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

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.

Both environmental conditions and intra- and interspecific interactions influence the movements of a marine predator

“…We found that social factors sometimes had similar effects on movement networks to environmental variables. While the bull shark has been found to form occasional aggregations around fish farms (Loiseau et al, 2016) or at artificial provisioning sites (Bouveroux et al, 2021), it is not recognized as displaying collective behaviours and strong patterns of spatial segregation have been found in our study population (Mourier et al, 2021). However, we found that more movements occurred through bull shark hotspots.…”

On the Movements of the Blacktip Reef Shark (Carcharhinus melanopterus)

“…This approach proved effective in habituating the blacktip individuals to the observer's presence, with the result that they exhibited fairly natural behaviour during encounters without food provisioning. The use of provisioning to facilitate the observation of sharks has since been used by other researchers in Polynesia [19,20] and various other regions [21].…”

Global genetic diversity and historical demography of the Bull Shark

“…However, FIJ was not the only isolated sampling location; Seychelles is ~1000 km apart from Madagascar, yet it did not show traces of genetic isolation. Sampling bias could explain FIJ genetic differentiation, as most samples used here came from intermittently resident females suspected to pup in the area (Bouveroux et al, 2021;Brunnschweiler & Barnett, 2013;Cardeñosa et al, 2017;Glaus et al, 2019). Given the suspected reproductive philopatric behaviour of C. leucas females (Devloo-Delva et al, 2023;Espinoza et al, 2016;, the Fijian genetic distinctiveness could stem from relatedness, as in Lemon sharks (Feldheim et al, 2014).…”