Examining coastal habitat co-utilization of chimaeras using social network analysis

The tendency of animals to form groups is a widespread phenomenon across many species. Group living, defined as animals that stay together and, therefore, share a space, is an inherent component of the biology of a species. The formation of groups arises from interactions between individuals, and the patterns of these interactions constitute a social structure. Group behaviour is a complex system shaped by an active process where individuals choose to associate with others and a passive process when habitat conditions such as, refuge or resource distribution, induce individuals to aggregate. In an effort to unravel the mechanisms of grouping behaviour, social network analysis has become a powerful method to unveil the complex process of sociality. Additionally, methodologies of social analysis in combination with technological advances, such as acoustic biotelemetry, have expanded our ability to investigate the grouping behaviour of marine species in their natural habitat. Taking these approaches, studies have revealed new insights into the social behaviour of sharks and rays (Class Chondrichthyes); however, there is a notable gap in knowledge around group living in other cartilaginous fishes, such as chimaeras.
This study focused on examining the grouping behaviour of adult elephant fish, Callorhinchus milii (Class Chondrichthyes, subclass Holocephali) in an estuary in southern Tasmania, Australia. Associations between individuals tagged with acoustic transmitters from 2012 to 2013 were explored to examine the group behaviour of this chimaera. Relating the associations with spatial analysis, a community of two clusters was revealed. The first cluster was made up of only females that remained for the entire time in the upper part of the estuary; they exhibited high association rates and long-lasting co-occurrences. The second cluster was a mixed-sex group characterized by a wide range of movement throughout the estuary, with a seasonal migration, leaving the estuary during winter.The female cluster played a crucial role in connecting the social structure observed, while the mixed-sex group exhibited fission-fusion dynamics, where the group split and re-joined seasonally. Furthermore, sex-segregation was evident where females were more connected to each other than males. This work presents strong evidence that reproductive patterns influence the habitat co-utilization of elephant fish in southeast Tasmania. This research significantly expands our knowledge of this understudied species and underscores the importance of essential habitats during critical life history events.