Offspring of one of world's biggest bull sharks found—why that’s so surprising

The 450kg mother, named Big Bull, could be one of the last of her kind on the U.S. Atlantic coast.

Published 14 Jul 2021, 10:17 BST
01-bull-shark
Bull sharks, which tend to hang out near coastal areas, can also tolerate fresh water.
Photograph by Andy Mann, Nat Geo Image Collection

Marine ecologist Neil Hammerschlag has caught and released a lot of sharks in his day. But there’s no question the most memorable was the 450kg female bull shark named Big Bull, one of the largest specimens on record.

“It literally took my breath away,” says Hammerschlag, director of the Shark Research and Conservation Program at the University of Miami. “It wasn’t so much the length, but the girth. It had this neck that was just bulging, like a wrestler.” Most bull sharks are around seven feet long—Big Bull was 10 feet.  (What can studying the world’s biggest sharks tell us?)

When Hammerschlag and his team took non-invasive blood and tissue samples of the bull shark, caught in 2012 off Marathon, a city in the Florida Keys, her blood chemistry indicated she had recently given birth. Despite years of searching, no one has reported another sighting of Big Bull.

But in an incredible turn of events, three bull sharks caught and sampled along the Florida coast in recent years were found to be offspring of the legendary mama shark, Hammerschlag says in the show “Biggest Bull Shark,” premiering July 20 on National Geographic’s SharkFest.

You’re going to need a bigger screen. “World’s Biggest Bull Shark?” premieres July 20 at 8pm on National Geographic as part of its ninth annual SharkFest.

Finding sons and daughters of a known shark is relatively rare. “I mean, you’re literally looking for—not a needle in a haystack—but a needle in the world’s oceans,” he says.

As amazing as the Big Bull story is, to the people who know these animals best, this saga also carries with it a worrying subtext: There are simply fewer mature bull sharks left to reproduce. Globally, the International Union for Conservation of Nature lists bull sharks as near threatened by extinction, and there’s evidence the species is declining in the Atlantic.

“You absolutely should not be able to throw out a line and catch sibling bull sharks. It shouldn’t be happening, but it is,” says Toby Daly-Engel, a molecular ecologist at the Florida Institute of Technology who performed the DNA work. “And so what we’re finding out, in general, is that there are really just not that many sharks left.”

Mother of giants 

As part of his job, Hammerschlag regularly catches bull sharks for research, hauling the predators onto a special platform on the back of a boat to take measurements and blood samples—and, in some cases, fit them with a satellite tag. During examination, the animals are given a constant supply of oxygenated water so they can breathe. 

This data is important, as it helps scientists learn about shark behaviour, track the animals’ regional movements, and assist with conservation planning. (These are the most fascinating shark discoveries of the past decade.)

In the summer of 2018, Hammerschlag’s colleagues caught and sampled two teenage sharks, a male and a female, near Palm Beach, nearly 200 miles north of Marathon. The team ran an analysis to see if the sharks’ DNA matched any in their database. Surprisingly, both young sharks were offspring of Big Bull.

In 2017, even farther north up the coast, a male one-year-old bull shark pup sampled from the Indian River Lagoon also had half of Big Bull’s genes, indicating she was his mother. 

All of this suggests that the massive female, who would likely be around 40 years old today, is a very successful mum. Bull sharks don’t reach maturity until 15 to 20 years of age and can live up to 25 years or more. A female generally gives birth to one to 13 pups every year, after which the pups are on their own.

In addition to Big Bull’s progeny, Daly-Engel found around half a dozen other bull sharks that were related to each other in some way. And that’s after sampling a total of only 50 sharks, she says. (Read how sharks are surprisingly social, dispelling the loner myth.)

“The populations seem to be dominated by some really big females. Because once you make it that big, you become kind of like a super reproducer, and you don’t have a ton of predators,” says Daly-Engel, who is also director of the Florida Tech Shark Conservation Lab. Since bull shark fishing is legal in Florida, she suspects most sharks don’t survive to Big Bull’s age.

These prolific mums are “interesting from a scientific perspective, but it doesn't say very many good things about the state of sharks in the ocean,” she adds.

‘Still some monster sharks out there’

As apex predators that can tolerate both freshwater and saltwater, bull sharks tend to spend most of their time near shorelines and estuaries. This also makes them more likely to come into conflict with humans. They’re one of the three shark species most likely to bite people, along with great white sharks and tiger sharks.

This can often lead to retaliation, notes Sébastien Jaquemet, a marine ecologist at the University of Réunion Island who has studied bull shark populations. For instance, when a spate of shark bites occurred on Réunion Island in 2011, the French territory in the Indian Ocean responded with a government program to increase harvests of tiger and bull sharks, Jaquemet says. The island also installed nets to exclude sharks from several beaches, banned swimming on others, and created a shark patrol system.

In 2019, Jaquemet and colleagues found a global decrease in bull shark population, in part due to such measures, he says. Living near the shore also puts bull sharks at risk of getting caught in fishing hooks and tangled in nets meant to protect swimmers.

Bull sharks may also be more vulnerable to human-caused pollution, says Hammerschlag. Since the animals hang out at the mouths of rivers, they are exposed to everything from mercury and other heavy metals to pharmaceuticals and the toxic algal blooms triggered by fertilisers. (Read how dolphins off Florida’s coast are getting sick from pollution.)

At the same time, the fact that a shark like Big Bull can still exist means there’s also cause for hope for the species.

“You know, Big Bull could potentially still be alive and reproducing,” says Hammerschlag. “So to me, it’s exciting to know there are still some monster sharks out there.”

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