Gear trials for a new gear for the IBTS

Abstract

The North Sea International Bottom Trawl Survey (NS-IBTS) is an internationally coordinated survey. The fisheries-independent indices for young herring, sprat, whiting, cod, haddock, Norway pout and other commercial fish species from the survey are used in fisheries management. As the complete catch is processed, the survey also provides marine ecosystem information. The survey is coordinated by the IBTS Working Group (IBTSWG) of the International Council for Exploration of the Seas (ICES). The survey setup, including the gear -Grande Ouverture Verticale (GOV)-, is standardised and written down in a manual. The results of an intensive evaluation of the GOV-gear used in the NS-IBTS by different institutes, indicated that all gears differed, and none of them matched the manual exactly. This, together with a wish for a new gear that would be using modern materials, would be cheaper, and able to fish in rougher areas, formed the start of the development of a new gear for the IBTS. Two new gears have been developed, one by the Marine Institute, Ireland (MI) and one by Marine Scotland Science (MS Science). Both gears have a very similar design, their main difference is the ratio in which the mesh sizes are attached to each other. The main differences with the current GOV are the netting materials used, and the removal of the lower wings. To test the two gears, MS Science organized a gear trial experiment, for two weeks in November 2019 on board of the Scottish research vessel Scotia. Other members of the IBTSWG were invited to participate, and the Netherlands accepted this, so the WMR gear technician and the RWS boatswain joined the field trial. This report provides the preliminary results of the trials, along with the observations of the Dutch participants. The preliminary results indicate that both new gears are fishing properly, and both target at least the same fish community as the GOV. There are differences observed in spreading and height of the net, that directly translate into differences in catch composition. The overall conclusion after the trials still is that both new gears would be suitable to replace the GOV. Having seen both gears operational, it is clear that both nets could be handled on board of the Dutch research vessel Tridens without real adjustments. The Dutch participants had a slight preference for the Irish net, because it is simpler to handle. However, in their opinion the Irish gear should be made of stronger materials to make it more durable and the smallest mesh-size (80 mm) should be made smaller (50 mm). The Scottish gear is currently too heavy for the sandy areas in the southern North Sea if the rigging used during the trials is being used, and should be made lighter. Both gear developers indicated that such adjustments are still possible. In the IBTSWG in April 2020, the preliminary results of the trials were discussed and it was concluded to continue the current road with the two new gears. Terms of reference were developed for a workshop with gear technicians end of 2020, early 2021 to discuss the materials, the rigging and further development of the gears into a single new gear for the North Sea IBTS. After this workshop a final net should be made, which can then be used on trial fisheries by each institute involved in the IBTS. The further roadmap for implementing the new gear in the near future, was drafted by the ICES Workshop on impacts of planned changes in the North Sea IBTS (WKNSIMP) in 2019, and includes a transition period in which both gears, the current GOV and the new gear, will be used in the NS-IBTS.