The use of self-feeders in rainbow trout (Oncorhynchus mykiss) production

The aim of the present paper is to evaluate the use of self-feeders in rainbow trout production. The basic idea with self-feeders is that the fish themselves control the feeding level, which is thereby set by their feeding motivation. To determine the limitations of the technique, this work has focused on characterising: (1) how self-feeding activity is related to food demand; and (2) how the self-service food supply is related to growth and feed conversion ratios. For the feeding system to function correctly, rainbow trout must first learn how to operate it. Trout reared in tanks in groups of 100–300 individuals require about 25 days to reach a stable level of self-feeding. Under large-scale rearing conditions (e.g. in cages with group sizes of 1000–2000 ind.), however, learning seems to be of minor importance for the ability of fish to operate the system. When reared in small tanks in groups of up to about 300 ind., self-feeding activity in trout is strongly influenced by the development of dominance hierarchies. Under such situations, a small number of fish will dominate the actuation of the trigger and thereby have a disproportionately greater influence on the food supply of the group as a whole. Any change in temperature is quickly followed by a corresponding change in self-feeding activity up to about 15 °C. Above this temperature, trout lower their activity, indicating that self-feeding activity reaches a maximum at around 15 °C. The reward level (amount of food received in response to one trigger actuation) is the single most important factor requiring proper adjustment in order to optimise growth and feed conversion ratios when using self-feeding systems. The optimal reward is discussed in relation to differences in dietary energy content, number of fish, and water temperature. High-density conditions reduce the self-feeding activity of trout, and the recommended maximum rearing density is about 30 kg m−3. An evaluation of growth and feed conversion data indicates that self-feeders have the potential to function well with rainbow trout under commercial rearing conditions.