Bleeding and residual blood in fish

Cod and haddock can have many serious quality flaws when they are brought ashore, usually caused by injuries inflicted by the fishing equipment used and when handling the fish on board, for example if the fish is trapped in the nets for a long period of time or through delayed bleeding or if it is not allowed to bleed out completely.

All stresses on the fish during capture and after bringing it on board before it is bled will contribute to insufficient bleeding and red fillets.

In commercial fisheries, some of the blood in fillets is caused by stress and suffocation during the capture; but a lot of it can also be attributed directly to delayed and inadequate bleeding after the fish is brought on board.

This is particularly challenging during large catches on board trawlers and with purse seines, as the crew will not have sufficient capacity to bleed out and gut the fish before it dies.

To achieve adequate quality, the strain on the fish during the capture and when bringing it on board must be minimised as far as possible. The fish must me anesthetised and bled out as soon as possible while it is still alive. The bleeding should ideally take place in chilled circulating water.

What should one keep in mind during capture?

• Both the type of the equipment used and how it is used will affect the quality of the fish. Minimising the time the fish spends on the fishing line and in the nets, as well as short towings and smaller nets, will have a positive impact on the fish quality. Conversely, fish that spends a long time in the fishing equipment (nets, cod-ends, purse seines) will be subjected to severe stress and even death before it is brought on board, resulting in more injuries from the fishing equipment and red fish meat.
• Catch volume restrictions and codend releasers will ensure better space in the purse seine and reduce the risk of crushing and suffocating the fish or inflicting damage.
• Repeated stress and crushing during capture, such as swimming to escape the fishing equipment, crowding or reduced oxygen supply leading to suffocation will cause an increase in the amount of blood in the muscle. A calm cod will have the whitest fillet.

What should you keep in mind on board?

• The fish should be brought on board as gently as possible, without using hooks or inflicting blows or crushing injuries.
• Fish that are under a lot of stress during capture (for example being subjected to prolonged trawling periods, being trapped in a net for a long period of time) will be even less able to withstand rough handling on board.
• The fish should be bled as soon as it has been hauled on board. It should not lie to “calm down” in dry receiving bins pending bleeding and slaughter.
• The use of electrical anaesthetisation makes it possible to bleed the fish immediately after being brought on board.
• Rapid cooling of the fish extends its shelf life.
• In specialised fishing vessels, most of the catch can be kept alive on board pending controlled slaughter on board or live delivery.

What affects the bleeding process and residual blood?

• Fisken som er stresset og belastet under fangst og ombordhåndtering pumper blod ut i muskelen mens den er i live. Dette blodet

• Fish that are stressed and crushed during capture and handling on board will pump blood into the muscle while alive. Neither this blood nor blood from injuries or burst swim bladders can be removed through bleeding.
• Fish that die before bleeding, either in the fishing equipment or in the reception container on board, will have a discoloured fillet.
• To avoid the increased blood flow into the muscle after bringing the fish on board, it needs to be bled immediately.
• It does not matter if it is bled in water or in air, but bleeding the fish in cool water contributes to rapid cooling and will wash away residual blood and grime before the fish is gutted.
• If the fish is bled out in water, the water temperature also has little bearing on the amount of residual blood in the fillet.
• Fish that are gutted immediately will actually have more residual blood in their fillets than fish that are drained of blood before gutting.

Three minutes of bleeding may be sufficient

How long should the fish be allowed to bleed out for? It turns out that most of the blood in the main arteries is emptied out after three minutes if the fish is left to bleed out into water.

Almost the same amount of residual blood is found in the fish whether it is left to bleed out for 3 or 30 minutes.

“These experiments were carried out on a small scale, but if we see the same results on a commercial scale, it may have a great impact on the bleeding process – cutting down on the processing time for each catch and reduced needs for equipment, space and water”, says Nofima scientist Torbjørn Tobiassen.