Weipa is young a coastal mining town in Queensland, located in the north-east of Australia, is particularly popular with sport fishermen for its annual tournament, the Weipa Fishing Classic. But in recent years, fishermen have reported an increasing number of cases of bull sharks running underwater, waiting for a fish to be pulled in and cutting it open. Some fishermen estimate that they lose up to 70 percent of their catch to these fish, which appear to target fishing boats.
(More disturbing posts in the gallery below.)
It’s a strange behavior for bull sharks and it raises an interesting question: is this proof that these sharks – known (unfairly) and thought to be aggressive “mindless killers” – are smarter than previously thought? This is one of the questions that shark biologists Johan Gustafson and Mariel Familiar Lopez set out to answer, and their preliminary work is slated for future research. Bull Shark gangspart of National Geographic’s 2023 SHARKFEST programming. SHARKFEST is four full weeks of “explosive, hair-raising and celebratory shark programs that … showcase the fascinating science, power and beauty of these great animals,” according to the government’s description.
Fishing is known as depredation. Among other things, Australia’s fish population has declined by 30 percent in the past decade, and it appears that sharks are adapting to their environment and teaching this behavior to their fellow fish.
“A lot of different species do this, including dolphins and orcas, top predators, but shark attacks are happening all over Australia right now,” Gustafson told Ars. “In areas where there are a lot of people fishing, this behavior happens more often or more. We call it a habit. They learn the habit, they learn from each other, and they spread it everywhere. [the population].”
bull sharks (Carcharhinus Leucas) can be found all over the world, usually preferring warm, shallow waters, along the shores of lakes and rivers. They are not strictly aquatic species, but females often give birth to their young on land because such spots provide the most sheltered nursery grounds. (Sharks do not raise their young; young sharks usually enter the water as adults when they are eight years old.) Bull sharks usually grow to an average of seven meters (for males) and eight meters (for females), and their powerful bite can produce up to 1330 pounds (5914 newtons) of force.
Bull sharks are considered opportunistic, meaning they eat slowly and digest for long periods of time during periods of scarcity. Their diet includes bony fish and small sharks (including other sharks), as well as turtles, birds, dolphins and crustaceans. They are also very territorial and solitary, preferring to hunt alone or sometimes in pairs.
Their reputation for aggression has been fueled in part by media reports of shark attacks, including 1916 Shark attack at the Jersey Shore that he inspired Jaws– all of them a book and Peter Benchley and 1975 blockbuster film (although they all had a great white shark). Bull sharks are the cause of many shark attacks near the beach, and they bite very hard. But the truth is many. “I always tell people that every animal, every human or every dog, we all have different personalities,” Lopez told Ars. “You can find a bull shark that’s aggressive, but you can find one that’s not. Their main goal is to eat food. But they’re not like, ‘Oh, I’m going to be mean to everybody. That’s one thing I see.’
It’s not clear in the various stories if it was actually bull sharks that steal the fish, because it all happens underwater. So Gustafson and Lopez’s first order of business was to confirm the anecdotal reports and try to catch a bull shark. He used a fishing line camera to capture a low-resolution image of a bull shark stealing a catch in just 20 seconds, but needed to go underwater to capture more images with a 360-degree drop camera. The shark cage was suitable, but most metal cages are quite noisy. It’s fine with local sharks that are used to cages, according to Gustafson, but the Weiba bull shark is aloof and can be swayed by noise.
So Gustafson and Lopez turned to underwater videographer Colin Thrupp, who created a soundproof shark cage out of polyethylene pipe with welded joints using electro-fusion welding. Plastic absorbs sound better than a metal cage, and the dark color also means less light. The barn served its purpose; The bullfish were initially cautious but the camera caught six or seven of them swimming close together as a pack – unusual behavior for a solitary and mobile species. “Part of our theory is that these are bull sharks that don’t migrate very much, because they have warm water year-round, it’s a tropical environment,” Lopez said. “This is probably one of the reasons [providing] opportunity for socialization. They may be compromising each other because they are getting easy food. “
The footage also showed the bull shark approaching the fish slowly at first, waiting for it to tire and struggle, then biting the fish’s tail end and drive, before turning back to take down the rest. This is a quiet, intelligent way of hunting, according to Gustafson and Lopez, as opposed to the mindless aggression often associated with bull sharks. In fact, it is very similar to how killer whales—known for their intelligence—hunt, using a precise surgical technique to save energy. “Being a carnivore means you put a lot of energy into all these foods,” Lopez said. “If that doesn’t come with a reward, you’ll have less power in the chase again. So they must be very wise [to determine] ‘Where do I put my energy in all these things?’ Sometimes, when they’re not sure, they bite this.”
The plastic cage did not work well, however, and began to shake and waver as the swell grew and the underwater turbulence increased. The top link came out again, making it a long wait until the cage was put back on top. “We were in the barn and it was sad and the water was big,” Gustafson said. “Then we saw the rope flying behind us and thought, ‘Oh, that’s not good,’ because that’s what’s holding the cage. [mesh] together. Then someone went. Then the cage began to deform. It was like living in a [trash] computer.”
Gustafson and Lopez also managed to tag several sharks with acoustic transmitters to track their movements. Having spotted a young shark among the adults, he also found a nursery that was supposed to be a baby shark in a nearby river, taking biopsies from the baby sharks for DNA analysis. After Thrupp prepared and strengthened the plastic cage for the shark, he sent it a second time to take biopsies from two other sharks for comparison, using harpoon-like tools.
The results showed that the pups they took up the river were related to half of the female bull shark they found in the lake, while the third shark was related to both. Therefore, all three sharks must have a common ancestor between them. This is more evidence that the population in Weiba is not cyclical, as there are many breeding opportunities, especially since sharks have multiple fathers, according to Gustafson.
The next step includes collecting more DNA samples from Weiba bull sharks to further genetic analysis, as well as tagging and tracking more bull sharks to determine their movements to determine how the end of the gulf ecosystem (Weiba) connects the west and east of the continent. “Does he go west to Perth or to Sydney?” said Gustafson. “Turtles like to return to the beach where they were born. he was born in, or in the same area where he was born.”
The data should also give them a good idea of the size of the male sharks in Weiba, since the popular opinion among fishermen is that there must be hundreds or thousands of them. And as fishermen continue to lose their catch, there is a greater risk of the shark herd becoming more aggressive, leading to less support for their protection and inviting more deaths. (This species is listed as endangered at IUCN Red List.)
“The more time you spend in the water, the more likely you are to encounter a shark,” Lopez said. “But that’s because you’re spending a lot of time in the water fishing. That doesn’t mean there’s a lot of sharks lurking in the water and they’re aggressive. It’s important to do this writing because you’re not going to change people’s minds. When you say sharks aren’t bad, you have to put a little bit of science in there, explain.. For projects like this we go there and talk to fishermen. some fishermen help us take samples.”
Bull Shark gangs now streaming on Disney+ and Hulu, premiering on NatGeo WILD on July 25, 2023.