how we found out what’s killing so many of the world’s biggest fish

More than 80% of international trade is carried out by sea. Much of what we use and consume every day has been or will be transported on huge ships that ply the world’s oceans.

The courses these container ships take are fixed sea routes known as maritime highways, which are not dissimilar to land highways. These highways, along which ships travel back and forth, connect distant ports, often on opposite sides of vast oceans.

Marine highways can also traverse the movements and migratory routes of marine animals. Giant whales and sharks that feed on plankton are particularly vulnerable to being hit and killed by large vessels when they spend long periods near the surface. Our new study found that this threat may be a bigger killer of the world’s largest fish, the whale shark, than anyone previously thought.

Whale sharks can reach lengths of up to 20 meters. Despite their robust appearance, their numbers have decreased by more than 50% in the last 75 years. In 2016, they were added to the growing list of endangered shark species.

Unlike most other shark species that roam the open ocean, intentional or bycatch by industrial fishing fleets is not thought to be a major cause of the whale shark’s decline. This is because major whale shark fisheries have been closed and the species has been protected by international trade bans since 2003. Instead, several factors point to shipping as a major, albeit hidden, cause of death.

Whale sharks spend much of their time cruising just below the ocean’s surface, often feeding on microscopic animals called zooplankton, which can put them in the direct path of a ship. If a large one collides with a whale shark, the shark is likely to have little chance of survival. There is often no trace of these events because, if a fatal collision occurs, the body sinks, as whale sharks evolved from smaller bottom-dwelling sharks and have retained their negative buoyancy.

This makes it difficult to detect and record collisions. Until now, the only available evidence was a sparse set of eyewitness accounts, news reports, and encounters with sharks harboring injuries from collisions with smaller vessels.

A ventral view of the back of a whale shark with a wide gouge.
The scars attest to how common collisions with ships can be.
Simon Pierce, Author provided

We set out to uncover the hidden deaths of whale sharks by bringing together an international team of more than 60 scientists from 18 countries. Our Global Shark Movement Project satellite tracked nearly 350 whale sharks by attaching electronic tags to them, mapping their positions across all major oceans in unprecedented detail. This revealed the most densely populated regions, which were often in coastal areas where the species is known to congregate.

A whale shark dorsal fin with an electronic tag attached.
Electronic tags allow scientists to track sharks using satellites.
brand erdmann, Author provided

eye-catching overlay

We compare these moves to a mandatory ship tracking system, which was initially developed to prevent ships from colliding with each other. This helped us track global fleets of cargo, tanker, passenger and fishing vessels – the kinds of large ships (over 300 gross tons) capable of hitting and killing a whale shark. We found that a staggering 92% of the horizontal space occupied by whale sharks and nearly 50% of their depth layers overlap with the activities of these fleets.

We then developed state-of-the-art models to identify collision risk within these overlapping areas and found that the Gulf of Mexico, Arabian Gulf and Red Sea posed the greatest risk to whale sharks. These regions are home to some of the busiest ports and sea passages in the world, and because our estimated levels of risk correlate with known fatal collisions here, they appear to be some of the most dangerous places in the world for whale sharks to inhabit.

A diver swimming alongside a whale shark.
Coastal seas are among the most dangerous regions for whale sharks.
sophia green, Author provided

Within high-risk areas, whale sharks regularly crossed ship paths and passed near ships traveling about ten times faster than they swam. This gave the sharks very little time to respond to an approaching ship, and these close-quarters encounters may be happening more frequently than we have the ability to monitor, which could end in fatal attacks.

Alarmingly, whale shark tag transmissions ended more often than we expected on busy shipping lanes. Even after accounting for random transmitter glitches, we found that 24% of tags stopped transmitting on busy shipping lanes, most likely because whale sharks were fatally struck and sank to the bottom of the ocean. ocean.

We may even have recorded whale shark deaths due to collisions. Some of the tags record depth as well as location, and showed sharks moving toward shipping lanes but then slowly sinking to the seafloor hundreds of meters below – the ultimate proof of a lethal attack on a ship.

A large container ship on the horizon.
Global shipping lanes are an underestimated threat to marine life.
Simon Pierce, Author provided

On the way to danger

The substantial threat to whale sharks uncovered by our study makes a strong case for urgent protection measures. Currently, there are no international regulations to protect whale sharks from ship strikes. In light of our study, this species faces an uncertain future if action is not taken soon.

As a first step in addressing this crisis, the International Maritime Organization (IMO) could develop a global reporting scheme that consolidates ship-wildlife collision records for whale sharks and other threatened species. Such a network would help regional authorities implement protective measures by providing evidence of where crashes are occurring.

Initiatives to reduce the risk of collisions with ships could emulate measures to protect whales from collisions, such as IMO regulations that require ships to slow down or navigate more carefully. Our study can help by identifying high-risk areas where these measures could be tried.

Fast action may be the only way to prevent whale shark numbers from sinking further toward extinction.

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