Percussive Stunning

The objective of percussive stunning is to induce immediate insensibility by administering a severe blow to the skull of the fish. The fish must then remain unconscious until death. The term ‘percussive’ describes the principle of striking the skull with a solid instrument, i.e. the forcible striking of one solid body against another.

Physiological effects of percussive stunning

When a fast, heavy blow is correctly applied to the skull it produces a rapid acceleration of the head, causing the brain to collide against the inside of the skull. This causes disruption of normal electrical activity resulting from a sudden, massive increase in intra-cranial pressure followed by an equally sudden drop in pressure. The consequent damage to the nerves and blood vessels causes brain dysfunction and/or destruction and impaired blood circulation. The duration of insensibility depends on the severity of damage to the nervous tissue and the degree to which the blood supply is reduced.

The initial effect on fish is immediate insensibility, accompanied by what is known as ‘tonic’ activity: the fish becomes rigid, loses opercular movement, its mouth opens, eye reflexes are lost and a ring of muscle near the pectoral fin contracts and bulges for a short period of time. This period of rigidity can vary in length depending on the force of the blow, as well as the age and species of fish. When a fish is hit with sufficient force and in the correct position the stun is normally irreversible. However, if insufficient force is used, or the position is not ideal, it may recover to some degree. If there is any uncertainty as to whether a fish is effectively stunned, it should be re-stunned immediately.

An effective stun can be defined as one which renders the fish immediately unconscious and insensible to pain. This condition should last until death.

Basic physics

When a severe blow is rapidly administered to a fish’s skull there is a transfer of energy from the percussive instrument to the head and from there directly to its brain, resulting in immediate unconsciousness.

Effective percussive stunning is achieved by delivering the maximum amount of energy to the correct part of the fish’s brain, in the shortest possible time.

Practical use

When using percussive methods, fish should be presented at such a rate that they are only on the table for a maximum of a few seconds prior to being stunned. This is more important for tables which do not hold water, as this will minimise the duration of stress experienced by the fish and will make them easier to handle. When left on the table for too long, fish will start to flip and become more difficult to place in the stunner correctly.

Fish should not be kept out of water for longer than 15 seconds before stunning. Where procedures result in longer periods than this, the rate of delivery needs to be slowed down to a rate appropriate to the stunning process. In some percussive stunning systems fish are electrically stunned before removal from the water so that they are unconscious, and easier to handle, during the process leading to percussive stunning.

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.

It is essential that all equipment is maintained and repaired as necessary to ensure that all fish are humanely stunned. Failure to maintain equipment will reduce its efficacy.

Next: Percussive Equipment