Social network analysis of behavioural interactions influencing the development of fin damage in Atlantic salmon (Salmo salar)
The development of fin damage in Atlantic salmon parr was investigated using social network analysis of behavioural interactions occurring under different feeding and stocking conditions. Four separate experiments were carried out in which groups of fish were subjected to a long food restriction period (30 days) described in Chapter 2, high (30 kg/m3) or low (8 kg/m3) fish stocking densities (Chapter 3), predictable or unpredictable food delivery regime (Chapter 4) or a short food restriction period (10 days) described in Chapter 5. Dorsal fin damage (erosion, splits and fin index) was significantly higher in groups of fish subjected to food restriction periods (short and long), held at high stocking density (30 kg/m3) or with an unpredictable food delivery regime. No other fins were found to be affected by fin damage irrespective of the treatment. The social networks based on aggressive interactions showed higher centrality, clustering coefficients, in-degree centrality, out-degree centrality and less dense networks in groups subjected to food restriction (short and long), low stocking densities 8 kg/m3), and unpredictable food delivery. The high centralities and clustering coefficients indicated separation of fish within the groups into initiators of aggression and receivers of aggression. This separation of roles was seen only in the food restricted group, high density groups and unpredictable food delivery groups. Initiators had higher out-degree centrality while receivers showed high in-degree centrality. Also, initiators of aggressive interactions had less fin erosion, higher final weights and higher body lengths than receivers of aggression. The severity of fin damage was significantly higher when this role differentiation occurred, and it was highly correlated with fin biting events. Additionally, overall aggression was higher in food restricted groups, low density groups and unpredictable groups. The dynamic analyses of networks over time (Chapter 6) showed that fish classified as initiators of aggression did not change this behaviour after normal/control environmental conditions were restored (i.e. ad libitum food delivery, low stocking density or predictable food delivery). The latter result indicates that individual fish maintained their behaviour irrespective of their social context, which is consistent with the definition of behavioural syndromes, personalities or temperament defined for other non-human animals. Overall these studies demonstrated the importance of using social network analysis to clearly identify and quantify roles that individual fish assume within their network group and through time based on their behavioural interactions leading to dorsal fin damage and differential physical characteristics. The results are potentially applicable within the commercial aquaculture industry as a valuable technique to evaluate and improve the welfare of farmed fish.