Somewhere off Fiji’s southern coast, a bull shark glides into Shark Reef Marine Reserve and settles alongside the same companion it swam with last season. Not a random neighbor. Not the nearest body in the water. The same individual, identified by the unique pattern of scars and markings on its dorsal fin, returning to the same partner across years.
That scene, repeated hundreds of times across six years of underwater observation, is the basis of a peer-reviewed study published in Animal Behaviour in early 2025. Led by Natasha D. Marosi of the University of Exeter, the research team tracked 184 individually identified bull sharks over 473 dives and 8,192 minutes of footage. Their conclusion: certain pairs of sharks showed up together far too consistently to be explained by chance. Bull sharks, long classified as solitary predators, appear to form stable social preferences that look a lot like friendships.
What the six-year study actually found
The researchers used social network analysis, a statistical method borrowed from studies of primates and birds, to measure how often specific pairs of sharks appeared together at the reef. After controlling for group size and seasonal fluctuations, they found that some associations were significantly stronger than random mixing would predict. In practical terms, certain sharks repeatedly chose to be near the same companions even when dozens of other individuals were available.
This was not a one-off observation. An earlier study at the same site, published in Frontiers in Marine Science in 2021, had already distinguished between sharks that were genuine “companions” and those that were merely “casual acquaintances” showing up at the same food source. That work laid the methodological groundwork for the newer paper, which expanded the dataset and strengthened the statistical case.
The site itself has been monitored for over two decades. Underwater surveys conducted between 2003 and 2009 recorded daily bull shark counts ranging from zero to 40 at the reserve. That variability is important: it means the population at the reef is fluid enough that any given shark has a genuine choice about when to show up and who to associate with. Acoustic tracking of tagged bull sharks between 2004 and 2011 confirmed that many individuals showed strong site fidelity, returning to the reserve across multiple years. Without that proof of long-term residency, the social findings would be far weaker, because researchers needed to verify that the same animals were actually available to form bonds over time.
The feeding tourism question
One of the most interesting tensions in this research is the role of food. Shark Reef Marine Reserve operates a provisioning site where bait is placed in the water to attract sharks for dive tourism. That raises an obvious question: are these “friendships” real social bonds, or are the sharks just crowding around the same buffet?
The 2021 Frontiers in Marine Science study addressed this directly, using network analysis to separate resource-driven aggregation from genuine social preferences. The researchers found that even after accounting for the pull of food, certain pairs of sharks associated more often than expected. The newer Animal Behaviour paper built on that framework with a larger dataset and longer observation window.
Still, a definitive test is missing. The published analyses do not isolate periods when no food was in the water as a separate condition. One multi-species assessment at the reserve did include a comparison period without provisioning during March 2008, but the social network analyses in the 2025 paper do not break out non-provisioning periods on their own. Until researchers can show that these partnerships hold up when the bait stops, the possibility that provisioning amplifies or even creates some of these associations cannot be fully ruled out.
What the data cannot tell us yet
Several gaps remain. The study does not break down the sex or maturity stage of paired individuals. Whether males bond with males, females with females, or mixed-sex pairs form preferentially is unknown. So is whether juveniles seek out adults or stick with age-matched companions. These details matter because social preferences could serve very different purposes depending on whether they reduce feeding competition, lower predation risk, or facilitate mating.
The raw association data and photographic archives used for individual identification have not been deposited in public repositories. That limits the ability of independent researchers to re-analyze the pairings, test alternative explanations (such as subtle habitat preferences or time-of-day effects), or apply machine-learning identification tools to the same footage. This is not unusual in marine biology, but it does mean the findings rest on the original team’s identification work.
There is also the question of generalizability. Shark Reef Marine Reserve is unusual: it combines long-running tourism, formal legal protection, and intensive scientific monitoring. Whether bull sharks at other reefs, particularly those without regular feeding or reserve status, form similarly stable social preferences is unknown. Research on other species offers some context. Studies of lemon sharks in Bimini, Bahamas, have documented non-random social associations among juveniles, and grey reef sharks in the Pacific have shown structured social networks. But comparable long-term social data for bull sharks outside Fiji does not yet exist.
Why social networks could reshape shark conservation
If bull sharks maintain preferred partners over years, then the consequences of disrupting local populations go beyond simple headcounts. Removing individuals through fishing pressure or poorly managed tourism could fracture social networks whose functions researchers are only beginning to understand. A shark that loses a long-term companion may not simply slot in a replacement.
Conversely, well-designed marine reserves and carefully regulated feeding operations might preserve not only shark numbers but also the social structure that organizes their lives at sites like Shark Reef. As of June 2026, no follow-up study has been published extending these findings to other locations, but the Fiji data has already shifted how marine biologists think about large predator sociality. The old model of the bull shark as a lone, indiscriminate hunter is giving way to something more nuanced: an animal capable of recognition, preference, and what increasingly looks like choice.
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*This article was researched with the help of AI, with human editors creating the final content.
