More than 80% of international trade is carried out by sea. The rates at which container ships sail represent
Highways of the sea can also crossroutes of movement and migration of marine animals. Giant whales and plankton-eating sharks are particularly vulnerable to attack and sinking of large vessels as they spend long periods of time near the surface.
Whale sharks can reach a length of 20 meters.But despite their solid size, over the past 75 years, their numbers have declined by more than 50%. In 2016, they were added to the growing list of endangered shark species.
Unlike most other shark species, intentional or accidental capture by commercial fishing fleets is not considered the main cause of whale shark declines.
This is due to the fact that the main whale fisheriessharks have been shut down and the species has been protected by international trade bans since 2003. But several factors point to shipping as a major but hidden cause of death.
Whale sharks spend most of their time traveling just below the surface of the ocean, often feeding on microscopic animals called zooplankton.
If a large shark encounters a whale shark,then the latter probably has little chance of surviving. Often there are no traces of these events, because in the event of a fatal collision, the body sinks: whale sharks evolved from smaller bottom sharks and retained their negative buoyancy.
The Global Shark Movement satellite tracked almost350 whale sharks, electronically tagged and mapped their positions in all major oceans with unprecedented precision. This revealed the most densely populated regions, which were often in coastal areas where these species are known to congregate.
We compared these movements with the mandatory systemship tracking, which was originally developed to prevent ships from colliding with each other. This has helped us keep track of the global fleet of cargo, tanker, passenger and fishing vessels – large vessels (over 300 gross tons) capable of hitting and killing whale sharks.
We have found that a staggering 92% of the horizontal space occupied by whale sharks and almost 50% of their depths are covered by the activity of these fleets.
We then developed state-of-the-art models forimpact risk in these overlapping areas and found that the Gulf of Mexico, the Arabian Gulf and the Red Sea pose the greatest danger to whale sharks.
These regions contain some of the mostthe world's busiest ports and seaways, and because our estimated risk levels correlate with known fatal encounters here, they appear to be among the world's most dangerous habitats for whale sharks.
In high-risk areas, whale sharks routinely crossed boat paths and passed alongside boats that were moving about 10 times faster than they were swimming.
This gives the sharks very little time to react to an approaching ship, so these encounters could potentially end in fatal collisions.
Transmission from tags on whale sharks is more common thanscientists expected, ended on busy shipping lanes. Even after accounting for occasional technical glitches in transmitters, we found that 24% of tags stopped transmitting on busy shipping lanes, most likely due to whale sharks being fatally stricken and sinking to the ocean floor.
Some tags record depth as well as location and show sharks moving along shipping lanes and then slowly sinking to the seafloor hundreds of meters below.
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