The story of a turtle caught twice in fishing nets reveals a dual threat facing many ocean animals.
The female loggerhead turtle, called Thunderbird, was entangled in abandoned fishing gear in the Mediterranean last year, and was subsequently fitted with a satellite tag and freed.
Researchers tracked the turtle as it swam through the Strait of Gibraltar and headed south along the coast of Africa.
But then signals became erratic, before a final position from the tag showed the turtle on land – near a major fishing port in Dakar, Senegal, suggesting it was unintentionally caught by a fishing vessel, probably a trawler.
“The turtle was found entangled in ghost fishing gear by Save the Med Foundation in July 2020 and taken to the Palma Aquarium rescue centre in Mallorca,” said Dr David March, of the universities of Exeter and Barcelona.
“We attached a satellite tag before she was released on August 11, which allowed us to track her epic journey of 6,000km through the Western Mediterranean and the waters off West Africa.
“Thunderbird raised an early warning when she entered into the Alboran Sea, a challenging area because of its strong currents and high density of marine traffic, which could result in a high risk of boat collision.”
At that time, a network of conservationists was asked to keep an eye out for potential stranding or bycatch.
Thunderbird managed to swim up-current out of the Mediterranean in November through the Gibraltar Strait.
Most adult turtles that move out from the Mediterranean swim towards America, as they were born in Florida or the Caribbean, but Thunderbird swam along the West African coast.
The satellite tag provided depth data, so the researchers could see the turtle was spending most of the time at the surface in the Mediterranean, then diving near the sea-bottom along the West African coast.
“In February this year, the turtle was off Senegal when we stopped receiving regular updates from the tag,” Dr March said.
“Then the final signal on March 17 was on land, near the main harbour in Dakar.
“It is always a challenging task to know what makes a tag stop transmitting data, so we checked tag data to find out more about potential causes.
“After checking that battery and tag sensors worked correctly, we used the Global Fishing Watch portal to overlap the track of the turtle with fishing vessels.
“We found out that the last dive recorded by the satellite tag was near a fishing ground used by trawlers.
“All this suggests the turtle was bycaught by a fishing vessel and taken back to the port.
“We don’t know if Thunderbird was released alive after capture or died as consequence of the bycatch event.”
Researchers are using satellite data on boat movements and working with partners in Senegal to try and find the boat that caught the turtle, in the hope of finding out more information about the fate of Thunderbird.
Dr March added: “The epic journey of this turtle illustrates two of the major threats that many marine species face – entanglement in ghost fishing gear, and bycatch in industrial fisheries.
“We must urgently address both issues to limit their impact on a wide range of marine species and ecosystems.”