to remain fully open when trawled through the water, even during large catches

Standard Diamond Net
The traditional net configuration with diamond shaped holes tend to close as the net is pulled through the water, making it difficult for small fish and prawns to escape and reducing towing efficiency

Dr Roberts said the project had enjoyed great support from industry. The idea to use the T90 net design was inspired by SA commercial fisherman Jim Raptis.

'Jim suggested we try the T90 design which he had been using with great success when trawling for fish in the Great Australian Bight. Until now, this new net design had not been tested in an Australian prawn fishery. Combined with Jim’s knowledge of fishing, we developed the new net for prawn fishing and tested the new net design on his commercial prawn boat during the March fishing run this year. It was a great success. We anticipate the rest of industry will follow suit, and adopt the new design soon, a move which will improve the overall environmental sustainability and economics of the SA prawn fisheries,' said Dr Roberts.

Mr Raptis said he hopes the prawn fishery adopts the T90 concept as soon as possible.
'South Australian king prawn fisheries will be able to produce higher catches once all trawlers use the new T90 concept because the small prawns easily flow out of the net in good, lively condition, providing the fishery with prawns for the future. The small prawns have time to grow and spawn, and then they can be caught when they are fully grown at maximum size and value per kilo.'

Dr Roberts added that research was continuing on a mega-fauna escape grid which aimed to reduce the capture of stingrays, sharks and sponges. 'Field tests are now underway and we will be refining its design and incorporating it with the T90 design over the coming year,' he said.

'It is worth noting that this new net design is just one of a number of initiatives taken by the South Australian prawn industry over the past decade to improve its ecological sustainability. Its use of real-time management harvest strategies have become a model for prawn fisheries around the world. Working closely with SARDI researchers, prawn fishers have refined their fishing patterns to trawl between just 25 to 50 nights a year targeting only traditional trawling grounds where prawns are ready for harvest – making the most out of a valuable resource while significantly reducing by-catch. Previously, prawn trawling took place on around 300 nights a year over less strategic fishing areas.'

The success of the net project is attributed to the close collaboration and financial support from industry (particularly Jim Raptis and the crew of Fishing Vessel Anna Pearl), SeaNet (Nathan Bicknell of OceanWatch), commercial net makers (Thorsteinn Benediktsson and Wally Hill of Fisheries Supply, QLD), SARDI and fishery managers (PIRSA). The project was further funded by the Fisheries Research and Development Corporation (FRDC) and the Adelaide and Mount Lofty Ranges Natural Resource Management Board.

Further information: Dr Shane Roberts, SARDI Aquatic Sciences Wild Fisheries, 8207 5483 or shane.roberts@sa.gov.au
A link to a video about by-catch reduction devices for prawn trawlers is below. Please note: information about the T90 net is about half-way through the 5.49 minute Click here to watch SARDI website