Electrical Stunning

The general principle of electric stunning is to pass sufficient current through the brain to cause an epileptic-like fit. This results in immediate unconsciousness and insensibility to pain. If the current flows for long enough (typically 30 seconds for trout) the fish will die of anoxia before the brain is able to recover sensibility.

The electric current also causes spasms in the fish muscle which can, under some circumstances, result in haemorrhages and other carcase damage. Stunning conditions therefore need to be carefully designed to ensure that the process causes neither pain nor carcase damage and that recovery before death is not possible. These conditions are known to vary widely between different species of fish. This section explains some basic electrical principles and how they apply to electrical stunning and killing of fish.

• An electric stun must cause unconsciousness within one second of application and the unconsciousness must last long enough to ensure that the animal does not regain consciousness before dying.

There are electrical stunning systems which stun fish whilst they remain in water and others which stun fish out of water using electrodes which make direct contact with the fish ('dry' or 'semi-dry' systems). Both methods have pros and cons. Stunning fish in water reduces the stress of exposure to air and light and reduces the likelihood of mechanical damage to the skin. However, 'dry' or 'semi-dry' stunning systems have a more consistent effect on the fish and can result in less electrical carcase damage.

In some instances, electrical stunning is used prior to application of a percussive stun. This renders fish unconscious for the duration of the handling process prior to percussive stunning. 

Current, voltage and conductivity

The flow of electricity through an object is known as the current and is measured in amps (A). the current is related to the number of electrons that pass in one second. Current can be either direct current (DC), as from a battery, which flows only in one direction; or it can be an alternating current (AC), from the mains supply or a generator, where the direction of the current flow changes many times per second. The driving force or pressure causing the flow of current is known as the voltage and is measured in volts (V).

The conductivity of a material is a measure of how easily an electrical current can pass through it. Conductivity is measured in micro-siemens (µS/cm). River water typically has a conductivity in the range 50-700µS/cm, while sea water may have a conductivity of up to 50,000µS/cm.

Frequency

The frequency of a current is how many alternating cycles of current occur per second. This is measured in cycles per second, or Hertz (Hz). Mains electricity has a frequency of 50Hz (i.e. 50 cycles per second). The frequency of the current determines the effect it has on fish.

Stun parameter selection

When fish are electrically stunned whilst they remain in water, electric current can pass around the fish as well as through them, therefore it is most useful to define the electric field which is required in the water rather than the electric current. If the water tank holding the fish is rectangular in shape and the electrodes cover two opposite walls of the tank then the electric field in a tank can be calculated as the difference in voltage between the electrodes divided by the distance between them. This is specified in units of volts per centimetre. The strength of the electric field required to stun fish is affected to some extent by the conductivity of the water. Trout in river water typically require 3V/cm whereas Halibut in seawater can be effectively stunned using 1V/cm.

As already mentioned, the effect of electricity on the fish is influenced by the frequency. Frequencies close to 50Hz have a greater effect on both the fish brain and muscle than higher frequencies, resulting in an effective stun. However in trout and salmon, due to the effect on muscle, a frequency of 50Hz is likely to cause unacceptable carcase damage. A higher frequency, at a slightly higher electric field strength, can still achieve immediate insensibility whilst also minimising carcase damage. A frequency of 1000Hz has been found to be suitable for trout. Higher frequencies above 1000Hz should not be used since the fish may not be rendered immediately unconscious and insensible to pain.

Next: Effects of Electricity