Two-stage Electrical Stunning/Killing

Rapid and permanent insensibility can be initiated in trout, without causing significant carcase damage, by using a two-stage electrical stun. Following an initial stun, in an electric field of 2.5V/cm at 1000Hz, fish are held within an electric field of 0.5-1.0V/cm with a frequency of 50Hz for 30-60 seconds to produce permanent insensibility (for water conductivity of 500µS/cm). This significantly reduces the power requirements of the machine in comparison with single-stage machines, lowering the capital cost of the equipment and enabling a wider range of mechanical configurations to be used.

To achieve humane stunning with a two-stage method it is necessary to ensure that:

• Insensibility is established rapidly;
• An irrecoverable stun is achieved using a low-voltage maintenance stun.

Fish stunned for a brief period of time can quickly regain full consciousness. It is therefore important that fish are moved into the low-voltage maintenance stun electric field within a few seconds.

The introduction of a pause between the two stages can reduce the effectiveness of the stun, but is necessary in continuous flow systems to prevent interference between the two electric fields.

Conductivity

The figures mentioned above are for water conductivities of 500µS/cm. When water of different conductivity is used, electric fields need to be adjusted to ensure that every fish is humanely stunned and killed.

The conductivity of water differs greatly across the country and can have a significant effect on the required field strength. An irrecoverable stun can be achieved for freshwater fish under the conditions detailed in Table 2.

Table 2: Lowest tested electrical field strengths found to produce a permanent stun when using a frequency of 1000Hz AC

Careful observation of the fish and their reactions must be made during the initial setup and regularly throughout the operation to ensure humane stunning.

Operator considerations

When using electrical systems it is important that the operator can monitor the machine at all times. It is also important that operators have unrestricted access to the safety stop controls.

Any person stunning and killing fish must know the:

• Voltage required for effective stunning;
• Correct duration of stun;
• Signs of an effective stun/kill;
• Signs of an ineffective stun/kill.

Contingency plans should be made for occasions when there is an equipment failure or other unexpected occurrence which could result in fish being left out of water or in the stunning machine. Manual percussion and gill cutting may be a suitable back up in these instances.