The utilisation of artificial and natural food sources by first feeding fry and small parr of Atlantic salmon (Salmo salar) reared in freshwater cages

Abstract

A study was conducted to evaluate the use of freshwater cages to rear small Atlantic salmon, Salmo salar. In particular, the study aimed to describe the feeding behaviour of such stocks with respect to artificial and natural food sources. Results showed that cage systems can be successfully used to first feed Atlantic salmon, or to maintain stocks transferred from tank systems after first feeding. The limiting factor to the method was considered to be the water quality conditions in the cages, which in turn are related to the physio-chemical characteristics of the cage site. Highly productive sites were considered unsuitable for salmon cage culture. Providing environmental conditions remain suitable, and proper mesh sizes and stocking densities are utilised, growth of caged fish can be comparable with those reared in tank systems. Except for a limited period after first feeding, the main food source of caged Atlantic salmon was determined to be the artificial diets. Crustacean zooplankton were the main food source at and immediately after first feeding, due mainly to their suitable particle size and mobile qualities. However, caged fish generally switched their diet choice to the artificial feed soon after first feeding; this was attributed to a recognition of this as the optimal feed choice. A proportion of the stock were observed to feed on the zooplankton until it became unsuitable with respect to particle size, upon switching, these fish were unable to feed on the artificial diets as effectively as those which switched earlier. The late-switchers were considered subordinates, and did not grow as well as the dominants. The degree of utilisation of natural feeds by fish introduced to cage systems after first feeding was determined to be dependent upon the size of Individual zooplankton in relation to the preferred particle size of the fish stock (PFR). As the latter equals and becomes larger than the former, natural feed importance in the diet decreases. It was not determined if this feeding behaviour had effects on the growth of the fish; however, it may have relevance in controlling cestode parasite infections. An evaluation of the feeding behaviour of the fish In the trials with respect to the concepts of an optimal foraging theory was undertaken.