A new study has found that tiger sharks (Galeocerdo cuvier), which ordinarily have a reputation as solitary creatures, have active social lives, based on research conducted at shark-feeding tourist dive sites in the Bahamas.
Published in Frontiers in Marine Science, the three-year study, led by scientists from the Institute of Zoology at the Zoological Society London and the University of Miami Rosenstiel School of Marine and Atmospheric Science, was carried out around Tiger Beach, situated on the northwestern edge of Little Bahama Bank in the Bahamas. The area is popular with divers for its regular encounters with several species of shark, including tiger sharks, often as part of baited shark dives, during which specially-trained guides carry boxes of bait used to attract the sharks and keep them at the site during the dive.
Baited shark dives are a controversial undertaking, with some conservationists critical of the practice claiming that it negatively impacts the sharks’ natural behaviour patterns. The new research, however, suggests that tiger shark behaviours in the studied area were not unduly influenced by the feeding, although it has been known to affect other species – such as great white sharks, which are known to deliberately change their ranges when chumming boats are present.
For reasons that remain unclear, large numbers of primarily female tiger sharks gather naturally in the waters off the northwest of Little Bahama Bank, particularly during the cooler winter months between November and April. It has been hypothesised that the area may be a refuge for female sharks to escape the aggressive attention of their male counterparts; that the area is a rich feeding ground for pregnant females, or that the warmer shallow water may aid in the gestation of their young.
The researchers used acoustic monitoring techniques to track the movement of the sharks during these aggregations, and using a social network analysis tool, found that the sharks exhibited distinct preferences for certain individuals over others – at least, while they weren’t being fed. The data suggests that, while the sharks form social bonds when deliberate human ‘provisioning’ is not available, this social structure may become a bit more ‘every shark for themselves’ when presented with food.
According to the study, only one in three of the possible groupings were ‘non-random’ when human provisioning was available, but four out of every nine groupings were found to be deliberate when there were no, or very few, shark-feeding dives taking place. Although the available data implies that tiger sharks give up their social bonds at feeding time, it is too limited to definitively say one way or another, but it does suggest that baited dives have little significant influence over tiger shark behaviour.
Interestingly, the research also found that the social bonds formed by tiger sharks were short-lived, with the sharks forming new associations each year, rather than hanging out with the same individuals should they both return to the same location. Although the sample is too small to determine if longer-term associations might be possible, it does confirm, as noted by the report’s authors, that ‘tiger sharks, often considered a solitary nomadic species, are highly flexible in their capacity to associate with one another.’
‘The boundary between wildlife and people is becoming increasingly thin,’ said ZSL Honorary Research Associate and the study’s lead author, David Jacoby. ‘So as well as observing a new social behaviour for the first time in what was once thought of as a solitary shark, we also measured the impacts of human activity on these predators’ interactions. Luckily, they seem to show some resilience to the bait feeding.’
The study recommends that impacts of tourist shark-feeding dives on shark biology and behaviour should continue to be assessed on a case by case basis. In the case of the tiger sharks, it seems that ‘provisioning of food for tourism can enhance gregarious behaviour, as well as subtly influence the level of social behavior within the population,’ but the overall effect on the sharks is – at least at current levels – not a threat to tiger shark behaviour in that region of the Bahamas.
‘By continuing to limit provisioning activities to certain times of year,’ concludes the report, ‘our study suggests that tourism is unlikely to be significantly disruptive to the structuring of the tiger shark population at Tiger Beach. The extent to which that may hold true elsewhere remains unclear.’
The full paper, ‘Social Network Analysis Reveals the Subtle Impacts of Tourist Provisioning on the Social Behavior of a Generalist Marine Apex Predator’ by David M P Jacoby et al is available in Frontiers in Marine Science, doi.org/10.3389/fmars.2021.665726