Cod caught by trawling are exposed to enormous stress during the final phase of the catch. When the trawl net is pulled up from the sea, many of the fish suffer compression damage or die as a result of the pressure. The catch is transferred from the net to a receiving bin, where it is kept until it is gutted. In many cases the fish have been dead for many hours before the gutting process, and the quality is significantly reduced. Consequently, cod caught by trawling is often damaged and miscoloured and cannot be sold as a top product on the market.
In order to see if the quality of trawled cod can be improved, scientists from Nofima and the Institute of Marine Research have conducted an experiment by keeping the fish alive when they leave the net up on deck. Keeping fish alive after being caught in Danish seine bags is referred to as capture-based aquaculture. This method functions well today and produces extremely good fish quality. However, this is the first time the method has been tested on cod caught by trawling.
In the experiment the cod were picked out of the trawl net and transferred to purpose-built water tanks. Senior Scientist Kjell Midling, who heads the National Centre of Excellence for Capture-based Aquaculture, reports surprisingly good results.
“Nearly 95 % of the cod transferred from the trawl net to our research tanks survived. This is sensational and provides great opportunities to exploit the quality of the fish raw material on board the trawlers.”
The scientists tested the fish immediately after catching and at various internals after the fish had been in the research tank to determine what condition they were in. The results were very similar to those for cod caught by Danish seine, both with respect to survival and the level of exhaustion. Capture-based aquaculture of cod caught by trawling can, therefore, produce good results.
A series of physiological parameters were studied equivalent to measuring the performance of an athlete. Among other things, the scientists tested the pH value, lactic acids in muscles and oxygen intake.
“When the cod, which is a demersal fish, is gathered from the depths of the sea relatively quickly by trawl, its swim bladder is punctured because of a difference in pressure at the bottom and up on the surface,” says Midling. “Twenty-four hours after the tough catching experience, all the values are completely fine again. However, it’s possible to slaughter the cod soon after it is put in the tank and get a first-class product.”
The quality of fish that are bled and gutted after they are dead is significantly reduced. When the fish can be taken directly from the tank to slaughtering, the product is completely different. During the experiment a series of samples of the fillet quality were taken to determine when it can be processed after slaughtering without damaging the fillet. After a period of freezing, these will be further analysed in Nofima’s laboratories and compared to normal trawled cod.
Giving notice of a new era for trawling
These good results give grounds for basing future trawling for cod on temporary storage on board. However, for the new knowledge to be implemented in practice, changes will be required to today’s trawler fleet, which is not designed for on board capture-based aquaculture.
“Gentle techniques need to be developed to get the catch on board or to pump it into a tank,” says Midling. “The vessels must also be fitted with tanks that are large enough as well as equipment to stun or cut the fish automatically. Nofima plans to test this equipment next year on Danish seine bags and the results of these tests can be transferred to the trawlers.”
The experiments were conducted on a large-scale research excursion under the auspices of the Institute of Marine Research.