Pre-Slaughter Handling

Most fish demonstrate an emergency response when threatened. This may follow a sudden disturbance from a net, a noise or other unexpected activity, or when they are removed from water. The response usually involves increases in stress levels which will have an adverse effect on the welfare of the fish and also the flesh quality. It is therefore essential that, regardless of the slaughter method employed, fish must always be prepared, handled and delivered to the stunning point in a humane way.

Withdrawal of food prior to slaughter

The withdrawal of food for a number of days before slaughter is believed by some producers to have beneficial effects on the final product in terms of food hygiene and flesh quality. Current research suggests that 72 hours maximum is sufficient for complete emptying of the gut whilst minimising adverse welfare implications. This period should not be exceeded.

Where cleaner fish, such as wrasse, are used, these should be removed before food is withdrawn in order to avoid predation.

Crowding

Crowding is the term given to the process in which the area available to the fish is reduced, usually in order to facilitate the removal of fish from the pond or cage. Net 'crowd pens' are typically used for the process. Crowding can cause suffering and stress for the fish but, with correct management and careful handling, it is possible to keep stressors to a minimum.

Unless crowding is carefully controlled, fish will be exposed to a decrease in oxygen levels, a rapid rise in stocking density, an increase in light intensity and abrasion from the net or other fish. For these reasons there must always be at least one member of the slaughter team monitoring the crowd pen. It is important that this person, who is solely responsible for the welfare of the fish, can recognise problems and knows what action to take to resolve them. 

Where possible, a crowd pen should be set up so that fish can swim against the tide towards the inlet pipe and preferably into a shaded area. Taking advantage of the natural behaviour of the fish in this way will encourage movement with minimal stress. 

Managing a crowd pen

Crowding should always be done in a gradual manner. It is not acceptable to pull nets tight, leave them and return to pull them in tighter. If any signs of escape behaviour are seen and movements become more vigorous, the nets should be loosened until the behaviour of the fish calms down. However, this can still cause problems, regardless of how quickly the nets are loosened and the fish return to more normal activity: the stress experienced will have an effect on the eventual flesh quality. There are two main concepts behind the shape of a crowd pen: either narrow and deep (the preferred option) or wide and shallow. Overhead covering nets can be used on either to reduce light levels and to protect against aerial predators.

Deep nets

Deep, narrow nets can provide a more relaxed environment for the fish. Due to the smaller surface area, the light intensity is similar to that found in the fish’s normal environment. As the net is not kept taut, fish have more freedom for movement without damaging themselves.

Figure 1: Deep net

Shallow nets

Large surface areas in the crowd pen expose fish to brighter light levels than normal, which can result in higher activity levels. This, combined with the taut net sides, can increase the amount of carcase damage. Even slight movements of a tight, shallow net can have a significant negative impact on fish, as a large number are in close contact with the net. This means it is especially important for this type of pen to be brought in very slowly and gently, rather than suddenly.

Figure 2: Shallow net

Raceways

As with crowd pens, the area in which fish are held in a raceway must be decreased gradually with no sudden or rapid movements which may cause excitement to the fish. Rapidly decreasing the area available to the fish forces them closer together and may increase stress levels.

Water quality

The water quality in a crowd pen can deteriorate in a short time. It is essential that clean and well-maintained nets are used for crowding and that the oxygen level is monitored. If the oxygen level falls below the critical level of 6mg/l then oxygen should be added to the water to alleviate stress. The addition of oxygen to the crowd pen has two advantages: it replenishes the oxygen content and attracts fish towards the diffuser. When correctly positioned it can help to move fish, passively, towards the exit. Careful consideration should be given to the type of diffuser chosen to release the oxygen, in order to ensure the oxygen level is suitable throughout the entire harvest.

Monitoring a crowd pen

Fish must always be crowded at an appropriate rate for the subsequent stunning operation. Where possible, fish should not be kept crowded for more than two hours. If the system requires longer than this, the process should be reviewed and the way in which the pen is split re-examined. Figure 3 shows how calm the water should be during crowding. Figure 4 shows a crowd pen that has been brought in too quickly, causing the fish to swim and burrow. A simple scoring system can be used to help train staff to recognise acceptable levels.

Follow this link for further information on Monitoring a Crowd Pen.

Figure 3: Good crowd

Figure 4: Bad crowd

See Also: Monitoring a Crowd Pen