Effective Percussive Stunning

Amongst other things, an effective stun is dependent on the blow being administered to the correct part of the skull. To ensure the maximum impact on the brain, the best position is where the brain is closest to the surface of the head and where the skull is thinnest. In salmon and trout this found directly above and slightly behind the eyes (figure 10). The blow does not have to penetrate the head to be effective.

Figure 10: Position for effective percussive stunning of salmon and trout 

Signs of an effective percussive stun:

• No opercular movement;
• No eye movement;
• Bulging of muscle ring near pectoral fin.

Failure to stun

If a fish is not properly stunned it must be re-stunned immediately and bled without delay. When using a priest there is a risk that the blow may not be positioned correctly. If positioned too far back, not only will this fail to produce an effective stun but it can also damage the flesh. If hit too far forward it may not produce an effective stun.

Mature and deformed fish can present problems when using automated mechanical stunning systems as they may not correctly activate the trigger. When these fish are found at the stunning point, consideration should be given as to whether to use the machine or a priest. If the machine is used, fish should be double checked for an effective stun. If there is any doubt that a fish has not been stunned effectively, the priest should be used to repeat the stun.

Operator considerations

The design of the stunning table and of the delivery method to the table/machine is of utmost importance for both fish welfare and the operator’s health and safety. Operators should not have to concentrate on keeping their balance, or to bend excessively when using the equipment, as this can lead to operator tiredness and inaccurate stunning, leading to poor welfare and damage to the fish.

Where manual stunning is employed, it is essential for both fish and operator welfare that the operators are given regular breaks and are allowed to work at a reasonable rate, otherwise accuracy and effectiveness will be compromised.

Humane slaughter using percussive stunning is dependent on:

• The use of an appropriately trained, competent slaughter team;
• Selection of suitable equipment/staff capable of producing an effective stun;
• Correct setting of the equipment;
• Correct delivery of the blow;
• Recognition of an effective/ineffective stun;
• Back-up stunning equipment being immediately to hand;
• Operator’s knowledge and competence in the use of back-up equipment;
• Regular maintenance and cleaning of equipment, i.e. daily.