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An opening at the bottom can prevent the trawl bag from bursting

A trawl with blue whiting

When fishing blue whiting, several hundred tonnes are fished.

Photo: Dagfinn Lilleng / The Directorate of Fisheries

When fishermen catch several hundred tonnes of blue whiting per trawl haul, incidents with too large catches occur. A fish outlet at the bottom of the trawl, in front of the codend where the catch accumulates, can solve the problem.

Blue whiting is a small gadoid fish that is fished largely west of the British Isles. But the fishermen have a problem with the shoal fish. 

Several hundred tonnes of blue whiting per trawl haul are fished, and accidents can occur. The trawl bag can simply burst because of the catch sizes. 

"In addition, the swim bladder expands when the fish is lifted to the surface from several hundred meters deep. This causes the codends to rise to the surface like a torpedo," says marine scientist Olafur Ingolfsson.

Animation shows the trawl under the surface 

Now IMR researchers may have found a solution to the problem. They began by animating the ascension of a trawl bag after it has caught fish. Using 10–15 depth loggers on the trawls of five fishing vessels, they received information on how the trawls with blue whiting move underwater. 

"We wanted to see the shape of the trawl beneath the surface. We were most interested in the final phase, i.e. the ascension," says Ingolfsson. 

He and his colleagues saw that as the trawl’s codend approaches the surface, its speed increases and it comes up almost vertically.

See how the trawl moves underwater:

Animation: Ringstad design

Must release surplus fish

With that knowledge, the researchers began to test out different solutions.  

"We had to find a way to dermine more accurately the amount of catches, so that we avoid blasting," says Ingolfsson. 

During a cruise in 2021, they tried various solutions to release surplus fish. Three or four different solutions with large openings at the front of the codend were tested. 

The challenge is to find a solution where the fishermen do not lose fish until the bag is filled up. 

"Now I think we have found a good solution. Blue whiting largely seeks up in the trawl, so openings in the bottom panel in front of the trawl’s codend result in little loss of fish before the bag is full," says the researcher.

In March, the researchers will embark on a new expedition for further testing of various solutions.