Fiji's friendly bull sharks

Fiji's friendly bull sharks make science headlines

An intensive study of Fiji's bull sharks threatens to modify the reputation of this marine species as a solitary apex predator. I hasten to emphasise that bull sharks are definitely not friendly to humans, but they may form social bonds with each other, choosing companions, swimming together, and even following one another in coordinated ways. But first, if you know nothing about bull sharks, you can catch up on the basics below.

Bull sharks at Fiji's Shark Reef Marine Reserve. Credit: Natasha D. Morosi

What you need to know about bull sharks

Bull sharks are:

• large, robust, and powerful like their namesake. Adult females range from 2.5 to 3 meters in length and weigh 130 kg, while males are slightly smaller.

• found globally in warm, shallow marine waters, brackish estuaries, and freshwater rivers.

• born live, typically in brackish water. The young are 81 cm long at birth and are completely independent.

• aggressive predators, employing a "butt and bite" technique, feeding mostly on bony fish, stingrays and small sharks, including other bull sharks.

• the strongest biters among all sharks, with a bite force reaching up to 5,914 newtons.

• the most likely sharks to attack humans, sharing this trait with tiger sharks and great whites.

Fiji bull shark behaviour project

Researchers from the University of Exeter, University of Lancaster, Fiji Shark Lab, and Beqa Adventure Divers studied 184 tracked individual sharks over six years at the Shark Reef Marine Reserve off Viti Levu. Fiji. (Watch a brief video of the site here.) Their findings were recently published in the journal Animal Behaviour.

Beqa Adventure Divers (BAD) operates one of the longest running shark ecotourism dive sites in the world. My own aquatic adventures in Beqa Lagoon many years ago were limited to snorkelling off Beqa Island, looking down on the occasional small white-tip or black-tip reef shark idly patrolling the fabulous drop-off wall. Although even the thought of interacting with bull sharks fills me with horror, thousands of BAD's customers feel quite the opposite! Fees from the bull shark dives not only compensate local villagers for not fishing in the reserve, but also fund research at the Fiji Shark Lab, which collaborates with Fiji's Ministry of Fisheries.

Research findings about Fiji bull sharks

Scientists reviewed six years of data (made up of 8,192 minutes of underwater observations) to spot patterns within the social interactions among 184 individual bulls sharks at the Shark Reef Marine Reserve in Fiji.

To determine whether the sharks had preferences in their choice of 'friends', the scientists looked to see if particular sharks were more likely to associate with each other – defined in the study as being within one body length of another subject.

“We quantified sociability using the following behaviours: 'join', 'lead', 'follow', 'parallel swim' and 'turn back',” says the study’s lead author Natasha D. Marosi, a researcher at Exeter University.  “As humans we cultivate a range of social relationships – from casual acquaintances to our best friends, but we also actively avoid certain people. These bull sharks are doing similar things.”

Adult bull shark ‘Chunky’ (foreground) parallel swimming with subadult female ‘Lady Lazarus’. Credit: Natasha D. Marosi

Both males and females preferred to associate with females, which grow larger than males (although males had more social connections). “One potential benefit they may gain is, by being more socially integrated, they are buffered from aggressive confrontations with larger individuals,” says Marosi, who also founded Fiji Shark Lab. 

Sharks were most likely to hang out with other individuals around the same size as themselves, and age plays a role, too. Fiji’s juvenile bull sharks usually live in rivers and estuaries while sub-adults find protection by staying close to shore. The sub-adults daring enough to visit the reserve have been seen to form associations with some of the adult bull sharks.

Older sharks didn’t feel the need to be as social. “These older individuals have many years of experience honing their skill sets, hunting and mating,” Morosi says. “Sociality may not be as integral to their survival as it is for an individual in their prime.”

Bull sharks aren’t the only shark species that might build friendships. Knowing more about these interactions can help researchers understand where and how to best protect these vulnerable animals. 

Personally, I've feared sharks from the day when, as a child on a popular Sydney beach, I heard the deafening shark horn blare a warning. My brother dragged me from the water and along with everyone else, we raced up the sand where he explained what the alarming noise was about. Fear and fascination have fuelled my interest, culminating in Death Beyond the Limit: Fiji Islands Mysteries 3, where Detective Inspector Josefa Horseman must investigate a human head found in a tiger shark's gut. Did the shark kill the victim, or was he already dead when those powerful jaws clamped around his neck?

Reference: Natasha D. Marosi, Samuel Ellis, David M.P. Jacoby, Juerg M. Brunnschweiler, Darren P. Croft. Rolling in the deep: drivers of social preferences and social interactions within a bull shark aggregation in Fiji. Animal Behaviour, 2026; 123511 DOI: 10.1016/j.anbehav.2026.123511

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Bernadette

B.M. Allsopp