The grizzly story of sharks and swordfish has fascinated marine biologists for several decades, but a recently released video has raised new fears about the impact of overfishing on ocean biodiversity.
A dead thresher shark nearly 15 feet long was discovered washed up on the Libyan coast in April, with an adult swordfish “leaf” 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 washed up and found dying on a beach in Valencia, Spain, with a swordfish sword embedded in its head – there have been at least six more cases of swordfish deaths among sharks 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, breaking off a swordfish leaf near its heart. Patrick Jambura, a PhD student at the University of Vienna, was part of a team of scientists who investigated the recent encounter after watching a video that was filmed and posted online by local community 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 belongs to the swordfish Xiphias gladius, “which is a very agile predator fish known to attack sharks, whales, humans and even boats.
The study stated that the length of the blade indicated that the swordfish was about six feet long and was therefore an adult – not the typical prey of the thresher shark. “The location of the injury, the timing of the wound and the absence of other obvious injuries lead us to conclude that the impaling was fatal and the ultimate cause of death for the thresher shark,” the team found.
The majority of shark species that fell victim to swordfish stab wounds in the Mediterranean were blue sharks or mako sharks, the study found. Both species fall victim to juvenile swordfish, suggesting that the juveniles were flogged during the attack.
Jambura and his team said that the age of swordfish in Libya “makes an attack unlikely as a defensive reaction. Instead, the team speculated that the attack may have been a deliberate attempt by the swordfish to get the shark away from its own prey.
“It cannot be ruled out that the staking was a result of a targeted attack against the shark in order to drive a competitor away from this resource,” the study said. “We put forward the hypothesis that competition could be a driving force for swordfish attacks on sharks or other potential competitors”.
The team fears that overfishing could force the two predatory fish to compete for increasingly limited resources in the Mediterranean in the future. The researchers call for future studies on stranded sharks to further investigate their encounters with swordfish and the possible lethality of the species.
In 2019, a United Nations (UN) report found that the combination of overfishing and climate change has led to a faster decline in marine biodiversity than at any other time in human history. About two-thirds of the marine environment has been significantly altered by human activities, 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, told Washington Newsday that there are many cases worldwide where overfishing has led to the functional extinction of endangered shark species. He says it is not too late to turn things around.
“The UN reported [in 2019]that commercial fishing has been the leading cause of marine biodiversity loss worldwide for the past 50 years,” Stone said.
“Sadly, there are many cases worldwide where overfishing has led to the functional extinction of vulnerable fish, shark and ray species. While this may sound bleak, there is a growing realization that fisheries can change and be managed in ways that are better for our oceans, coastal communities and consumers and that actually allow biodiversity to recover.
“Fishing limits and adequate protection of important habitats are an obvious start, but we should now make much better use of technology to fish smarter, not harder, and to better monitor catches. There is huge potential to transform our fisheries by putting sustainability, science and technology at the heart of the solutions, but we need to move away from traditional approaches and short-termism”.
Industrial fishing of large fish – including sharks, swordfish, tuna and mackerel – could also have an impact on global warming, according to a recent study. These fish are a carbon sink that consumes carbon dioxide (CO2) from fishing boats without releasing it back into the atmosphere. When they die, they sink to the bottom of the ocean and take the greenhouse gas with them.
In October, an international team of scientists found that the amount of CO2 that these species reduce has been overlooked and that overfishing could “wipe out” the natural phenomenon called a “blue carbon pump”. In light of their findings, the researchers called for more sensible and thoughtful fishing practices.
The lead author of the study, published in Science Advances, Gael Mariani said: “New conservation and management measures need to be introduced to ensure that more large fish remain a carbon sink and no longer become an additional source of CO2”.