The sight of a basking shark's brooding silhouette gliding through the waters off western France is more than just a rare treat for sailors -- it is a boon for scientists trying to trace its secretive migrations across the globe.
It may be the world's second largest fish, growing to more than 10 metres, but the basking shark is an enigma for scientists eager to help preserve the plankton-eating giant after centuries of overfishing.
"It's a shark that remains very mysterious," said Alexandra Rohr of the research group APECS, which is based in the western French region of Brittany and dedicated to the study of sharks and rays.
Rohr said population estimates, the age of sexual maturity, and where and when the sharks reproduce are still not known for certain.
But new tracking technology has allowed APECS researchers to discover evidence of a much greater migratory range than previously thought.
Tracing migration routes
APECS has tagged four basking sharks so far this year with its new tracking devices, after deploying three in 2016.
The group said 77 were spotted in France last year from February to September, with most seen in the Bay of Biscay and the Mediterranean.
One tagged female was tracked off the coast of northern Scotland in September 2016, and then resurfaced four months later south of the Canary Islands. By May 2017, the shark was back in the Bay of Biscay, south of Brittany.
APECS relies on crowd-sourced information from divers, sailors and other members of the public.
Alain Quemere sighted a basking shark during a fishing trip in the Glenan archipelago off the coast of Brittany and reported details to APECS, enabling a research team to find the shark and fit it with a satellite tracker.
"I just saw the tip of his fin," said Quemere, still enraptured by the memory of his five-hour encounter.
"One moment it grazed the front of the boat, which made me laugh because my boat is barely five and a half metres and the shark was eight."
The International Union for the Conservation of Nature (IUCN) now lists the basking shark as "vulnerable" across the world and "endangered" in the North Pacific and Northeast Atlantic.
In 2003, basking sharks and whale sharks were added to the Convention on International Trade in Endangered Species (CITES) "Appendix II", which bans all trade in parts, unless under stringent conditions.
An estimated 100 million sharks are killed every year, according to a 2013 study by the journal Marine Policy. An amount that conservationists say threatens the survival of many shark species.