Development of selective trawls for important Danish fisheries (4313)

The project was structured in following sub-projects:
1. Development of a North Sea haddock trawl: to design a trawl with reduced by-catches of cod.
2. Improved size-selection of the Norway lobster trawl used in Kattegat: to reduce the catch of undersized Norwegian lobster.
3. Improved species-selection in Norway lobster trawl used in Kattegat: to design a trawl that selectively catches Norway lobsters while letting cod and other unwanted by-catch escape.
4. Improved size selection in Baltic cod trawls: test of T90 meshes in the cod end in comparison to BACOMA trawl.

The cod stock in the North Sea is on a low level, with little signs of recovery during the most recent 20 years. By-catch of cod in different fisheries is a problem at this low stock size and therefore technological modifications of gears used in fishing fleets with significant by-catch of cod are requested. Sub-project 1 aimed at the development of a haddock trawl with reduced catchability of cod. Making use of the different behavior of haddock and cod during the catching process, modifications of the trawl groundrope were tested for their effect on cod catchability.

The Norway lobster population in the Kattegat is doing well, and the Norway lobster fishery is the most economically important fishery in the Kattegat. However, there is a substantial catch of undersized Norway lobster in the fishery and improving the size selectivity of the trawl in use was the goal of sub-project 2. This included designing and testing of different mesh sizes and sorting mechanisms.

In contrast to Norway lobster, the cod population in Kattegat has declined severely in the last 20-30 years. Without reducing the by-catch of cod through a more selective trawl, the Norway lobster fishery would have to be reduced significantly in order to protect the cod. Within sub-project 3, the aim was to develop a trawl with significantly improved selectivity, allowing enhanced escapement of cod. The traditional round cod end was replaced with a cod end shaped like a square mesh box. This box proved to be more stable in the water enabling to take advantage of the different behavior of cod and Norway lobster. While cod tend to move up-wards in the tunnel of a cod-end, Norway lobster remains passive at the bottom. Placing a 180 mm escape panel into the upper panel of the box, allowed to improved escapement of cod.

A simple way to increase the mesh opening in a cod-end is to turn the mesh 90° (T-direction, henceforth T90) because the knots will determine the initial mesh bar angle. A T90 cod end was introduced in the legislation for the Baltic Sea cod fishery and the aim of sub-project 4 was to test for differences in cod selectivity in comparison to the standard BACOM (having a sorting window in the top-panel of the cod end).

The project was coordinated by DTU Aqua.

Research area: Fisheries Technology