THE EFFECT OF TEMPERATURE, DIETARY ENERGY CONTENT AND REWARD LEVEL ON THE DEMAND FEEDING-ACTIVITY OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The present study included three experiments, the objectives of which were to assess the effects of temperature (Expt. 1), dietary energy content (Expt. 2) and reward level (amount of food received in response to one trigger actuation) (Expt. 3) on the demand feeding activity of rainbow trout. Fish (initial size 30-120 g) were reared in 0.8-m(3) indoor tanks and fed using demand feeders. Each of the three experiments lasted about 60 days. The main parameters recorded were bite activity (number of trigger actuations), growth rate and feed conversion. Bite activity was significantly higher in trout reared at 15 degrees C than in those reared at 5 degrees C; consequently, food supply and growth rate were higher in the former. However, as evaluated by the growth and feed conversion rates, only fish raised at 5 degrees C showed a bite activity that was close to the optimal, whereas the bite activity was too low at 15 degrees C. Groups of fish fed diets with either a low (19.8 MJ/kg), medium (20.7 MJ/kg) or high (22.0 MJ/kg) gross energy content did not show any significant difference in bite activity or growth rate. This indicates that trout in aquaculture conditions are unable to adjust the trite activity in a way that matches the energy content of the food. The reward level significantly influenced the ability of the trout to meet their demand for food by regulating its supply. Among groups offered either a low (0.36 g), medium-sized (0.80 g) or high (1.65 g) reward, the high-reward groups exhibited the highest growth rates. The trout were, to some extent, able to compensate for a low reward by increasing their bite activity. However, at low or medium-sized rewards, fish were unable to compensate to the degree necessary to meet their food demand.