Shark Killed by Swordfish Prompts Fears of Overfishing Impact on Ocean Predators

The grizzly tale of shark versus swordfish has caused intrigue for marine biologists for a number of decades but a recent video has led to new fears over the impact of overfishing on ocean biodiversity.

A dead thresher shark measuring almost 15 feet was discovered washed up ashore along the coast in Libya in April with an adult swordfish "blade" stuck in its back after what scientists called a "deadly interaction".

Swordfish are known to attack sharks, with the first documented incident dating back to 1960, but since 2016 - when a blue shark was found washed up and dying on a beach in Valencia, Spain, with a swordfish sword embedded in its head - there have been at least six more cases of death by swordfish recorded in sharks found along the Mediterranean coast, researchers told the New York Times.

In the latest example, the adult 15-foot thresher shark washed up in the Gulf of Sidra with a swordfish blade broken off near its heart. Patrick Jambura, a graduate student at the University of Vienna, was among a team of scientists who looked into the latest encounter after watching a video filmed and posted online by local citizen biologists and picked up by Marine Biology Libya.

In the video, a man approaches the shark on the beach and pulls a sword from its back. Jambura's study, published in the journal Ichthyological Research, found that the sword belonged to the swordfish Xiphias gladius, "which is a highly mobile, predatory fish known to attack sharks, whales, humans and even boats".

The study said the length of the blade suggested the swordfish measured about six-feet-long and was therefore an adult - not the typical prey of the thresher shark. "The location of the injury, timing of wound infliction, and lack of other apparent injuries lead us to the conclusion that the impalement was fatal and the ultimate cause of death for the thresher shark," the team found.

The majority of shark species falling prey to swordfish stabbings in the Mediterranean have been blue or mako sharks, the study found. Both of those species prey on young swordfish, suggesting the young animals lashed out while under attack.

Jambura and his team said the age of the swordfish in Libya "makes an attack as a defensive response unlikely". Instead, the team speculates the attack may have been a deliberate attempt by the swordfish to get the shark away from its own prey.

"The potential for the impalement to be a result of a directed attack against the shark to drive a competitor away from this resource cannot be excluded," the study found. "We hypothesize that competition could be a driving force for swordfish attacks on sharks or other possible competitors."

The team fears overfishing may force the two predators to compete for more and more limited resources in the Mediterranean going forward. The researchers are calling for future studies on stranded sharks to further establish their encounters with swordfish, and just how deadly the species could be.

In 2019, a United Nations (UN) report found that the combination of overfishing and climate change has seen the biodiversity of the world's oceans declining more rapidly than at any other time in human history. About two-thirds of the marine environment has been significantly altered by human actions and climate change has the potential to make the situation much worse, the report said.

Sam Stone, head of fisheries and aquaculture at the Marine Conservation Society, tells Newsweek there are many cases around the world where overfishing has resulted in vulnerable shark species becoming functionally extinct. He says it's not too late to turn things around.

Mako shark on the fishing hook
Sharks are fighting over fewer resources, like this mako shark in Florida, researchers argue Ronald Modra/Getty

"The UN reported [in 2019] that commercial fishing has been the leading cause of loss of marine biodiversity globally over the last 50 years," Stone says.

"Sadly, there are many cases around the world where overfishing has resulted in vulnerable fish, shark, and ray species becoming functionally extinct. Whilst this sounds grim, there is growing recognition that fisheries can change and be managed in ways that are better for our seas, coastal communities, and consumers, and that actually allow biodiversity to recover.

"Fishing limits and proper protections for important habitats are an obvious start, but we should now be making much better use of technology to fish smarter, not harder, and to better monitor catches. There's huge potential to transform our fisheries, putting sustainability, science, and technology at the heart of solutions, but we must move away from traditional approaches and short-termism."

Industrial fishing of big fish - including sharks, swordfish, tuna, and mackerel - could also have an impact on global warming, a recent study found. These fish are a carbon sink, consuming carbon dioxide (CO2) from fishing boats without releasing it back into the atmosphere. When they die, they sink to the bottom of the ocean, taking the greenhouse gas with them.

In October, an international team of scientists found that the amount of CO2 these species reduce has been overlooked and overfishing could be "annihilating" the natural phenomenon – dubbed a "blue carbon pump". The researchers called for more reasoned and thoughtful fishing practices in light of their findings.

Lead author of the study, published in Science Advances, Gael Mariani said: "New protection and management measures must be put in place so that more large fish can remain a carbon sink and no longer become an additional CO2 source."