Abstract
Group living determines how animals interact with others and their habitat. Elephant fish ( Callorhinchus milii ) are chimaeras (Class Chondrichthyes, subclass Holocephali) that migrate to coastal waters for reproduction throughout its southern Australian range. However, there is limited information about how they co-utilize these areas. This study used social network analysis (SNA) and passive acoustic monitoring data to examine associations between adult elephant fish in an estuary in southeastern Tasmania, Australia (42°53.710′S 147°34.228′E), specifically Pitt Water and Frederick Henry Bay (FHB) from January 2012 to December 2013. The influence of sex on the social structure was also investigated. As a result, two distinct groups of animals (clusters) were found across both years: a resident female cluster that remained in the upper estuary (UP) and a mixed-sex group whose movements varied seasonally. These clusters formed a cohesive community, with the female cluster potentially playing a critical role in connecting the population members. The mixed-sex group split into temporary aggregations, a so-called fission-fusion society that relies on seasonal reproductive patterns. Furthermore, by investigating all the individuals (including both clusters), spatial sex segregation was evident. The suitable conditions of the upper part of the estuary for egg-laying and the mating dynamics may shape the association structure observed. These findings highlight the importance of these essential habitats for elephant fish reproduction and emphasize the need for management strategies that incorporate space co-utilization patterns, address sex-based segregation, and recognize the role of females in maintaining social structure to ensure population stability and connectivity.