B0602.PDF (177.16 kB)
FTC Summary of Conclusions
reportposted on 2002-01-01, 00:00 authored by ICESICES
1. Trawl performance modelling was reported to have progressed to such an extent that software is now commercially available that allows graphical visualization of a trawl with geometry, construction details and operational parameters specified by the user. Although it was recognized that trawl simulation could reduce the need for field trials, the Group was sceptical about whether enough realism could be incorporated into the models to capture the variability seen in field trials.2. The use of archival tags on groundfishes was reviewed with emphasis on determining the timing and extent of daily and seasonal depth migrations, which were considered as possible factors affecting the catchability of trawl surveys.3. The use of technical measures, such as codend mesh size and selection grids, and the use of codend protection bags on size and species selectivity was reviewed. Although technical measures are usually tested under actual fishing conditions, concern was expressed that fishermen alter their gear in ways that thwart the intent of the technical measure or that inadequate enforcement allows fishermen to ignore legally required measures.4. The use of codend strengthening bags was shown to reduce the size of 50% selection and therefore to reduce the effectiveness of a specified minimum codend mesh size. In some cases strengthening bags are truly justified (e.g. to prevent seal damage or the bursting of netting with large catches), however other cases may be an attempt by fishermen to reduce the effective codend mesh size. The effectiveness of technical measures will be addressed again as a theme session at the 2003 ASC.5. A subgroup is now working on a manual for the measurement of the selectivity of static gears, similar to the manual for the selectivity of towed gears, and expects to have a draft within one year.6. The subgroup that reviewed the IBSFC scientific meeting on technical measures reported that, for Baltic sea cod, trawls with diamond codends made of 4 mm double strand twine having a mesh size of about 140 mm would produce the same L50 as trawls fitted with 120 mm BACOMA windows. This represents an increase of 10 mm above the minimum mesh size for polyethylene codends.
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