142. Fry leave the hatchery at a size of about 5g for distribution to farms. They are transported in specially designed tankers fitted with oxygen monitoring equipment and aeration or oxygenation apparatus. Larger fish are also transported in this way. Some farms start with fish of 50 to 100g in weight and, for stocking fisheries, fish may be up to
1 kg in weight or more when released. Trout reared for the table are killed at approximately 350-400g.
143. On our visits we saw trout stocked at levels up to 60kg/m³ although the more normal range was between 30-40kg/m³. During the visits we noted a high incidence of fin injury which suggests that stocking density may be too high in all trout farms. It is a highly complex task to set a maximum stocking density for trout because of the variety of systems in use. Many factors such as oxygen level, fish size, water temperature, water flow, available space and carbon dioxide and ammonia levels affect the acceptability of stocking densities. We note these difficulties but believe that work needs to be done urgently to determine a maximum acceptable stocking density for trout. It has been difficult to discover the mortality rate of farmed trout but it is 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.