Report from a cruise onboard RV G. O. Sars 22.11 – 03.12.2008: Comparing the impact of two bottom trawls

The final research cruise described in this report had the objective to compare the physical and biological impact of the bottom trawl modifications developed during the EU DEGREE project to a standard bottom trawl used in the Barents Sea cod fisheries. The “new” trawl was fitted with the last modification of the plate gear developed during the project as well as trawl doors (standard Thyboron doors) rigged to barely touch the bottom. The commercial trawl used for comparison was rigged with a conventional rockhopper ground gear and the doors were rigged to go heavy on the bottom as normally rigged during commercial bottom trawling. We looked for combinations enabling us to fish with the doors off, or nearly off, the bottom, while the ground gear was still on bottom and the door spread was maintained. This enabled almost no impact of the doors on the seabed while hopefully keeping an acceptable fishing efficiency for species not sensitive to herding effects of doors. More fishing experiments are therefore needed before it may be concluded that this rigging can be recommended for commercial use by the fishing fleet. The physical and biological impact on the bottom habitat of the two trawls was compared. Only two valid impact hauls was done with each trawl, both on very soft sediments. This is of course too few hauls to obtain a full statistical comparison between the two trawls. However, all parameters measured indicated that the plate gear trawl had a lower impact on the bottom substrate and benthic organisms than the conventional rockhopper trawl. The physical impact on the bottom was visually inspected and measured by ROV technique.Although the number of hauls was low, the results indicated that the rockhopper dug up more living material than the plate gear. This tendency was confirmed by the ROV investigations.