Social stability, information use and shark sociality inferred from telemetry-derived movement data

David M.P. Jacoby, Institute of Zoology, Zoological Society of London, Yannis P. Papastamatiou, Department of Biological Sciences,, Florida International University, Thomas W. Bodey, Environment and Sustainability Institute, University of Exeter; School of Biological Sciences, University of Aberdeen, Jennifer E Caselle, Marine Science Institute, University of CA Santa Barbara, Santa Barbara, CA, Darcy E. Bradley, Bren School of Environmental Science & Management, University of California, Santa Barbara, Santa Barbara, CA, Robin Freeman, Institute of Zoology, Zoological Society of London, United Kingdom and Alan M. Friedlander, University of Hawaii, Manoa, Honolulu, HI

Background/Question/Methods
Efforts to determine the influence of sociality on the ecology, evolution and conservation of wide-ranging marine fishes is often hampered by our ability to gather long-term data on interactions between individuals. Consequently, methods have been adopted that estimate association based on the proximity of individuals to one another in space and time, from discrete tracking data. Acoustic tracking of site-attached fishes such as reef sharks, is commonplace within aquatic ecology, and on many reefs has revealed clear patterns of residency and central place foraging behaviour in these animals. Yet, reconciling the space use and foraging strategies of individual sharks with putative social, aggregatory behaviour at these core areas remains a challenge. Here we utilise movement network analyses to demonstrate that grey reef sharks tagged around an isolated Central Pacific Ocean atoll, can be delineated into distinct groups based on the similarity of their movement patterns. We then adopt a proximity-based inference method to construct social networks of sharks, testing for weighted assortativity and long-term stability of sociality at the group and dyadic level. Finally, we use an individual based modelling (IBM) approach to explore the Information Centre Hypothesis as a possible mechanistic driver of sociality in these fission-fusion reef predators.
Results/Conclusions
Movement networks, generated from individuals moving between receiver locations, suggested five distinct spatial ‘communities’ of sharks within this small, isolated atoll. Dynamic social networks then revealed non-random, year-on-year social structure assorted by community membership, with long-term social stability between specific dyads extending over the four years of the study. Telemetry data and video observation from animal-borne camera tags confirmed groups of sharks formed and increased in size throughout the day, before sharks dispersed to feed off-shore in deep water at night. We cite local enhancement through leader-follower behaviour (i.e. using social information while foraging) as a likely explanation for the regularity and stability of social groups at this location; a strategy that that outperforms non-social and non-central place foraging agents in our individual based models. In lieu of direct measurement of social interactions in wild fishes - something that we are still some way off achieving at scale - we demonstrate that proximity-based social network inference, in combination with spatial network approaches, can still generate important insight into the ecological and evolutionary drivers of population structuring in cryptic species.