Percussive Equipment

For many years the priest (figure 7) has been used as an effective way of stunning fish, but in order to be humane it relies heavily on the strength, skill and consistency of the slaughter team. The subsequent development of automated systems in the 1990s mechanised the stunning operation, introducing greater consistency. Research, showing that sufficient percussive force alone causes insensibility and indeed death in the majority of fish, has led to the development of individual stunners (figure 8) and flow-through machines (figure 9) which deliver a non-penetrative blow to the fish.

Figure 7: Priest                            Figure 8: MT4 stunner

Figure 9: SI5 stunning machines

Manual percussive stunning

The priest still remains a humane option for slaughter when used appropriately. However, due to the pressures associated with a commercial harvest, it is very difficult to maintain enough accuracy to ensure 100% effective stunning with the first blow of the priest. Therefore it is advised that for commercial harvesting this method of stunning is used only as a back-up, for casualty fish or very small numbers.

Automated percussive stunning

The most commonly used automated stunning machines are powered by compressed air with an air pressure range of 90-120 p.s.i. (6-8 bar). The operator gently grasps the fish near the middle of the body (not by the tail), guiding it into the opening of the machine to ensure the fish is upright. The fish activates the trigger system, resulting in the piston striking the fish on the head, rendering it immediately unconscious.

More recent models avoid the need for the operator to handle the fish as the design of the table encourages fish to swim into the entry channels. These tables need to be set up carefully and adjusted for the site’s individual circumstances. They should always be used in line with manufacturer’s guidelines. Incorrect set-up will not encourage the fish to flow through and intervention will be required.

A percussive stun may also be delivered by a handheld captive-bolt device designed for poultry. This will ensure sufficient impact energy is delivered to each fish, but may cause unacceptable carcase damage in some species.

To enable back-up stunning it is imperative that all staff stunning and bleeding have quick access to a priest and that all staff are properly trained and competent in its use.

Limitations of automated machines

The automatic systems currently available have been developed for large salmonids such as salmon and trout (over 1kg). They are not suitable for fish with a significantly different body shape to these fish. Problems can arise when the machines are used for deformed or mature fish as they do not always activate the trigger at the correct time, resulting in incorrect positioning of the blow.

Next: Effective Percussive Stunning