In the practice of prawn-trawling, the traditional otter board (the device that spreads the net) scrapes the seabed in a plowing process that occurs along its heavy steel shoe. The benthic (ocean-bottom) organisms and other material dislodged from the seabed by the edge of the otter board are transferred inward along the shoe and into the trawl net, where they accumulate as a significant part of the bycatch.
The Batwing improves on the traditional otter board by opening prawn-trawling nets in a way that causes substantially less seabed impact and towing resistance. Instead of towing a large door at a 40-degree angle across the bottom, the heavy contact shoe is aligned with the direction of tow – in a straight line. This eliminates the scraping action of the board and any serious disturbance to the seafloor. As a result, there is significantly less damage to benthic ecosystems, reduced bycatch of sedentary benthic animals, and lower fuel consumption.
Proven Results
Standardized comparison tests between the prototype CP1 Batwing boards and traditional prawn-trawling boards showed the Batwing producing half the drag and a 90 percent reduction in the accumulation of benthic material in the net. This was achieved by eliminating the impact of the trawl gear on the seabed – a more effective solution than simply filtering the disturbed seabed materials from the catch (e.g., by using longer sweeps between the otter boards and net to allow the material to pass by on the outside of the trawl wingends).
Subsequently, a second commercial prototype (CP2) was devised. It revised implementation of the Batwing technology, using different construction materials and an alternative strategy for dealing with the harsh operating environment of commercial trawling. The potential reduction of towing resistance for the CP2 Batwing board is 70 percent compared to current prawn-trawling otter boards -- a big incentive for fishermen to use the new technology, and a win for both fishermen and the environment.
CP2 Batwing otter board
Dr. David Sterling, Australia
