Finishing bycatch from prawns

The UK’s Sea Fish Industry Authority (Seafish) has developed a new trawl which uses the way certain finfish in front of a trawl to stop them being caught and it believes this will reduce the bycatch.

Seafish said the work started in late 1997 through an industry-funded programme to improve the selectivity in trawls and this led to work by the Fisheries Development Group on new approaches instead of through technical measures. Fishing trials in 2002-2003 (from Mallaig in the West of Scotland, Troon in the Clyde estuary and from Blyth in Northeast England) demonstrated a considerable reduction in the numbers of haddock and whiting in the catch of the new prawn trawl when compared with the catch of a standard net, Seafish says.

“The results showed that the design of the new prawn trawl had affected the catches of fish across the entire size range. In other words, the new design of prawn trawl had demonstrated that it was being more species selective than the standard trawl which Background,” it added.

Finfish evaders

The background problem is that targeted fisheries for prawns (Nephrops norvegicus) frequently involve high bycatch rates of finfish. The latter are often unmarketable because of small size, poor quality or a lack of quota availability and fishing mortality is often significant.

The new trawl approach compared catches of Nephrops, haddock, whiting, cod and various bottom-living species and showed it could reduce the bycatch of haddock and whiting without losing out on prawns.

Certain unwanted finfish, says Seafish, exhibit known behaviour patterns in front of a trawl and it was this behaviour which the new design of trawl has taken advantage of. The idea was to get one step ahead of existing techniques which rely on releasing the unwanted part of the catch after it had entered the net.

The approach also had to work for prawn fisheries with a legitimate and valuable whitefish bycatch. Because the development was done with two established trawl designers and manufacturers, the credibility gained from this partnership was expected to increase the prospects of commercial uptake of any successful designs resulting from this work. In the long term, the trawls will still have to go through any national and EU regulatory clearance procedures.

The main changes researched in the development were: reducing the headline height to allow fish to rise up and escape from the mouth section of the net, shortening the wing length to reduce herding of fish as far as possible, reducing the cover section of the net by cutting it back to allow a greater area for fish to escape upwards and increasing the mesh size in the upper panels of the net, to encourage fish escapes.

Tests were initially done in the flume tank at the Fisheries Development Centre in Hull, UK and after various modifications two designs were built at full scale for sea-trial evaluation.

Seafish says all was not easy going to begin with. “At the beginning of these trials it was very difficult to maintain prawn catches with the new trawls that were comparable with those from the standard trawls. After several modifications, however, the new prawn trawls had become sufficiently well tuned to be able to optimise the trade-off between bycatch and prawn catch. The fishing trials demonstrated that one of the designs produced a considerable reduction in the numbers of haddock and whiting in the catch of the new prawn trawl when compared with the catch of a standard net.”

Prawns up finfish down

The results showed that the design of the new prawn trawl had affected the catches of fish across the entire size range. In other words, the new design of prawn trawl had demonstrated that it was being more species selective than the standard trawl. The catch difference with the new trawl for haddock was a 63 per cent reduction, whiting — 65 per cent reduction, Nephrops (by weight) 20 per cent gain, though go only showed an 11 per cent reduction. The catch composition of bottom-living species lemon sole (Microstomus kitt) and plaice (Pleuronectes platessa) were not significantly affected by the design of the new net.

However, the results showed that fishermen would not lose out on commercial catches of Nephrops by using the new design of net. In addition, Seafish says, the new design brought a reduction in catch-sorting times for fishing crews and therefore a corresponding increase in catch quality.

One of the skippers engaged for the final trials was so convinced, that the new design would consistently maintain (or improve) the catch rates of Nephrops whilst avoiding bulk catches of unmarketable whiting and haddock, that he purchased the net to use it in preference to his ‘normal’ Nephrops trawl.

The next stage is to open discussions with fishermen’s associations, fisheries departments and the European Commission to consider how the benefits of the new prawn trawl design could be incorporated into the stock conservation regime.

A strategic approach needs to be combined with a robust funding base.

There also needs to be wider evaluation including tackling the question of cod bycatch.