Multi year social stability and social information use in reef sharks with diel fission-fusion dynamics: Raw acoustic detection

Animals across vertebrate taxa form social communities and often exist as fission-fusion societies. Central place foragers (CPF) may form social groups, from which they will predictably disperse to forage, either individually or in smaller groups, before returning to fuse with the larger group. However, the function and stability of social associations in predatory fish acting as CPFs is unknown, as individuals do not need to return to a shelter, yet show fidelity to core areas. Using dynamic social networks generated from acoustic tracking data, we document spatially structured sociality in CPF grey reef sharks at a Pacific Ocean atoll. We show that sharks form stable, social groups over multi-year periods, with some dyadic associations consistent for up to four years. Groups primarily formed during the day, increasing in group size before sharks dispersed from the reef to forage at night. Our simulations suggest that multiple individuals sharing a central place and using social information while foraging (i.e. local enhancement), will outperform non-CPF social foragers. We show multiyear social stability in sharks and suggest that social foraging with information transfer could provide a generalisable mechanism for the emergence of sociality with group central place foraging.

Methods
Acoustic listening stations (VR2W) were placed on the reef and listening continuously for sharks that have been tagged with long life acoustic transmitters. Every time a shark swam within range of a listening station, the time and date of detection was recorded along with the unique transmitter ID number. Raw data is provided