Companions and Casual Acquaintances: The Nature of Associations Among Bull Sharks at a Shark Feeding Site in Fiji

Thibaut Bouveroux,Nicolas Loiseau,Juerg M Brunnschweiler,Natasha D Marosi,Adam Barnett

doi:10.3389/fmars.2021.678074

Abstract

Provisioning activities in wildlife tourism often lead to short-term animal aggregations during the feeding events. However, the presence of groups does not necessarily mean that individuals interact among each other and form social networks. At the Shark Reef Marine Reserve in Fiji, several dozen bull sharks (Carcharhinus leucas) regularly visit a site, where direct feeding is conducted during tourism driven shark dives. On 3,063 shark feeding dives between 2003 and 2016, we visually confirmed the presence of 91 individual bull sharks based on external and long-lasting identification markings. We measured the intensity of associations between pairs of individuals by calculating the Simple Ratio Index (SRI) and calculated Generalized Affiliation Indices (GAIs) to distinguish true associations between dyads from structural predictor factors. Although the resulting mean SRIs were low, ranging from 0.01 to 0.12 (SRImean = 0.06; mean SRImax = 0.21), preferred long-term companionships were observed between individuals. Avoidances were also observed within pairs of individuals during the second half of the study. The best fitting model describing the temporal association patterns of bull sharks revealed a social structure which is characterized by preferred companionships and casual acquaintances. Our results suggest that the aggregation resulting from direct feeding has served to facilitate the development of social associations.

Highlights

The formation of aggregations and groups can be found occurring in taxa throughout the animal kingdom with considerable intra- and interspecific variation (Elgar, 1989; Whitehead, 1997)
Observation periods were defined as the cumulative number of days between the date an individual bull shark was first observed at the Shark Reef Marine Reserve (SRMR) and recorded in the database, and the last day the individual was observed visiting the site
At the Shark Reef Marine Reserve, the ongoing wildlife tourism activity of shark diving has resulted in the formation of temporary aggregations of bull sharks

Summary

Introduction

The formation of aggregations and groups can be found occurring in taxa throughout the animal kingdom with considerable intra- and interspecific variation (Elgar, 1989; Whitehead, 1997). The first type is driven by non-mutualistic forces, whereas the latter by forces which are typically mutualistic, in that some benefits are derived from group membership (Whitehead, 2008a). Aggregations can form for a multitude of underlying non-social. Animal groups, often termed “social groups” arise by way of underlying social drivers, with individuals actively preferring to associate with each other, displaying social preferences. Group members are more highly connected with other members in the group than with other individuals within the broader network (Croft et al, 2008). Social groups have been empirically investigated in a wide range of taxonomic groups including insects, fish, birds, and mammals (Chepko-Sade et al, 1989; Elena et al, 1999; Fewell, 2003; Lusseau and Newman, 2004; Cross et al, 2005; Wittemyer et al, 2005; Croft et al, 2006; Naug, 2008; McDonald, 2009; Vital and Martins, 2011)

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