RESEARCH ON LIMITING BLUE WHITING CATCHES IN PELAGIC TRAWLS

Research on limiting blue whiting catches in pelagic trawls. In the fishing for blue whiting, it is at times a problem that the catches become too large, and that there is thus a risk that the trawl will be blown up with loss of catch and damaged gear as a result. Since 2019, efforts have been made to develop methods to prevent this from happening, and a catch restriction system for the blue whiting fishery will now be tested further on the blue whiting fields.

Through the ‘Scheme for Fish Experiments and Development Measures’, which the Directorate of Fisheries manages, and in cooperation with the Fisheries and Aquaculture Industry Research Fund and the Institute of Marine Research, the Directorate of Fisheries is now developing better methods for catch regulation in the fishing for blue whiting.

“Since the start of the project in 2019, we have tested a catch restriction system, which was initially developed for use in cod seine fishing,” says adviser Hermann Magnus Hagen Pettersen from the Environmental Section of the Directorate of Fisheries.

The catch limitation system involves installing a separate section with slits/incisions at the top of the side panels and an opening at the bottom of the trawl bag. Behind these, a fish lock is installed to prevent setbacks and loss of fish from the bag in the sea surface.

-When the trawl bag fills up with fish up to this section, the bag will expand, the fish lock will close the bag and the slits will open when the bag is full. Surplus catches are led out of the trawl through the discharge holes,” says Hagen Pettersen.

A strap is placed around the bag at the desired distance from the end of the bag, to regulate how much catch one wants to get. The strap is held back with a mechanical trigger that releases the strap at a preselected depth.

“When this strap is released, more volume is also released in the bag, so that too much pressure is avoided on the bag when the fish expand, as a result of air in the swim bladder due to rapid ascent from the fish depths,” says Hagen Pettersen.

This allows the trawl bag to have a better ascent to the sea surface, and the risk of “blasting” of sacks is significantly minimized.

“The catch limiting system also helps the vessels to regulate the catch so that the last haul before full load does not exceed the load compartment capacity,” says Hagen Pettersen.

Facts about fishing for blue whiting

Blue whiting is a small codfish whose distribution extends over large parts of the North Atlantic.

The Norwegian fishing fleet fishes blue whiting using flotation trawls. Fishing mainly takes place at a depth of 300-600 m along the continental slope west of the British Isles in the period February to April.

The catches in each trawl tail vary, but it is not uncommon to have far more than 500 tonnes of fish in the tail. The large catches combined with demanding weather conditions can in the worst case lead to blown up trawl bags with loss of catch and extra expenses as a result. With too large catches, issues related to safety (HSE) on board also easily arise.

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