Changes in demand feeding behaviour in Arctic charr, Salvelinus alpinus L., caused by differences in dietary energy content and reward level

The objective of this study was to determine whether Arctic charr, Salvelinus alpinus L., are able to adjust their demand feeding behaviour in accordance to differences in dietary energy content (experiment 1) and reward level (amount of food received in response to one trigger actuation) (experiment 2). Fish (initial size 215 g in experiment 1 and 88 g in experiment 2) were reared in 0.8‐m3 indoor tanks at commercial stocking densities and fed using demand feeders for 57 and 180 days, respectively. Demand feeding activity did not differ significantly between groups of charr fed diets with a low (19.8 MJ kg‐1) gross energy content and those given high‐energy (22.0 MJ kg‐1) feed. As a result, fish offered the high‐energy diets grew significantly faster. The results show that charr held under culture conditions are unable to adjust their demand feeding activity based on the energy content of the food. On the contrary, Arctic charr are able to adjust their demand feeding activity to either low (0.33 g), medium (0.87 g) or high (1.52 g) rewards, and thereby, regulate their food supply to fit their needs. However, it took about 90 days before charr in the low‐reward treatment released a daily food ration as high as that released in the high‐reward groups. Consequently, there was a significant positive relationship between the size of the reward and final weights. To avoid any depression of initial growth rates, the optimal size of the reward should be 0.1 g per kg fish and trigger actuation.