Scientists have found what kills the largest fish in the world

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

The routes of these container ships are fixed shipping lanes, known as highways of the sea, which are no different from land highways. These highways, on which ships travel back and forth, connect distant ports, often located on opposite shores of vast oceans.

Highways of the sea can also cross the movement and migration routes of marine animals. Giant whales and plankton-eating sharks are particularly vulnerable to being struck and killed by large vessels as they spend extended periods near the surface.

Our new research has shown that this threat may be a bigger killer for the world’s largest fish, the whale shark, than previously thought.

Whale sharks can reach a length of up to 20 meters. Despite their powerful appearance, their numbers have declined by more than 50% over the past 75 years. In 2016, they were added to the growing list of endangered shark species.

Unlike most other shark species found in the open ocean, intentional or accidental fishing by industrial fishing fleets is not considered the main cause of whale shark decline.

This is because large whale shark fisheries have been closed and since 2003 the species has been protected by international trade bans. Instead, several factors point to shipping as a leading but hidden cause of death.

Whale sharks spend most of their time swimming 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 vessel collides with a whale shark, it will most likely have little chance of surviving. Often there are no traces of such events, because in the event of a fatal collision, the body sinks, since whale sharks evolved from smaller, bottom-dwelling sharks and retained negative buoyancy.

This makes it difficult to detect and register collisions. So far, the only evidence available has been scant eyewitness accounts, news reports and sightings of sharks injured in collisions with small boats.

We have made it our mission to uncover the hidden death of whale sharks by bringing together an international team of over 60 scientists from 18 countries.

Our Global Shark Movement project tracked nearly 350 whale sharks from satellite, electronically tagged them, and displayed their location in unprecedented detail across all major oceans. This allowed the most densely occupied regions to be identified, which were often in coastal areas where the species is known to congregate.

We compared these movements with the mandatory ship tracking system, which was originally designed to prevent ships from colliding with each other. This has helped us track the global fleets of cargo, tanker, passenger and fishing vessels – the types of large vessels (heavier than 300 gross tons) capable of hitting and killing whale sharks.

We have found that a staggering 92 percent of the horizontal space occupied by whale sharks and almost 50 percent of their deep layers intersect with the activities of these fleets.

We then developed state-of-the-art collision risk models in these overlapping zones and found that the Gulf of Mexico, Persian Gulf and Red Sea pose the greatest threat to whale sharks.

These regions have some of the busiest ports and sea lanes in the world, and because the levels of risk we assessed correlate with known fatal encounters, they are among the world’s most dangerous habitats for whale sharks.

In high-risk areas, whale sharks regularly crossed and passed alongside ships, moving about 10 times faster than they swam.

This gave the sharks very little time to react to an approaching vessel, and such close-range encounters can occur more often than we have the ability to track, which can be fatal.

Worryingly, the transmission of whale shark tags was interrupted more often than we expected in busy shipping lanes. Even after accounting for occasional technical glitches in transmitters, we found that 24% of the tags stopped transmitting in busy shipping lanes, most likely due to whale sharks being fatally stricken and sinking to the ocean floor.

We may even have recorded the death of whale sharks as a result of collisions. Some of the tags record depth as well as location and show the sharks moving along the shipping lanes but then slowly dropping to the seabed hundreds of meters below – a smoking pistol for a ship’s fatal blow.

The significant threat to whale sharks identified in our study is a compelling argument for urgent protection action. There are currently no international regulations to protect whale sharks from ship strikes. In light of our research, unless action is taken soon, this species has an uncertain future.

As a first step to address this crisis, the International Maritime Organization (IMO) could develop a global reporting scheme that would consolidate records of ship collisions with wildlife for whale sharks and other endangered species.

Such a network would help regional authorities implement protective measures by providing evidence of where clashes are taking place.

Initiatives to reduce the risk of ship collisions can be similar to measures to protect whales from collisions, such as IMO regulations requiring ships to slow down or navigate more carefully. Our research can help identify high-risk areas where these measures can be tested.

Quick action may be the only way to prevent whale sharks from declining further down the road to extinction.


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