116. Salmon parr become smolts naturally between March and May but the process can start much earlier. The smolts are then moved to sea. They are more easily damaged during transport and more susceptible to fungus if they are maintained in freshwater. Smolts that go to sea after one year are called S1s and may weigh anything between 30 and 120g. Parr that do not smolt and are held for a further year are called S2s and may weigh up to 400g going to sea. There is a small but increasing production of S½s, which are induced to smolt in the autumn by using temperature and photoperiod control.
117. Almost all further growth takes place in sea water cages. During this period the main priority is to achieve optimal growth. Feeds may vary but the most common feed consists of a dry pellet with a high oil content (25-30% oil) (see paragraph 74).
118. Stocking densities are a crucial factor affecting fish welfare. A balance must be struck between optimising the use of cages and the needs of the fish. Fish need sufficient space to show most normal behaviour with minimal pain, stress and fear. Some of the fin injury which was evident in most of the salmon we observed may be a consequence of stocking densities which were too high. Farmers formerly aimed for a maximum density of about 25-30kg/m³ in sea cages but recent practice has been to aim for a maximum of about 15kg/m³ because this reduction helped to limit the impact of disease. The actual figure chosen is dependent on local conditions but the trend towards lower stocking densities is welcomed. However, it was noted that this reduction was part of a strategy to control disease and that as farmers make more use of vaccines there may be a temptation to increase stocking densities. In cages which were studied carefully, smaller fish have been observed to adopt peripheral positions and this may occur generally. Some fish can be prevented from feeding and forced to take evasive action by more dominant fish. Good scientific information concerning the effects of stocking densities on welfare is lacking, but in the absence of this, high stocking densities must be avoided until it can be demonstrated that they do not have harmful effects on fish.
119. Apart from movements between cages to group fish of similar sizes following grading, fish will normally remain in the same cages at sea until harvest. This may be anything from one to three winters at sea depending upon the maturity of the fish and the size required at slaughter.
120. Salmon mortality after the parr stage can be much higher than that which occurs in other farmed animals. High mortality is likely to be associated with poor welfare and efforts should be made to reduce it in existing management systems and to develop systems which improve fish welfare and reduce mortality.