In a study published in the journal PNAS, an international group of scholars sounds the alarm for whale sharks. Although they are found in every ocean, Rhincodon typus are largely found in tropical waters. They like to swim near the surface of the sea — and their numbers have been falling.
The study, which lays out the results of a global tracking project, explains why.
Using satellite-linked tags, the team tracked movements of 348 whale sharks. Then they matched the data with information about the movements of the kinds of ships capable of killing the massive animals. More than 90 percent of the whale activity overlapped with busy shipping corridors.
There’s no reporting requirement for vessels that strike whale sharks, but the researchers were able to pinpoint hot spots for the animals. The tags were designed to pop off animals that had remained at a constant ocean depth for a long period of time — a signal that the animal was lying dead on the seafloor. After ruling out transmitters with technical issues, the researchers learned that the tags popped off most often in busy shipping areas.
“We propose that ship strikes may have been responsible for a substantial proportion of these [transmitter tag pop-offs] but were undetected or unreported by vessels,” they write.
“Incredibly, some of the tags recording depth as well as location showed whale sharks moving into shipping lanes and then sinking slowly to the seafloor hundreds of meters below, which is the ‘smoking gun’ of a lethal ship strike,” David Sims, a University of Southampton marine ecology professor and co-author, said in a news release. “It is sad to think that many deaths of these incredible animals have occurred globally due to ships without us even knowing to take preventative measures.”
It is unclear how many whale sharks are in the world’s oceans, but the population is thought to have declined about 50 percent in the past century. The animals are an important part of the marine food web.
Ship speed limits and collision reporting requirements could help, the authors write. Although local conservation laws cover whale sharks in parts of their range, they are not covered by any international laws.