Experts say three key factors create the perfect storm for 'dinner bell' shark attacks on humans
A shark attack survivor has helped uncover three specific conditions that combine to trigger aggressive bull shark attacks on humans, a phenomenon researchers are calling the 'dinner bell'.
Paul de Gelder, an Australian military veteran, was attacked in 2009 while taking part in a counter-terrorism exercise in Sydney Harbor.
He lost his hand and leg in the near-fatal incident and has since become a conservationist and a regular face during Discovery's annual Shark Week.
His latest special, 'Bull Shark Dinner Bell', which aired on Monday, saw him travel to Florida to work alongside apex predator specialist Rosie Moore to figure out exactly what causes bull sharks to swarm and bite.
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According to Moore, the answer rarely comes down to a single trigger. "With bull sharks, there's usually a number of factors that lead up to that bite," she said on the Discovery Channel.
Their investigation eventually identified three key elements working together to create the conditions for an attack: rainfall, the presence of oily or fatty fish, and human activity that generates vibrations in the water.
Why are bull sharks responsible for so many attacks?
While most of the roughly 500 known shark species tend to avoid humans, bull sharks are considered 'ambush predators' that thrive in temperate waters and are the leading suspect behind most shark attacks worldwide.
According to the Global Shark Attack File, a public database tracking incidents back to the 1940s, ten biting incidents had already been attributed to bull sharks in 2026 alone.
De Gelder said the numbers back up the species' fearsome reputation.
"If you go through the Global Shark Attack Files since 2025, bull sharks are responsible for twice as many attacks as tiger sharks, and even 25 percent more than great whites," he said.
He added, however, that the sharks are not simply mindless killers, noting: "If they chose to, they could kill you, but they more often than not don't."
To understand what changes that calculation, de Gelder and Moore traveled to Jupiter Ledge near Palm Beach, a known bull shark hotspot, where the animals gathered around their boat almost immediately.
What triggers a bull shark attack on humans?
Rainfall
The first factor the pair examined was rainfall. Researchers believe storms can agitate bull sharks by creating vibrations across the ocean's surface. To test this, Moore dove into the Atlantic near a group of five to six bull sharks while De Gelder activated a water pump simulating rain. The sharks reacted almost instantly, moving faster and gathering in greater numbers as the simulated rainfall continued.
"I think the bull sharks might associate the raindrops on the surface with other fish. They detect the tiniest vibrations. They can literally see with their eyes closed," Moore explained, adding that these heightened senses allow the sharks to hunt effectively even in total darkness.
"The rain has definitely triggered a more aggressive response. They're faster, bolder, more willing to bite," she said. "Rain is definitely a factor we need to ring this dinner bell."
Food
The second factor centered on food source. The team lowered three separate crates of meat into the water, including one filled with pork intended to simulate human flesh.
The bull sharks ignored it entirely. Instead, they aggressively bumped and head-butted a crate containing oily, fatty fish, a behavior researchers say typically precedes an attack.
Moore said understanding seasonal fish migration patterns that overlap with bull shark territory could help determine when it is safest to avoid the water.
Human activity
The third factor was human activity itself, particularly actions like spearfishing that generate vibrations in the water.
During one test, de Gelder speared a fish from the boat while Moore, in scuba gear, watched a bull shark charge in to attack the struggling catch. Within moments, nearly a dozen more bull sharks moved into the area. Moore said this reaction reflects a kind of competitive instinct among the animals.
"Bull sharks are interesting because it's almost like they get a sense of FOMO," she said. "It triggers an uptick in aggression from the other sharks around it because if you're a shark in the ocean, there's not necessarily a grocery store for you to go to; you have to compete for that available prey."
In a final test combining all three conditions, de Gelder jumped directly into the water during simulated rainfall near bait.
"As soon as I hit the water, they came to investigate," he said.
The sharks were seen lowering both pectoral fins, a recognized sign of agitation, and one bit down on de Gelder's scuba fin.
"As soon as I hit the surface with the rain, the bait in the water, it was like the perfect storm," he said.
Moore said she had never witnessed anything quite like it.
"I have never seen bull sharks behave like this," she said. "You can literally see how if all these elements lined up and you had somebody in the water that was not prepared, they would not stand a chance. The dinner bell has rung."
Despite the intensity of the experiment, Moore said peaceful coexistence with bull sharks is possible provided people remain aware of the conditions that raise risk.
De Gelder echoed that message directly to viewers.
"Truly assess the conditions before you enter the water, it could save your life," he warned.
Thomas Bamford