Search for sociality in white sharks: investigation of the co-occurrence of juvenile white sharks (Carcharodon carcharias) using passive acoustic telemetry data

Context Social network analysis (SNA) as a quantitative measure of animal grouping and social behaviours has been increasingly used to study the group dynamics and potential sociality of elasmobranch species. Aims Despite their mainly solitary swimming and hunting behaviours, white sharks (Carcharodon carcharias) are known to aggregate at seasonal feeding sites as adults, and in spatially defined nursery areas as juveniles, exhibiting yearly site fidelity. This raises the question, does sociality, defined as active association or avoidance preferences with conspecifics, influence white shark movement behaviours when in close spatial proximity at aggregation sites? Methods In this study, detections of juvenile white sharks on a non-overlapping passive acoustic array along a single beach within a nursery area off New South Wales (NSW), Australia were analysed hourly using SNA over a 9-year period (2010–18). Tagged individuals were defined as co-occurring if they were detected within the same hour up to 2 km away from each other. This co-occurrence data was used to construct yearly association networks. Key results Contrary to some previous studies, the likelihood of sharks co-occurring was not found to be correlated with sex or age, and the observed co-occurrences were not found to be significantly different from expected based on random association. However, there was a strong tendency of individuals that were tagged at the same time in the same location to co-occur across all years. Conclusions This could indicate that certain cohorts of sharks visit the NSW nursery area at consistent times over the years of the study. Implications Although no evidence of non-random co-occurrence was found in this study, the fact that groups of the same individual sharks appear to move to and from the NSW nursery area at the same times and are detected in close spatiotemporal proximity indicate that social behaviours may be present in the nursery aggregations. Owing to the large spatiotemporal definition of co-occurrence and the fact that only tagged individuals could be observed in this study, further research is needed at smaller spatiotemporal scales to quantify the impact of sociality on white shark behaviour when aggregating in the nursery area.