The substantial threat to whale sharks uncovered by our study makes a strong case for urgent protection measures. Simon Pierce, Author provided En route to danger Global shipping routes are an underappreciated threat to marine life. Some of the tags record depth as well as location and showed sharks moving into shipping lanes but then sinking slowly to the seafloor hundreds of metres below – the smoking gun for a lethal ship strike.
We may even have recorded whale sharks dying due to collisions. Even after accounting for the random technical failures of transmitters, we found 24% of tags stopped transmitting in busy shipping lanes, most likely due to whale sharks being lethally struck and sinking to the ocean floor. 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.Īlarmingly, whale shark tag transmissions ended more often in busy shipping lanes than we expected. Within high-risk areas, whale sharks regularly crossed vessel paths and passed close to ships that were travelling about ten times faster than they were swimming. These regions are home to some of the world’s busiest ports and sea passages, and because our estimated levels of risk correlated with known fatal collisions here, they appear to be some of the most dangerous places in the world for whale sharks to inhabit.Ĭoastal seas are among the most dangerous regions for whale sharks. We then developed state-of-the-art models to identify the collision risk within these overlapping areas and found that the Gulf of Mexico, Arabian Gulf and Red Sea posed the highest risk to whale sharks. We found that a staggering 92% of the horizontal space occupied by whale sharks and nearly 50% of their depth layers overlapped with the activities of these fleets. This helped us track global fleets of cargo, tanker, passenger, and fishing vessels – the types of large ships (heavier than 300 gross tons) capable of striking and killing a whale shark. We compared these movements with a mandatory ship tracking system, which was initially developed to prevent ships colliding with each other. Mark Erdmann, Author provided Striking overlap Before now, the only evidence available was a sparse set of eyewitness accounts, news reports and encounters with sharks harbouring injuries from collisions with smaller vessels.Įlectronic tags allow scientists to track sharks using satellites. This makes detecting and recording collisions difficult. 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. If a large one collides with a whale shark, the shark probably stands little chance of survival. Whale sharks spend a lot of their time cruising just below the ocean surface, often feeding on microscopic animals called zooplankton, which can put them in the direct path of a ship. Instead, several factors point to shipping being a leading, yet hidden cause of death. This is because major whale shark fisheries have been closed down, and the species has been protected by international trade bans since 2003.
Unlike most other species of shark that roam the open ocean, intentional or accidental catches by industrial fishing fleets are not thought to be a leading cause of whale shark decline. In 2016, they were added to the growing list of endangered shark species. Despite their robust appearance, their numbers have declined by over 50% in the last 75 years. Whale sharks can reach lengths of up to 20 metres. Our new study found that this threat may be a greater cause of death for the world’s largest fish, the whale shark, than anyone previously realised.
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. Marine highways can also cut across the movements and migratory routes of marine animals. These highways, which vessels travel back and forth, connect distant ports, often on opposite sides of vast oceans. The courses these container vessels take are fixed shipping routes known as marine highways, which are not unlike motorways on land. Much of what we use and consume everyday either has been or will be transported on huge vessels which plough the global ocean. Over 80% of international trade is carried by sea.
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