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To Track Bull Sharks, First Hit Their Snooze Button

Posted Nov 6, 2023Updated Oct 17, 2026

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For these researchers in Southern Africa, tagging the apex predators starts with putting them in trancelike tonic immobility.

Daly and colleagues catch the famously ferocious fish off the coast of southern Mozambique and fit them with transmitters as part of a long-distance tracking project to reveal more about the animals’ habits. The team uses strong lines and circle hooks—harmless to the sharks—baited with striped bonito fish, thick rubber mats to protect the inflatable boat from sharp teeth, and something called tonic immobility: essentially, a shark’s Achilles’ heel.

“Once you can get a shark upside down, they pretty much just fall asleep,” says Daly. “It’s pretty incredible.”

It’s still unclear why sharks are prone to this state, but it may have evolved around mating behavior, with the male shark grabbing the female shark by its pectoral fin and flipping her over so that she relaxes. However, both male and female sharks experience tonic immobility—it may, says Daly, simply be that sharks are not wired to be on their backs.

“I think it just confuses them,” he says. “But it’s really helpful for us to be able to work on the sharks and tag them.”

Tonic immobility is a “weird and wonderful thing,” says Toby Rogers, a doctoral candidate and research manager with the Cape Town-based conservation NGO, Shark Spotters. He suspects it may have evolved as a way for sharks, especially smaller species, to play possum, feigning death when threatened. “It’s potentially a survival trait,” he says.

Knowing a shark’s immobility trigger is one thing, but for Daly’s team, turning the creature upside down while aboard an inflatable craft out on the ocean is another.

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To Track Bull Sharks, First Hit Their Snooze Button

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