Researchers filming a new Shark Week special have identified three factors that combine to trigger bull shark attacks on humans: rainfall, the presence of oily fish and human activity in the water.
The findings come from Paul De Gelder, an Australian military veteran and shark attack survivor who lost his right hand and leg in a 2009 attack during a counter terrorism exercise in Sydney Harbor. De Gelder, now a conservationist, worked with apex predator specialist Rosie Moore on the special, called Bull Shark Dinner Bell, which airs Monday at 8pm ET on Discovery.
Bull sharks are considered the top suspect in most of the world's shark attacks. The Global Shark Attack File, a public database tracking cases back to the 1940s, has already logged ten bull shark biting incidents in 2026.
"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," De Gelder said during the special. "If they chose to, they could kill you, but they more often than not don't."
Testing the triggers off Florida
De Gelder traveled to Florida, which he and Moore describe as the bull shark capital of the US, to investigate why attacks have been rising. The pair worked at Jupiter Ledge, a bull shark hotspot near Palm Beach, where sharks gathered around their boat almost immediately.
"With bull sharks, there's usually a number of factors that lead up to that bite," Moore said on the Discovery Channel.
Rain has long been linked to bull shark attacks, and researchers believe storms may agitate the animals through vibrations on the ocean surface. Moore dove into the Atlantic near a group of five to six bull sharks while De Gelder ran a water pump that simulated rainfall. The sharks moved in faster and in greater numbers as the simulated rain fell.
"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 said, adding that the sensors let sharks hunt even in the darkest water.
"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."
Bait tests point to oily fish
The team also lowered crates of different bait into the water to see what drew the sharks in. They tested lean fish meat such as grouper, oily and fatty fish such as bonito, and a crate of pork meant to simulate human flesh and blood.
The bull sharks bumped and headbutted the crates, a behavior tied to pre-attack aggression, but ignored the pork entirely. They targeted the oily, fatty fish instead, a result Moore and De Gelder called a key clue to what sets off the "dinner bell."
"By knowing what seasonal fish migration patterns overlap with the bull shark populations, that can help us advise people on when they should stay out of the water because the sharks might be hunting," Moore said.
Spearfishing and a dive into the water
The researchers also examined how human behavior can set off an attack, since bull sharks, like other shark species, are highly sensitive to vibrations. Spearfishing is one activity frequently tied to bull shark incidents.
While De Gelder caught a fish with a speargun from the boat, Moore watched from the water as a bull shark charged the struggling fish. Nearly a dozen more sharks moved in to bump Moore and her cameraman.
"Bull sharks are interesting because it's almost like they get a sense of FOMO. 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," Moore said.
In a final test, De Gelder jumped into the ocean to create a large splash and vibration. "As soon as I hit the water, they came to investigate," he said. The sharks lowered both pectoral fins, a sign of extreme agitation, and one bit De Gelder's scuba fin.
When researchers combined all three factors, rainfall, oily fish bait and a human diving into the water, the bull sharks moved immediately to attack De Gelder. "As soon as I hit the surface with the rain, the bait in the water, it was like the perfect storm," he said. The sharks grew more agitated, drew more of their kind to the area and began attacking one another.
"I have never seen bull sharks behave like this," Moore 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."
Moore said peaceful coexistence with sharks is possible, but that rainfall, the presence of fatty fish and heavy human activity in the water together create dangerous conditions.
"Truly assess the conditions before you enter the water, it could save your life," De Gelder said.
