However, due to the practical issues of performing quantitative behavioural observations on individual fish at such high rearing densities, the indirect measures of aggression obtained by Laursen et al. (2013) serve as an indication of increased aggressive behavioural interactions at lowdensities. Here, it is therefore speculated that fish reared at density of 25 kg m−3 were more engaged in aggressive social interactions, and that this behaviour incurs the additional energetic cost.