More than 2,700 seals, porpoises, dolphins and whales have been spotted in the Thames in the past decade, according to a survey by the Zoological Society of London.
ZSL, the conservation charity behind London Zoo, launched the Thames Marine Mammal Sightings Survey in 2004 to investigate how marine mammals use the Thames Estuary, and how they can be helped.
Harbour seals were the most commonly spotted marine mammals, with 1,000 reported sightings in the 10 year period, some as far upstream as Hampton Court Palace. Harbour porpoises and dolphins, meanwhile, ventured up to Teddington Lock.
The best place to spot marine mammals seems to be Canary Wharf — more sightings were reported here than anywhere else in the Thames Estuary, an area stretching from Harwich to Whitstable on the coast, and down the Thames to Teddington.
Many sightings were recorded in central London too, especially between the Houses of Parliament and the O2 Arena in Greenwich, as well as Hammersmith.
The presence of marine mammals in London is "a good sign that the Thames is getting cleaner and supporting many fish species", says Joanna Barker, European Conservation Projects Manager at ZSL.
ZSL is still collecting data. If you see a marine mammal in the Thames, report it to the survey here.