Batzina, A., Drossos, I.-P., Karakatsouli, N. 2019. Effects of familiarity on individual growth and feeding behaviour of European seabass Dicentrarchus labrax. Applied Animal Behaviour Science 211, 112-117.

Available literature indicates that fish are able to recognize familiar conspecifics and that familiarity may offer important benefits (e.g. more stable social environment, improved foraging behavior, growth, survival). In the practice of intensive rearing of European seabass Dicentrarchus labrax mixing of familiar and unfamiliar conspecifics is very common following grading and fish with no previous contact with each other are forced to coexist. The aim of the present study was to investigate whether living with familiar or unfamiliar conspecifics is advantageous for European seabass. To mimic a grading procedure, two size classes were tested (small: 70.2 g and large fish: 96.2 g). Seventy two fish from mixed groups with prior nine-months familiarity were distributed, in groups of six, in twelve tanks to form familiar groups of small (FS) and large (FL) fish (fish selected from the same tank), and unfamiliar groups of small (US) and large (UL) fish (fish selected from six different tanks). All fish were individually marked and individual food consumption was used to rank individuals in the feeding hierarchy. Each treatment was triplicated and the experiment lasted for 75 days. Familiarity had no effect on fish growth performance. In all treatments a feeding hierarchy was established; fish of the highest feeding ranks had the higher mass and specific growth rate, consumed the majority of food offered with little daily variation and made more efforts to catch their food. This hierarchy was weaker in US compared to FS fish (lower correlation coefficients). Correlation coefficients between mean share of meal and coefficient of daily food consumption variation, which have been used as indicators of the strength of the social hierarchy, indicate US groups as the ones with the milder social environment. Familiarity effects on large fish social environment were minor The results of the present study suggest that, within the range of body mass examined, the mixing of unfamiliar conspecifics after performing a grading procedure is not indicated to be neither beneficial nor detrimental for European seabass productive traits. Moreover, milder social conditions seem to be established, at least in the smaller size class, which is of importance form the fish welfare point of view.