Effect of grading on energy consumption in European catfish Silurus glanis

In aquaculture, fish grading is usually used to balance the resource availability for conspecifics in a stock. As a result, the social hierarchy does not limit the growth of fish in a size-homogeneous stock. However, grading also leads to the interaction of individuals without any previous contact. The interaction of these unfamiliar individuals is followed by increased aggression that is suppressed in the case of familiarity based on previous experience. Under laboratory conditions, we observed the spatial distribution of juvenile European catfish using passive integrated transponders while simultaneously measuring individual energy consumption using physiological sensors. We observed energy consumption during the interactions between familiar and unfamiliar conspecifics.Three types of spatial distributions, referred to as direct contact, visual contact and contact on the basis of chemical or acoustic cues, were observed. We assumed that familiarity would reduce energy consumption for the benefit of the group of fish with previous experience with one another and would be reflected in the spatial distribution of conspecifics. Our results show that energy consumption during contact with familiar conspecifics was lower compared to energy consumption during contact with unfamiliar individuals. For all three types of spatial distributions, the level of energy consumption was established in less than 1 min following first contact and did not vary with further passage of time. Furthermore, familiar conspecifics always took up positions closer to each other than did unfamiliar conspecifics.Our results correspond with the generally known fact that grading stresses fish whilst familiarity reduces stress, as shown by levels of energy consumption. The presented results provide further evidence that the familiarity of stocked fish represents an important factor influencing the welfare of fish in aquaculture.