it was became widely used in the 1930s has been estimated at 1.3 million tonnes.

Industrial accidents, leaks from landfill sites and deliberate discharges saw large amounts of the chemicals get into the sea, which has often been used as a waste dumping ground.

As indicated by their name, POPs do not degrade easily and are believed to be able to persist in the environment for decades at least.

It is thought the chemicals got into the trenches as plastic debris sank to the sea bed or as contaminated animals died and floated down from the surface. The POPs accumulate in the food chain so that by the time they reach the deep ocean, concentrations are many times higher than in surface waters.

In an associated article in the same journal, DrAfrica. Animals in decline Leatherback turtle (Dermochelys coriacea) Where: Indonesia, Malaysia, Mexico, Costa Rica. What: Numbers are down in both the Atlantic and Pacific. It declined by 95 per cent between 1989-2002 in Costa Rica. Why: mainly due to them being caught as bycatch, but they’ve also been affected by local developments. Alamy Animals in decline Wandering albatross (Diomedea exulans) Where: South Atlantic. What: A rapid decline. One population, from Bird Island, South Georgia, declined by 50 per cent between 1972-2010, according to the British Antarctic Survey. Why: being caught in various commercial longline fisheries. Alamy Animals in decline Saiga Antelope (Saiga tatarica) Where: Kazakhstan, Mongolia, Russia, Turkmenistan, Uzbekistan. What: fall in populations has been dramatic. In the early 1990s numbers were over a million, but are now