New knowledge on stunning and bleeding fish

The aim of the project “Stunning and bleeding fish onboard vessels,” funded by the Fishery and Aquaculture Research Fund (FHF), has been to obtain new understanding of the process of slaughtering fish to ensure good exsanguination, regardless of catch size.

The project focused on what happens after the catch have been taken onboard.

The approach was that fish stunned and killed immediately after catch will not pump blood into muscle. This contrasts with commercial fishing, where large quantities of fish can be left in dry receiving bins to die, before being bleed.

In commercial fishing, a lot of blood residue in fish muscle can be a result of stress in the fishing gear and delayed or poor blood drainage during slaughter. This typically applies to large single catches. The crew does not have the capacity to stun and bleed all the fish before they are dead.

“By ensuring good exsanguination of all fish, the quality will often be improved and the value of the catch increased”, says researcher Torbjørn Tobiasen of Nofima.

Several methods

There are several ways of handling fish when they are taken onboard and processed. Generally, the slaughter method of fish has been based on simplicity and operating expenses.

Bleeding techniques such as cutting the throat (ventral aorta) or the arteries in the neck (dorsal aorta), and even direct evisceration are frequently used methods to accelerate death and drain most of the blood from the muscle. This is often done after suffocation in air to simplify the slaughter process.

“On some fishing vessels the fish are headed and gutted directly, while on other vessels they are allowed to “calm down” before being bleed, as it is often laborious and dangerous work to bleed the fish while they are still alive and active,” says Tobiassen.

Some vessels have started to stun the fish with electricity, prior to bleeding, but they still have problem of dealing with large catches onboard.

The results of the trial show that the amount of residual blood in the muscle increases, with increasing physical activity in the fishing gear and from the time that elapses, prior to stunning and bleeding.

Previous trials have shown that fish stunned/killed by a blow to the head and stored for 30 minutes before bleeding, did not increase the amount of blood in the small veins of white muscle.

In this project, the researchers tested both kinds of stunning – percussive and electric stunning. Percussive stunning is too slow when large amount of fish is taken in a single catch. That made it necessary to see if electric stunning could be used as substitute to percussive stunning, withregard to residual blood in the muscle.

The result of the stunning experiment showed that fish stunned by percussion or electricity had a flat or gradual increase in the amount of residual blood in the muscle during short-term storage. This is because some fish recovered before being bled in both groups.

“We can therefore not recommend short-term storage of fish after electric or percussive stunning pending bleeding, in terms of fish welfare. Controlled slaughtering (stunning, bleeding and gutting) of fish immediately when they come onboard, or live storing of the catch onboard prior to slaughter, works well in terms of residual blood. In addition, stunning fish prior to bleeding/gutting is safer for the crew and better for the welfare of the fish,” says Tobiassen.

Three minutes

In the case of bleeding and exsanguination in clean seawater, the results show that most of the blood is drained of the fish within three minutes after cutting.

The results also show that the temperature on the seawater during exsanguination does not appear to be important regarding the amount of residual blood in the fish muscle, after ice storing and filleting.

“Nevertheless, cooling the seawater during exsanguination can be beneficial. A small portion of the blood is still draining after 3 minutes and chilling can prevent blood coagulation. Clean seawater also help to remove blood residuals from the gills and skin of the fish. If the seawater is chilled, it will also contribute to rapid cooling of the fish and prolong the shelf life of the fish during cold storage”, states Tobiassen.

The trials were conducted on wild cod at the Aquaculture station in Tromsø.

An objective instrumental method called hyperspectral imaging was used to determine residual blood in the fillet after slaughtering. The method is based on measuring how much light is absorbed at different frequencies by the fillet. The instrument can indicate residual blood both on the surface and inside the cod fillet.

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