British endurance swimmer Lewis Pugh has completed long-distance swims in each of the Seven Seas to campaign for marine protected areas.
Mr Pugh has become the first person to complete long-distance swims in each of the classical Seven Seas, the Mediterranean, Adriatic, Aegean, Black Sea, Red Sea, Arabian Sea and finally the North Sea, swimming up the Thames to the Thames Barrier.
Mr Pugh is backing calls by the United Nations for 10% of the world's seas - both around countries and on the high seas - to be declared marine protected areas by 2020 to safeguard fish and other wildlife. Just 3% of the world's marine areas are protected, compared to around 13% of the world's land area. Mr Pugh, who has previously undertaken swims at the North Pole and in a glacial lake in Everest to highlight rising global temperatures, said seas were threatened by pollution, overfishing and climate change.
Speaking after he finished his final swim, in the Thames up to the Thames Barrier, he said: "I've experienced some things I will never forget and seen some things I wish I could erase from my memory, but which will haunt me for the rest of my days.
"I will never forget the people I met along this journey, the literally hundreds of people from all walks of life who helped us and supported us and jumped in the sea to swim with us, just to be part of this mission, just for their love of the sea.
"And then there are the things I would rather forget, such as the sea floor under me as I swam the Aegean, which was covered with litter; I saw tyres and plastic bags, bottles, cans shoes and clothing - but absolutely nothing that qualifies as 'sea life'."
He said that in the Arabian Sea, he swam through "spectacular" shoals of turtles, but did not see many other fish species which should have been there, and that he never saw fish bigger than his hand in any of the Seven Seas.
"The larger ones had all been fished out," he said.
He added the Black Sea was full of jellyfish, which did not belong there, when he swam in it and that he saw no sharks in any of his swims.
"But I did see something astonishing in the Red Sea. It was when I swam through a marine protected area, and experienced a sea as it was meant to be: rich and colourful and teaming with abundant life.
"And then, just two kilometres on, outside of the protected area, the picture changed again. There was no coral and there were no fish. It looked like an underwater desert," he said.
Mr Pugh, who completed all seven swims in a month, called for a world with well managed marine protected areas to protect coastlines and the high seas.
United Nations Environment Programme (UNEP) executive director Achim Steiner said: "UNEP applauds Lewis Pugh's latest expedition, which will spotlight the importance of MPAs and increase global attention to the plight of the world's oceans.
"Land-based pollution, poorly managed coastal development, overfishing and climate change are all major threats which can be reduced if governments work together and set ambitious targets."