Social network analysis of the behavioural interactions that influence the development of fin damage in Atlantic salmon parr (Salmo salar) held at different stocking densities

Social network analysis of behavioural interactions was used to quantify the effect of high (HD, 30kgm^-3) and low (LD, 8kgm^-3) stocking densities on the frequency and severity of fin damage in Atlantic salmon. Dorsal fin damage (erosion, splits, fin index) was significantly higher in HD compared to LD groups with higher amounts of dorsal fin erosion. The prevalence of dorsal fin splitting was also significantly higher in HD groups. No other fins were affected by fin damage irrespective of density. Social networks based on aggressive interactions showed that HD groups exhibited higher centrality, clustering coefficient, in-degree centrality, out-degree centrality and were less dense than LD groups. High centralities and clustering coefficients indicated a distinctive separation of fish within HD groups into initiators of aggression (out-degree four times higher than in-degree) and receivers of aggression (in-degree four times higher than out-degree). This separation of roles was seen only in HD groups where initiators had higher out-degree centrality while receivers showed high in-degree centrality. Initiators of aggressive interactions had less fin erosion, higher final weights and higher body lengths than receivers of aggression. Fish in HD groups were significantly less aggressive than fish in LD groups in terms of the total number of aggressive behaviours observed but they exhibited significantly more overt aggression in terms of biting frequency. This can explain the occurrence of higher levels of fin damage in HD groups. Fish in LD groups had lower final weights, final body lengths and body condition than those in HD groups. This study shows that fish grew better and were in better condition when held at higher densities, but has significantly more overt aggression and fin damage than fish at lower densities. Density therefore has a differential detrimental effect upon performance and welfare depending upon the choice of welfare indicator (e.g. growth and condition vs. aggression and fin damage).