Unacceptable Methods

Recent work on the perception of pain in fish has shown that they have mechanisms for pain perception like those in other vertebrates, including mammals and birds. They  should therefore be afforded the same welfare considerations as any other animals kept for food. Therefore the HSA does not recommend the use of any of the following methods: death in ice slurry; live chilling; gill cut without stunning; or carbon dioxide narcosis. If these methods are currently used as standard practice, they should be replaced as soon as possible with a more humane method.

Death in ice slurry

This process involves fish passing over a de-waterer and into ice slurry. The fish are left until they die through lack of oxygen. In some cases, loss of consciousness can take over nine minutes. When fish are placed in ice slurry it is difficult to use normal reactions (such as escape behaviour or vigorous swimming) as indicators of welfare as the ice can have an immobilising effect on the fish. In these circumstances fish will be relatively still, apart from sporadic flips. The long period for the onset of unconsciousness with this method could result in fish being bled and eviscerated whilst still conscious, but immobile. If fish are not left for long enough in the ice slurry, or are not bled out effectively, they are likely to recover and regain muscle movement and brain function as they warm up.

Live chilling

This method immobilises fish and reduces the carcase temperature to allow quicker processing. Fish are introduced to temperatures of 2-6°C, where they may show violent movement and escape behaviour. This movement gradually subsides as they become exhausted and/or immobile. After about 30 minutes they are removed from the water and their gills are cut whilst still fully conscious. Where chilling is used, the rate of chilling should not exceed a drop of 1.5°C at any time. It is essential that the water quality is maintained and that oxygen, carbon dioxide and ammonia levels are measured and controlled by changing the water throughout the day.

Gill cut without pre-stunning

This method involves removing fish from water and then cutting the gills without any pre-stunning. On removal from water, the fish show escape behaviour and flip their tails. Once the cut into the gills is made these reactions are dramatically increased and vigorous head shakes and tail flaps are seen for at least 30 seconds. This movement slowly subsides and after several minutes most fish stop moving.

Carbon dioxide narcosis

Loss of consciousness in fish immersed in carbon dioxide saturated water (pH level 4.5), which is highly aversive, can take 7-8 minutes. Fish will show head shaking and vigorous tail shaking for up to two minutes after immersion in the solution. Movement then subsides and the fish become still after approximately 5 minutes. This is due to exhaustion as opposed to insensibility. Unless fish are kept in a high concentration solution for 7-8 minutes, recovery will begin soon after removal from the solution, i.e. on the table or in the bin.

High concentrations of carbon dioxide must be used to maintain a pH level of 4.5 for a period of at least ten minutes, to cause unconsciousness in every fish before the gills are cut. If removed before then, or if the pH is altered, signs of recovery may be seen, especially when the gills are cut. It is essential when using this method that the gas concentration is measured and replenished as required.