Mix 'n' match

Trawlers fishing the Clarence River near Maclean NSW, for prawns.
Photo: Wade Labe

Fisher experiences will play a crucial role in extending the use of new gear designed to reduce prawn trawl bycatch and improve fleet efficiency

By Catherine Norwood

After more than 20 years of research, Australian prawn trawl fisheries already have an extensive ‘toolbox’ at their disposal to improve efficiency and reduce environmental impact. Trawl nets are the only way to catch Australian prawns so these innovations are vital if we want to continue to eat Australian wild-catch prawns. But the adoption of different gear modifications and operational strategies has varied considerably between fisheries. 

To help industry better match and modify existing solutions to suit specific prawn fisheries, the Australian Council of Prawn Fisheries (ACPF) is developing a plan to take the existing ‘toolbox’ of developments to ports around the country.

This follows a two-day workshop in Sydney in February, attended by industry representatives from all prawn trawl fishing states, as well as fisheries managers, researchers and gear modification experts. The aim of the workshop was to identify further research needs, particularly those related to reducing bycatch and improving fuel efficiency.

At the workshop, an overview of work already undertaken and discussions of industry issues, state by state, identified that extension of existing bycatch mitigation measures and their suitability should be the priority, rather than new projects. The workshop report is available on the FRDC website.

Shared experiences

The ACPF and the FRDC hosted the workshop and ACPF executive officer Rachel King says it proved a good opportunity to exchange ideas between fisheries, and between researchers and industry. “We found the fishers attending were keen to share their experiences and learn from each other,” she says.  

Workshop participants were also keen to get a better understanding of research being done overseas which might have applications in Australia. This included initiatives in Europe as a result of the implementation of the Landing Obligation (often referred to as the “Discard Ban” involving the phasing-out of discarding of certain species).

The ACPF is planning portside preslp evaluate the onboard performance to ensure the solution fits the need and to increase adoption where appropriate”. 

The ACPF is planning portside presentations of the prawn trawl research to help fisheries “gear match” existing tools to address specific operational issues. Dates are in the process of being finalised.

“It’s important that these aren’t just ‘talk and leave’ events,” Rachel King says of the proposed presentations, for which dates are still being finalised. “We need co-investment from industry and research agencies to help evaluate the onboard performance to ensure the solution fits the need and to increase adoption where appropriate”

One such project has been undertaken in the Northern Prawn Fishery (NPF) where industry aims to reduce bycatch by 30 per cent. The NPF Industry recently trialled a number of innovations aimed at reducing bycatch and worked with the Australian Fisheries Management Authority and CSIRO to ensure scientific validation of the results. 

Among the designs trialled is a new bycatch reduction device (BRD) known as the Kon’s Covered Fish Eyes (KCF) device.  The CSIRO analysis of the data collected during the scientific trials showed the KCF BRD reduces small fish bycatch by an average 36.7 per cent compared to the Square Mesh Panel BRD, with no prawn loss. 

In spite of the success in the NPF, these results may not translate directly to the same level of effectiveness in other prawn fisheries. The KCF BRD could be trialled in other fisheries but further modification of this and other BRD designs may be required to suit different fishery conditions.

The sharing of the onboard experiences between fishers can provide valuable information to improve adoption of new gears – what worked, what didn’t and any tweaks which might have been made, which is all often undocumented. Greater networking between fishers and fisheries is expected to be an important part of these future extension efforts.

FRDC program manager Crispian Ashby says there have already been significant reductions in bycatch and improved fuel efficiency in prawn trawl fisheries over the years. BRDs for a wide range of species are already widely used although, as the NPF project indicates, further improvements can be made.

Escape options

Crispian Ashby says most modifications have focused on the cod-end of the net and helping non-target species to escape. Prawn nets use smaller mesh than general fish trawl nets, because prawns are generally much smaller than fish.

“The unintended consequence is that the nets retain small fish that might be in the same area as the prawn trawl’s path, and we end up with fish that are not targeted, but which are brought onboard."

“The public perception of this – fish that are discarded – is one of a potential protein source being wasted. There are also other potential ecosystem impacts that might come from reducing quantities of fish."

“We have done a lot of research looking at the back end of the net, the codend where the catch aggregates. BRDs – fisheyes, radial escape sections, square mesh panels, square mesh cod-ends, grids, turtle excluder devices – they are all about trying to sort the catch as it starts to collect towards the back of the net." 

“This is sometimes achieved by manipulating water flow characteristics in the net. As the net is towed through the water, the way the net displaces water creates areas of high and low velocity. By understanding how fish behave in the net, and swim differently to prawns, you can use water flow to encourage fish to move toward the exits.” 

