Negative effects of handling the fish
Even if handling is necessary, especially in intensive farming, it includes a number of possible adverse effects. Before selecting handling routines and equipment, this must be taken into consideration. Handling creates a stress response in the fish, which may affect the production results negatively. When the fish become stressed, the primary and secondary effects will not normally be discovered unless special measurements of heart rate, oxygen consumption or blood characteristics (for instance cortisol or glucose) are taken. The farmer normally registers the secondary or tertiary effects of stress manifested by reduced growth and reduced immune defence, which again may directly reduce productivity.
It is also important to consider the possible stress response involved in pre-harvest handling. This may increase the consumption of glycogen stored in the muscle (part of the stress response). The results of this may be an earlier occurrence and a shorter duration of rigor mortis after slaughtering, which again will reduce the fish quality.
How much the fish is affected by handling is species dependent: some species are more tolerant of handling than others. Results also show that fish may adapt to handling procedures, and the stress responses will gradually be reduced. This can be seen, for example, when the fish tanks are washed.14The first time the tanks are washed it is possible to measure a high stress response, but this will gradually decrease as the fish begin to tolerate this pro-cedure. Breeding programmes may also be used to adapt the fish to more and more of the normal handling operations in fish farming.23–25When starting to rear a new species, it is collected from natural wild stocks and put into farming conditions. The behaviour of such stocks differ from that of wild stocks that have been farmed and bred for genera-tions as is clearly seen when looking over the edge of the tanks containing farmed and wild stock; difference in behaviour is also shown by the number of involuntary collisions between the fish and the tank walls.
Fish may also suffer physical damage if handled too roughly. Tolerance here, of course, also depends on species and life stage. The fish may be wounded, by rough handling leading to fungal attack. It is especially important to avoid physical damage in pre-slaughter handling because it may reduce the flesh quality and hence the price of the product.
All handling includes some kind of human work, which requires time and creates costs. The total economic cost and possible negative effects of handling must therefore be compared to the positive effects handling will have on the production. For this reason it is very important to use effective handling procedures and handling lines which affect the fish as little as possible.