Reducing bycatch using modifications to sweeps and lines anterior to the trawl mouth - collaboration with the Technical University of Denmark: Final FRDC Report

Prawn trawling is among the world's least selective fishing methods and there has been a great deal of work done over the past few decades to develop modifications that reduce unwanted bycatches. Much of this work has focussed on modifications at, or near, the codend (at the aft section) of trawls, but more recent efforts have examined ways to stop fish entering the trawl at all—via modifications to their anterior components (or forward section).

New South Wales (NSW) Department of Primary Industries (DPI) Fisheries Conservation Technology Unit (FCTU) has led such work with prawn trawls in Australia. Another group based in Denmark (the Danish Technical University's team in Hirtshals – DTU Aqua) has grown to be among the European leaders with similar work directed at Nephrops and fish trawls and is a major centre for work being done to underpin the European ‘landings obligation’ (often termed the ‘discard ban’).

The current project took advantage of a travel grant for a PhD student at DTU Aqua to: (i) establish a link and exchange of ideas between the Australian and the Danish teams; whilst (ii) exploring ways of refining anterior-trawl modifications to reduce bycatch in our prawn fisheries. The research involved two experiments in Lake Wooloweyah (part of the Clarence River prawn-trawl fishery).

The first experiment examined how a modification based on the theory behind the FLEXSELEECT design involving counter herding (developed by the DTU Aqua group) performed in combination with the SAFE modification developed by the NSW DPI FCTU during the FRDC project 2011/010 “Reducing the environmental impacts and improving the profitability of prawn trawling through a structured framework of anterior gear modifications”.

The FLEXSELECT concept uses a series of lines extending from the otter boards to the trawl in a cross configuration to herd fish outwards, instead of inwards as occurs with conventional sweeps in fish trawls—effectively making these lines ‘reverse sweeps’. In the second experiment, we examined the utility of simply lowering headline height at the otter boards which, if it reduces bycatch as hypothesised in the earlier FRDC project, could provide an adaptive and rapid tool for fishers to use in situ.

The results confirmed the utility of the SAFE design as developed previously, but its combination with the FLEXSELECT concept did not perform any better in reducing bycatches. However, the lowering of headline height in the second experiment dramatically reduced the bycatches of small fish without any effects on prawn catches.

This result showed that a very simple modification could be used by fishers to adapt their practices at key locations and times of large fish abundances—which may be useful, for example, in avoiding the triggering of bycatch caps that lead to fishing closures in some of NSW’s estuarine and oceanic prawn-trawl fisheries.

The collaboration with DTU Aqua was a significant success in establishing a relationship with one of the most important gear technology groups in the world—at a time when Australia’s jurisdictions are (unfortunately) reducing priorities in this area. This places Australia in a good position to learn of any new European efforts in bycatch reduction as their ’landing obligation’ is implemented.

The project led to a very useful cross-fertilisation of gear concepts and analytical treatments while allowing us to continue to test anterior modifications to prawn trawls. We therefore recommend ongoing communications with DTU Aqua via the PI on this project (IC Independent Consulting) and/or other gear technologists in Australia (including Smart Fishing Consulting) to develop collaborative opportunities.

Notwithstanding the success of this collaboration, we also concluded that like previous work done with BRDs, anterior trawl modifications should ideally be developed on a fishery-by-fishery basis. For this reason, it is important that Australia maintains its local expertise to develop bycatch reduction technologies.