Scientists Have Figured Out What's Killing The World's Biggest Fish

International trade is carried by sea. Much of the stuff we use and consume is transported on huge vessels which travel around the globe.

The courses these container vessels take are not like motorway courses on land. These highways connect distant ports on opposite sides of vast oceans.

Marine highways can cut across the migratory routes of marine animals. Giant plankton-feeding whales and sharks are particularly vulnerable to being struck and killed by large vessels as they spend long periods near the surface.

The threat to the whale shark may be more serious than previously thought.

Simon Pierce is an author.

There are scars that show how common ship strikes are.

There are whale sharks that can reach lengths of up to 20 meters. Their numbers have declined by over 50 percent in the last 75 years. They were added to the growing list of shark species that are in danger.

The decline of whale sharks is not thought to be caused by intentional or accidental catches by industrial fishing fleets.

The whale shark has been protected by international trade bans since 2003 and has been closed down. Shipping is a leading cause of death.

Whale sharks spend a lot of their time cruising just below the ocean surface, feeding on zooplankton which can put them in the path of a ship.

A whale shark is unlikely to survive if it collides with a large one. If a fatal collision occurs, the body sinks as whale sharks evolved from smaller, bottom-dwelling sharks and have retained their negative buoyancy.

It's difficult to detect and record a collision. The only evidence available before now was a sparse set of accounts, news reports and encounters with sharks.

We brought together over 60 scientists from 18 countries to uncover the deaths of whale sharks.

Almost 350 whale sharks were tracked by a satellite and fitted with electronic tags, which mapped their positions across all major oceans in unprecedented detail. The most densely occupied regions were in coastal areas where the species is known to congregate.

Mark Erdmann is the author.

Scientists can use electronic tags to track sharks.

Striking overlap

The mandatory ship tracking system was developed to prevent ships from colliding. The types of large ships that are capable of striking and killing a whale shark helped us track global fleets of cargo, tanker, passenger, and fishing vessels.

We found that almost all of the horizontal space occupied by whale sharks and almost all of the depth layers were occupied by the fleets.

The Gulf of Mexico, Arabian Gulf, and Red Sea were found to have the highest risk of whale sharks.

These regions are home to some of the world's busiest ports and sea passages, and because of our estimated levels of risk correlated with known fatal collision here, they appear to be some of the most dangerous places in the world for whale sharks to live.

Within high-risk areas, whale sharks crossed vessel paths and passed close to ships that were 10 times faster than they were swimming.

This gave the sharks very little time to respond to an oncoming ship, and these close-range encounters may be happening more often than we have the capacity to monitor, potentially ending in fatal strikes.

We expected whale shark tag transmissions to end in busy shipping lanes. After accounting for the random technical failures of transmitters, we found that 24 percent of tags stopped transmitting in busy shipping lanes, most likely due to whale sharks being lethally struck and sinking to the ocean floor.

Sofia Green is an author.

The most dangerous regions for whale sharks are the coastal seas.

We may have recorded the deaths of whale sharks. Some of the tags show sharks moving into shipping lanes but then sinking to the bottom hundreds of meters below the surface, which is the smoking gun for a lethal ship strike.

There is a strong case for urgent protection measures because of the threat to whale sharks uncovered by our study. There are no international regulations to protect whale sharks. If action is not taken soon, this species faces an uncertain future.

The International Maritime Organization could develop a global reporting scheme that consolidates records of ship-wildlife collision for whale sharks and other threatened species.

Regional authorities would be aided in implementing protection measures by providing evidence of where crashes are happening.

Efforts to lower the risk of ship strikes could be similar to measures to protect whales from accidents. Our study can help identify high-risk areas.

Rapid action may be the only way to save whale shark numbers.

David Sims is the Professor of Marine Ecology at the University ofSouthampton.

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