Fish Slaughter

Current European legislation requires that all farmed animals are spared avoidable suffering and pain. Although there are few countries that specifically mention fish in their welfare at slaughter legislation, the current general animal welfare laws cover all vertebrates including fish. Besides the legal imperative, humanely killed fish may attract a higher premium and may be of better quality than conventionally killed fish. As the scientific knowledge on stress and pain in fish increases, the ethical consumer will demand fish that are killed in a humane manner.

The method used to kill fish will depend on the species as well as the resources available to the fish farmers. Traditionally, the method of slaughter was chosen for its ease of application and cost reduction. Most traditional methods of killing fish such as live gutting, suffocation (in air or on ice) hypothermia and carbon dioxide narcosis are now considered inhumane as they cause the fish to suffer unnecessary stress and pain. It is now recognised that reducing the stress at harvest maintains the product quality when compared to stressful methods. There is therefore a possible financial incentive for adopting humane alternatives to traditional methods of slaughter. As with mammals, a humane slaughter is one that results in an immediate loss of consciousness, or if slow acting, induces unconsciousness without discomfort or pain. This unconsciousness should persist until death intervenes.

The welfare of fish during harvest should also include the pre-slaughter handling processes. These include food withdrawal (fasting), crowding and removal from water. When fish are handled in a rough manner, they exhibit their emergency (stress) response and this may result in some reduction in the product quality. Further guidance may be found in the HSA publication, Humane Harvesting of Salmon and Trout: Guidance Notes No. 5.

Withdrawal of food

Most farmers withdraw food from fish prior to slaughter to enable the gut to empty its contents. This reduces the chances of the fish being contaminated by gut contents during processing, and thus maintains the keeping quality and hygiene of the final product. Current research suggests that 72 hours is adequate for the complete emptying of the fish gut whilst minimising adverse welfare effects. Fish should therefore not be fasted for longer than 72 hours.

Crowding

Crowding exposes fish to a rapidly increasing stocking density. As a result, oxygen availability and general water quality may also decrease rapidly. The fish are brought closer to the surface thus increasing their exposure to sunlight. There is also an increased risk of damage from abrasion through contact with nets or other fish. The adverse effects of crowding may be reduced by slowly crowding to reasonable densities. Clean nets and additional oxygen reduce the rate at which the water quality deteriorates. The fish must be brought up and crowded slowly. Fast crowding may result in burst swim bladders in species kept deep in the water. When fish are crowded too densely, they show vigorous burrowing, gasping and splashing. On the other hand, a good crowd is calm with only the occasional fins breaking the surface. Fish should only be crowded for a maximum of 2 hours.

Removal from water

Removing fish from water will involve some amount of stress. This can be minimised if the time the fish are kept out of water is kept to a minimum. As a guide, fish should not be held out of water for longer than 15s as after that they will show aversive behaviour and become progressively difficult to handle. Pumps and other equipment used to move fish should be regularly checked and maintained. Pipes should be kept as short and straight as possible and should be flushed during extended breaks and at the end of each harvest. Brail nets should have a lining that keeps some water in and reduces damage by abrasion. Loaded brail nets should be moved slowly and should only be emptied after making contact with the killing table. Over-filling the nets my result in physical damage and suffocation.

The killing process is potentially very stressful. Modern methods of stunning fish reduce the stress and may produce a product quality that is superior to traditional methods. The HSA currently recommends percussive and electrical stunning where relevant systems have been developed. Chemical anaesthetics (not carbon dioxide) are potentially very useful for so called rested harvests but have not been licensed in the EU as yet.

Percussive stunning

The traditional method of stunning by a concussive blow to the head remains an inexpensive manner to dispatch fish. A strike of sufficient energy to the head of a fish results in an immediate loss of sensibility. The blow should be targeted at a position where the brain is closest to the surface of the head and where the skull is at its thinnest. An effectively stunned fish will immediately stop rhythmic movement of the opercula and the eye roll reflex. If placed into water, it cannot maintain its balance and it may show muscular quivers. Such a fish will not be responsive to handling and will not exhibit escape responses. Percussive stunning may be irrecoverable, but it is advisable to bleed the fish immediately after a stun to prevent recovery (and possibly improve appearance, taste and keeping quality of the product).

Percussive stunning may be achieved using a priest for small numbers of fish, or for emergency or back up purposes. For routine commercial operations, automated percussive stunners are advised. It is difficult to maintain the required accuracy with priest stunning when large numbers of fish are being processed. Moreover, mechanical stunners that use fish behaviour to minimise handling are now available and these offer a huge welfare advantage. Percussive stunners may not be as effective when used on deformed fish, or those of a size other than that for which the machine was designed. A priest should therefore always be available for dispatching deformed fish and in case of machine failures. Machines are currently available for stunning salmon and trout.

Electrical stunning

The general principle of electrical stunning is to pass sufficient current through the brain to cause an epileptic -like fit. This may be achieved by directly applying electrodes across the fish's head or by exposing the fish to an electrified waterbath. The latter method has the advantage that the fish can be processed in water and do not suffer as much from handling stress. On the other hand, in water stunning requires higher currents. The electrical parameters will vary depending on the species of the fish and the conductivity of the water. The effect of the electrical treatment will depend on the strength and duration of the current as well as the frequency of the electrical waveform. Generally the effectiveness of the stunning increases with increasing current strength and decreasing frequency. The effects of electricity on fish range from immobilisation, narcosis to electrocution. Low currents can result in the fish becoming immobilised by the tetanising effects of electrical stimulation. This is undesirable as the fish may experience pain or distress but are unable to express their condition due to the immobilisation. Stunning with the correct parameters should result in the fish losing consciousness and sensibility to pain (narcosis). Some currents are of characteristics that results in the fish failing to recover consciousness (narcosis). This is desirable for animal welfare as it guarantees that the fish do not recover during subsequent processing.

All stunning treatments should render the fish immediately insensible to pain. In the absence of complex brain activity measuring equipment, some behaviours may be monitored as indicators of brain function. These behavioural tests are listed in the table below.

Use of incorrect electrical parameters may not only result in poor animal welfare, but may also result in poor product quality through haemorrhaging. It is therefore important that only parameters that have been determined by scientific investigation be used in stunner design. Electrical stunning has potential as a humane method for killing large numbers of fish for disease control. It is also possible to effectively stun deformed fish and flatfish.

Unacceptable methods

Some traditional methods of killing fish result in lengthy periods to loss of consciousness and may involve pain or distress. The methods include death in ice slurry, live chilling, gill cutting without stunning and carbon dioxide narcosis. The HSA recommends that alternative methods (eg percussive or electrical stunning) be used where available. The HSA support research on humane alternatives to traditional methods of killing fish.

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