Effects of Electricity

Stunning with electricity is known as electronarcosis, and killing with electricity is known as electrocution. Electronarcosis is a fully reversible procedure, immediately disrupting normal brain function for a short period only. Electrocution leads to complete dysfunction of the brain which prevents the breathing reflex working. This means that fish die through lack of oxygen whilst still in an unconscious state. This section of the guide describes what happens to fish during electrocution, detailing the parameters required to ensure that each stun is immediate and effective. Electronarcosis by itself is not suitable for fish that are not percussively stunned or bled immediately after stunning. This is because they would recover from the stun and be fully conscious during processing.

Electrocution

The purpose of electrocution is to kill by causing immediate insensibility and loss of consciousness and preventing the respiratory system working effectively. When held in an appropriate electric field for sufficient time the brain is severely damaged and the breathing reflex is abolished. Fish therefore die due to lack of oxygen.

When a fish is electrocuted, it becomes rigid with slight body tremors, then gradually relaxes and shows no further movement. In practice, fish are killed by electrocution using equipment which exposes the fish to an electric field that causes an immediate stun and which then, through extended exposure, results in permanent and fatal brain damage. Following electrocution, some fish will show strong sporadic muscular spasms in which the mouth gapes and gills flare whilst they are unconscious. Following each spasm the fish relaxes. These spasms normally cease within five minutes. They are uncontrolled irregular movements which should not be confused with regular movements indicative of a return to consciousness.

In order to stun and kill fish with electricity, sufficient current at a suitable frequency must pass through the brain for sufficient time. Factors such as species, size, stress levels, temperature, water conductivity and the number of fish in the stun tank may affect the duration of insensibility resulting from a stun. These factors therefore need to be carefully considered when selecting equipment parameters.

Signs of an effective stun:

• Eye movement stops;
• Small muscular twitches;
• No opercular movement;
• Fish turns upside down.

Animals should be regularly monitored during operation of the stunning machine. All fish should be effectively stunned on exit from the stunner and remain so until death. Signs of recovery should be looked for when fish are on the bleed table. Regular monitoring allows problems to be identified and responded to quickly.

Inappropriate stunning

If an electrical field of inappropriate voltage, frequency, current or duration is applied to fish they may not be effectively stunned and may instead be paralysed whilst still conscious. Under these circumstances the fish cannot show typical pain responses or escape behaviour. Alternatively, exposure to a sufficient current, but for insufficient time, will result in a stun with only a limited period of insensibility and a high risk of recovery before death.

Stunning equipment must display the stun parameters being applied to the fish in a way that is clearly visible to the operator. The equipment should be designed to provide a visible or audible warning if the correct parameters are not being delivered.

Corrosion can build up quickly on the electrodes of the stunner, especially in saltwater systems. This can impact on the amount of current delivered to the fish and result in an ineffective stun. Regular cleaning and maintenance of electrodes is essential.

It is essential that all equipment is maintained and repaired as necessary and in line with manufacturer’s guidelines to ensure that all fish are humanely stunned. Failure to maintain equipment will reduce its efficacy.

Next: Two-Stage Electrical Stunning/Killing