Conspecific presence affects the physiology and behaviour of developing trout.

Interactions between conspecifics during embryonic development have the potential to influence phenotypic traits of individuals, with social interaction being particularly important for the realisation of normal behaviours. Here, the effect of increasing numbers of conspecifics (from isolation to groups of 20, 50, 100 and 200) on the physiology and behaviour of developing trout was investigated with the hypothesis that a) conspecific presence would alter physiological processes and b) that development in the absence of conspecifics would affect subsequent behavioural performance. During development, mass, oxygen consumption, sodium uptake and ammonia excretion rates were measured at a variety of time points from fertilisation to after hatching. After first feeding, behaviour of individuals raised in social isolation was compared to behaviour of juvenile fish raised in social groups. Embryos and larvae raised in social isolation had lower oxygen consumption rates, lower sodium uptake rates and higher ammonia excretion rates compared to those raised in groups. Social isolation delayed the response of a juvenile fish to its own mirror image but did not affect the probability of it winning a paired encounter with an individual raised in a social environment. In conclusion, the presence of conspecifics alters physiological processes during trout embryo and larval development and subsequent social behaviours of juveniles.