Enlarged mesh in trawl top sheets hits the sweet spot of gear technology research

Industry-led trial on repurposed trawls ⁠​​​​​​​‌​​‌‌‌‌​​‌​​​​‌​‌​​‌‌‌‌​‌‌​‌⁠demonstrates substantial improvements in Nephrops catch\nrates, fuel efficiency and bycatch reduction\n\nBy Ronán Cosgrove and Matthew McHugh, BIM\n\nBIMs Fisheries Conservation Sections works at sea onboard commercial fishing vessels in close\ncollaboration with the fishing industry to address key environmental and related economic issues.\nMost of our gear trials are focussed on reducing unwanted catches or bycatch, improving carbon and\nfuel efficiency, while maintaining commercial viability. We regularly make gains in two of these areas\nbut technical solutions which hit the mark on all three are rarer events.\n\nThomas Whelahan, owner of the MFV Cisemair had some spare trawls following decommissioning of\nanother vessel and was keen to try modifying them to improve fuel efficiency. Reduced twine\nsurface area from increased mesh size is known to reduce hydrodynamic drag. Previous BIM trials of\nfour-panel ‘box trawls’ with extensive sections of enlarged mesh in the top sheet demonstrated\nsubstantial improvements in energy efficiency but ultimately requires purchase of new nets.\nModifying existing two-panel nets potentially negates the need for new nets while contributing to\nthe circular economy through repurposing of fishing gear.\n\nStandard trawls are typically constructed with 80 mm top sheets. Thomas previously tested 160 mm\nand was pleased with the results. BIM assisted him in taking this a step further and set up a trial\nwhich compared the performance 300 mm with 160 mm mesh top sheets. Assisted by Matthew\nMcHugh and Martin Oliver from BIM, a total of fourteen hauls were conducted on board the MFV\nCisemair with skipper David Reid in the Western Irish Sea in June 2024.\n\n \n\n

\n\nNephrops catches were up 38% while catches of undersize whiting were down by 39% in the trawl\nwith the 300 mm mesh top sheet. The twine surface area was reduced by 8%. While it was not\npossible to assess fuel consumption during the trial, Niall Connolly also fitted 300 mm top sheets and\nreported a fuel saving of around 10% compared with 80 mm top sheets over a number of trips\ntargeting Nephrops onboard the MFV Mellifont with skipper Robert Dawe.\n\nThe increased Nephrops catch is a surprising result and may be due to improved ground contact of\nthe trawl associated with increased water flow. We plan to do more testing at sea and possibly in\nthe flume tank next year to confirm and improve knowledge around this result.\n\nThe reduction in undersize whiting is also an important result. BIM has conducted extensive research\non reducing whiting in the Irish Sea Nephrops fishery due to poor stock status and requirements to\nimprove whiting selectivity. Measures such as the Swedish grid and increased codend mesh size\nhave been shown to help in this regard but are also associated with losses in targeted Nephrops.\n\nAlso, it is not clear how well small whiting survive the process of escaping through diamond mesh\ncodends which raises questions over the efficacy of such a measure in boosting stock conservation.\nIncorporating 300 mm mesh panels in the top sheet allows whiting to escape early in the capture\nprocess through meshes which are much larger than their body size likely resulting in excellent\nsurvival of escapees.\n\nIncreased Nephrops catches and reductions in fuel use can lead to\nimprovements in profitability while reducing carbon emissions as part of the energy transition. All in\nall, enlarged top sheets hit the sweet spot of gear technology research with major potential to become a management measure supporting the sustainable development of the Irish Nephrops\nfishery.\n\n MFV-Cisemair\n\nThomas Whelahan and crew deserve credit for driving forward the development of enlarged top\nsheets, as do Niall Connolly and crew, and Pepe Trawls Ltd for taking the initiative and doing their\nown gear research.\n\nThis work is funded by the Irish Government and part-financed by the European\nUnion through the EMFAF Operational Programme 2021 – 2027. A full trial report is available at\nwww.bim.ie/publications/fisheries