How solitary are white sharks: social interactions or just spatial proximity?

Abstract: White sharks (Carcharodon carcharias) are circumglobally distributed large apex predators. While ecologically important, there is very limited study of their social behaviour. Although evident in other large, apex marine predators (e.g. toothed whales) and smaller elasmobranchs (e.g. blacktip reef sharks), the ability of any large pelagic elasmobranch to demonstrate social preferences, tolerance or grouping behaviour is largely unknown. Here, we test whether white sharks in a near-coastal environment form non-random associations with other conspecifics or simply share the same space at the same time. We photo-identified 323 individuals—74 % juvenile females (175–300 cm)—during chumming events at six different sites in Mossel Bay, South Africa, over a 6-year period (2008–2013), and tested for grouping behaviour. We found evidence for random associations among individuals, though spatio-temporal co-occurrence of white sharks in close proximity was weakly structured according to sex and, potentially, body size. Such biological traits may play a minor part in structuring co-occurrence of individuals at fine spatio-temporal scales, which could reflect ontogenetic preferences in diet and site fidelity, or differing tolerance levels for conspecifics of different sexes and sizes. Our study strengthens the evidence that large pelagic shark species are generally solitary and display limited social behaviour. Significance statement: Large pelagic shark species are important top predators, but we know little about their social behaviour. We tested the ability of white sharks (C. carcharias) to form groups and display social preferences for other individuals when they congregate at scavenging events in a coastal environment, where social interactions may be more likely. We found that white sharks co-occur at random, displaying no preferred or avoided associations for other individuals. Nevertheless, there was a minor influence of biological traits, with individuals aggregating according to gender and, possibly, body size. While we hypothesise these effects could represent preferences in diet and site fidelity, or more tolerance for similar-sized individuals of the same sex, our study strengthens the evidence that white sharks are mostly solitary foragers.