Strange animal behavior: sharks found “stabbed” by swordfish

A corpse that washed ashore last week near Benidorm, eastern Spain, is another example of an increasingly common phenomenon that has scientists stumped.

Last week, a blue shark had to evacuate a beach in Benidorm, eastern Spain. The shark, which was about two meters long and weighed about 60 kilograms, became disoriented and ended up in an area where there were many bathers. Rescuers from the Oceanogràfic of Valencia managed to catch the shark, after tests it was declared healthy and returned to the sea.

On Saturday, August 21, the same specimen was found in the municipality of El Campello, 30 kilometers south, where it was fished dead the next day. According to a necropsy carried out by Jaime Penades, a researcher of the marine zoology department of the University of Valencia, the cause of death appears to be a small wound near the eyelid, which was already visible in the videos taken a few days before by the local police marine service; a wound that is compatible with a swordfish attack. Until recently, this type of aggression was a complete mystery to scientists, but a series of cases has prompted research into the relationship between these two marine animals.

The first scientifically documented case of a deadly swordfish attack on a shark occurred in Valencia in 2016. The individual was found dying on a nearby beach. Subsequent tests revealed an 18-centimeter swordfish blade fragment embedded in its brain. “The sword completely pierced and split the shark’s brain in two,” explains Penades, a co-author of the study.

“Aggressive interactions between young swordfish and blue sharks in the Western Mediterranean are common.”

“At the time, we were amazed. We didn’t expect to find anything like that.” This first reported case was a wake-up call to the scientific community to see if there were any other signs of this type of attack. Since then, similar cases have been reported in Italy and Libya.

By June 2019, Penades’ team had managed to document five more attacks. Since then, new cases have been discovered and will be documented in the near future. All of the swordfish shark kills follow the same pattern.

“The blows are struck in the head, so the swordfish acts with intent,” Penades says. “It’s not a random attack. It’s also very precise because it hits the eyes or brain, which are the vital organs of sharks.”

Because of the limited data available, it’s hard to say whether attacks on other areas of the body are common. Researchers are also unsure whether this is active aggression on the part of swordfish or self-defense, although Penades suspects that because of their smaller size, swordfish are most likely trying to defend themselves.

The main obstacle to finding new examples of such attacks is that dead sharks don’t swim. “It’s very difficult for a shark to be stranded,” Penades says. “It’s easier with dolphins because they swim, but sharks usually sink.” In addition, the tests required to detect swordfish “swordfish” fragments are not routinely conducted.

“The blows are made in the head, so the swordfish acts with intent,” Penades says. “It’s not a random attack. It’s also very precise because it hits the eyes or the brain, which are vital structures.”

Because of the limited data available, it’s hard to say whether attacks on other areas of the body are common. Researchers are also unsure whether this is active aggression on the part of swordfish or self-defense, although Penades suspects that because of their smaller size, swordfish are most likely trying to defend themselves.

The main obstacle to finding new examples of such attacks is that sharks don’t swim. “It’s very difficult for a shark to get aground,” Penades says. “It’s easier with dolphins because they swim, but sharks usually sink.” In addition, the tests required to find swordfish blade fragments are not routinely performed. “Normally in a necropsy, the cerebrospinal fluid is removed, but the skull is not dissected and a full autopsy is not performed,” Penades says.

Fortunately for the researchers, the Valencia Community Beach Network has records of specimens found in the region’s waters, allowing them to analyze their physical condition upon arrival. “We were able to look back and see sharks with similar injuries – ones we didn’t know where they came from,” Penades says. With the help of the Oceanogràfic Foundation, they were able to do some CT scans and X-rays. “We found a shark that had a swordfish fragment inside its skull, but it had already healed; it was an animal that had survived a swordfish attack,” he says.

The reason this type of aggression is being discovered more and more often may be because no one has paid attention to it until now, as Penades notes. However, he also believes that something, such as global warming, could be the cause of the attacks, although he admits that there is currently no data to support this.

“We’re all very quick to mention climate change these days, which is certainly a reality,” he says. “But in this case, I think it probably has to do with the fact that we haven’t taken it into account as a possibility; we haven’t even tested it.”

Science has recorded numerous cases of turtles, whales and small boats impaled by swordfish. The tips of swordfish blades have even been found stuck in the floats used for anti-submarine mines popular during the Second World War. But confrontations between swordfish and sharks only ever appeared in storybooks; until now. “Traditional knowledge tells us that fishermen had already encountered sharks with their heads impaled by a sword,” says Penadés. “Scientists are lagging far behind popular knowledge; sometimes we have to get closer to the fishermen.”

But Àlex Bartolí, a biologist with the marine biodiversity conservation organization SUBMON, disagrees. In his opinion, these attacks are coincidental and occur during hunting. “Where there are schools of fish, all the predators are trying to catch what they can,” he says. “The swordfish go around swinging their sword to hit the small fish which, once stunned, will be eaten. In this maelstrom, the predators pass by at high speed and I think there are collisions.”

According to Bartolí, there is little motive for an attack; sharks and swordfish do not fight over the same habitat, as they are migratory species; they would not attack to defend their young, since their maternal duties end once they lay their eggs; neither does he see the sense of an attack in which one animal sees the other as prey. “The energy required for a shark to attack a large animal means there’s no point in it,” he says. Still, he admits he may be wrong, as the sample of encounters is still small. “I think it is a hypothesis that is still to be proven; from here on, we will have to contrast evidence, but until there are more cases, I find it hard to imagine a fight between a blue shark and a swordfish.” For now, the mystery is yet to be resolved.

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