Front end strategies

More recent work has focused on modi cations to the anterior trawl – the front of the net – looking at the trawl boards, foot ropes and headlines to reduce drag and disturbance of the sea floor.

This includes modifying the net spreading mechanisms, the positioning of boards used to hold the nets open, the number of nets, net tapers, hanging ratios, twine size, knot orientation, sweeps and ground gears.

The more drag, the more power the trawlers need and the more fuel they use. Strategies to reduce drag include using a ner twine, increasing the mesh size, changing the net mesh orientation and even using di erent knot con gurations. 

Research in NSW has found that combinations of many smaller nets rather than one larger net can reduce drag and fuel use, while maintaining the same ‘swept area’ of each trawl and therefore the same potential catch. 

The trawl or otter boards used to hold openthe nets are essentially solid objects being towed through the water. the larger the net, the larger the boards and the greater the drag they create. Reducing the time the net is towed through water has also been shown to reduce bycatch. Less bycatch reduces the potential damage to the targeted prawn catch and increases the value of the catch. 

“There is a lot of knowledge about what can be done,” says Crispian Ashby. “Now the issue is getting that out to the industry, and helping to trial modi cations that will work for them.” 

To be properly tested, gear and BRD trials need to happen during actual fishing periods. e trials need to be well designed and scientifically robust to make best use of sher’s time and investment.

Table 1: Summary of gear modifications for Australian prawn trawl fisheries
Modification
Description 
Purpose 
Physical and behavioural-type bycatch reduction devices
Grids, square mesh panels, full square mesh cod ends, funnels, super-shooters, T45, T90
Provides a physical stimulus to induce an escape response by an animal.
Turtle exclusion devices (TEDs)
Modifications to TEDs to increase water turbulence and help fish, turtles and other creatures escape via the TED.
Originally designed as a physical stimulus guide turtles out of the net, these devices are also useful for sharks, rays and fishes. TEDs are a form of grid.
Fisheyes
Including new design from the Northern Prawn Trawl, Kon's Covered Fisheyes.
Creates areas of high and low water flow, as well as an opening to stimulate fish to swim out of the opening.
Adjusting tow speed (and water flow)
Improves the effectiveness of TEDS and bycatch reduction devices (BRDs).
Increasing or decreasing the speed that the gear is towed at can improve the performance of the BRDs and trawl net by keeping nets open.
Visual clues to promote fish escapes
In clearer water different contrast of nets can be used to visually stimulate animals to escape. St night lights could be used.
 
 

Modifications to the net of opening of the trawl

Overall trawl design Single. double, dual, triple, quad and penta-rigs, beam trawls, W trawls and tongue trawls. The trawl net can come in many different styles and configurations, such as single (one trawl net) to penta-rig (five smaller trawl nets). These can reduce drag, open meshes and potentially improve water flow. 
Otter board design to improve efficiencies Multi-foil doors, batwing boards, Bisons, cambered doors. Otter boards are used to spread the mouth of the net. They come in several different shapes and sizes that can reduce drag and bottom contact.
Ground gear The possible use of wheels and sleds on boards. Ground gear is placed on the trawl to help the stability of multi-trawl nets ( such as penta-rig) and also stimulates prawns to move into the trawl. Different types of ground gear can have different footprints on the bottom-the aim is to minimise disturbance.
 

Improved performance by using fewer boards, and more sleds etc.

Different ground gears including different-sized chains, soft-brush gear, and so on.

 
  Net construction Simple lines at the mouth of the trawl (the simple anterior fish excluder). Different types of trawl net construction can change the way the net operates and can reduce drag or reduce the amount of unwanted catch in the net. Things such as twine size, twine type and tapers can influence this.

The role of twine size, net length, sweep length, low-drag netting, etc. in reducing frag and bycatch.

Wing mesh height and orientation (T45 and T90),  and trawl body taper, mesh size, seam number, and hanging ratios.

Trawl body taper, mesh size, seam number, and hanging ratios.

 

More is less

Line illustration of a triple rig

Research by the NSW Department of Primary Industries suggests that using more, smaller rigs in prawn trawling can improve fuel ef ciency and reduce bycatch.

It evaluated the fuel ef ciency of a single trawl rig at 2.88 litres per hectare, which was reduced to 2.44 litres per hectare for a double rig, and even further to 2.13 litres per hectare for a triple rig.

Bycatch in the double rig was reduced by 26 per cent compared to the single rig, and by 44 per cent in a triple rig.


More information

Rachel King,
02 8006 0498, 0425 237 566,
acpf.eo@gmail.com