According to a ten-year marine mammal survey by the Zoological Society of London, there have been more than 1,000 harbour seal sightings along the river Thames over the past decade, along with dozens of harbour porpoise sightings and even the occasional whale. ‘People look at the Thames and see murky water,’ says Sofia Castello y Tickell, co-author of the study, ‘but in fact, it is full of life ranging from invertebrates to fish and top predators such as seals.’
“Harbour seals have been seen at Hampton Court, porpoises and dolphins at Teddington Lock and whales as far as Gravesend”
The tidal Thames had been used as the London’s drainage system for centuries before it was declared biologically dead by the Natural History Museum in 1957. The waters around the most developed areas of the city had become anaerobic – devoid of oxygen and unable to sustain life. Following the introduction of tough legislation in the 1990s, the ‘big stink’ began to make a biological comeback. Most harmful effluents are now prevented from entering the Thames and its tributaries, while sewage is treated and exported elsewhere. Given this chance, 400 invertebrates and 125 species of fish, have returned to the grey-green waters. ‘Marine mammals are most likely following their food,’ says Joanne Barker, marine biologist at the ZSL and co-author of the study. ‘Harbour seals have been seen at Hampton Court, porpoises and dolphins at Teddington Lock and whales as far as Gravesend.’ Canary Wharf is the best place to see marine species as more sightings were reported there than anywhere else along the Thames estuary.
While the water quality has improved, harmful pollutants are still adrift in the form of plastics. A study by Royal Holloway found 70 per cent of flounder had plastic in their guts. Plastics are thought to cause the bioaccumulation of toxins in marine species. ‘Although much cleaner than in the 1950’s,’ says Barker, ‘the Thames still has a number of problems and has a long way to go before I would describe it as being clean.’
This article was published in the January 2016 edition of Geographical magazine.