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MARINFORSK-Marine ressurser og miljø

Managing trawl catches by improving the hydrodynamic performance of sorting grid sections and codends

Alternative title: Managing trawl catches by improving the hydrodynamic performance of sorting grid sections and codends

Awarded: NOK 5.5 mill.

The large densities of Atlantic cod in the Barents Sea in the last years together with malfunctions of mandatory sorting grids have caused that the fleet occasionally takes excessively large trawl catches. These excessive large catches result in reduced fish quality, and imply an increased risk for discards, gear damage and safety concerns. Therefore, various prototypes of catch control devices have been developed in the last years. Although some of them have shown potential for controlling catch size, they have often failed when simultaneously working with mandatory sorting grids. The explanation for this failure is mainly related to the significant reduction of the water flow inside the grid section caused by the various combinations of grid, lifting and guiding panels. This project has aimed at developing sorting grid sections that provides high selective performance and simultaneously good control over catch sizes even when exposed to extremely high catch rates. The new sorting grid sections that have been developed in this project have been shortened, reducing the deformation problem observed in old grid sections. The cross-section area of new sorting grid section has been increased by approximately 30%, thus optimizing the sorting area. By removing, or even the replacing the lifting panel by the sorting grid itself, the water flow inside the section has been significantly improved without compromising selectivity. A stronger water flow inside the new grid sections reduce the accumulation of fish in front of the grid and transport fish easily to the codend. Finally, the effect of using artificial light to stimulate escape behavior inside sections was studied and showed great potential for reducing small haddock from the trawls. Most of the results of this project have been published in scientific journals. Seven scientific papers have been published in peer-reviewed journals and three-four other manuscripts have been submitted, and /or are under review.

The large densities of Atlantic cod in the Barents Sea in the last years together with recently discovered malfunctions in the mandatory sorting grids have been leading to excessively large trawl catches. These excessive large catches result in reduced fish quality, and also imply increased risk for discards, gear damage and safety concerns. Therefore, at the request of the industry and management authorities (to try to solve this problem) various catch control devices have been tested in the last two years. Although some of them have shown potential for controlling catch size, they have often failed when simultaneously tested with mandatory sorting grids. The explanation for this failure is mainly related to the significant reduction of the water flow inside the grid section caused by the various combinations of grid, lifting and guiding panels. The main objective of this project is to develop sorting grid section(s) (combined with a codend) for the Barents Sea demersal trawl fishery that provides high selective performance and good control over catch sizes even when exposed to extremely high catch rates. The following topics have been identified as the main challenges: - Design 4-panel grid sections (Sort-V og Flexigrid) in which the components (lifting panel(s), sorting grid(s) and guiding panel(s)) keep constant geometry. - Increase the grid section's cross section area without (or minimally) increasing the size of the grids. - Design a lifting panel that minimally reduces water flow and that increases grid contact probability to secure effective size selectivity. - Design a guiding panel that minimally reduces water flow. - Test alternatives to the lifting panel in order to improve grid contact. - Ensure that the new grid section does not increase the risk for occupational accidents when operated.

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MARINFORSK-Marine ressurser og miljø