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Commercial Fishing Marine Science



Research into improving quality of cod fillets. Shorter crowding time and more space during trawl capture followed by lower slaughter stress, can improve the quality of cod fillets, according to research by the Norwegian organisation, Nofima.

One solution may be waterfilled tanks where the cod can be kept alive after they are caught.

“Reduced crowding for the cod, and a shorter time in the trawl had been good for the cod and for us the consumers,” says Nofima researcher Ragnhild Aven Svalheim. She wrote a doctoral dissertation entitled, Stress responses influencing fillet quality of trawled Atlantic cod and haddock, where she looks at how stress responses in cod and haddock affects the quality of fish fillets.

Trawl fishing is highly stressful for the cod. The stress response already begins when the fish attempts to avoid the fishing gear by swimming away from the approaching net. Eventually, the cod tires and is swallowed by the trawl net. As more and more cod are captured, space becomes scarce, pressure increases, and the fish are heavily crowded.

Svalheim has investigated how the combination of fatigue and crowding in the trawl net influence the quality of the cod fillets.

As part of her dissertation, she has done three experiments using a “trawl simulator”; one experiment with haddock and two experiments with cod where she exposed the fish to similar conditions as those in a cod end. In the trawl simulator the fish swim until they are exhausted and then end up in a net, where they are retained.

“I have isolated different phases of the trawling process, from the fish trying to swim away, to capture and crowding in the trawl net, and finally, the slaughtering process,” explains the Nofima researcher.

But even though the cod is worn out after having swam, it isn’t this fatigue that is most stressful. It is when they are forced together in the end of the trawl for a long period of time, that the stress level really increases.

“During the crowding phase, stress levels and mortality increase and fillet quality is reduced. And, by quality I mainly mean the colour of the fillet. When the fish is denied the opportunity to move about, it cannot continue pumping blood back into the large veins. Instead, the blood remains in the small vessels in the white part of the muscle, from where it cannot be removed by bleeding.”

A large amount of blood in the fish can affect how the fish tastes and smells, and is thus negative to the quality. In addition, it represents an aesthetic problem and can make the fillet less attractive for a consumer.

In another experiment, Svalheim investigated what happens with cod just before and during slaughter. “The current recommendation is that fish should be bled within 30 minutes after slaughter to reduce the amount of blood in the fillets. The problem with these recommendations is that they are based on fish that are not stressed.

“On board modern trawlers the fish are often left lying in air for a long time before they are killed. A cod can have brain activity for up to two hours when it is kept in air, and thus the practice of leaving fish in air represents yet another stress factor for the fish. The cod is already stressed because it has tried to escape the gear and has been crowded in the trawl.

“In my last experiment the cod was subjected to moderate crowding for hours, with limited ability to move about. We also investigated how the stress level and fillet colour changed in response to the crowding and when the fish was left in air for up to 30 minutes without being killed first. We then saw that the moderate crowding did not trigger any particular stress response. Interestingly, it was not the stress level itself that was decisive to quality, but the type of stress. As long as the fish had the possibility to move about, then they kept their good quality.

“The combination of limited space in the trawl and the cod being kept in air before it dies, is detrimental to quality. Because the type of stress the fish experiences plays a role in fish quality, and this possibly has to do with movement.

“The worst thing one can do with a fish, in terms of quality, is to prevent it from moving about. Either by crowding or leaving it in air to die,” emphasises the researcher.

“The solution may be to have water tanks onboard the boat, so that the fish recover and swim around before bleeding. There is currently only one trawler that has invested in this type of water filled bins for live holding of fish, in order to provide better quality.”


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