Kristi Tange, 47, initially thought she had seen a dog in the water when she spotted the seal swimming near Barnes railway bridge.
Tange says: “It was low tide, and I saw the seal come up out of the water before disappearing. It must have swum quite a distance because I didn’t see it resurface again.
It was the first time I had seen a seal in that location, which was pretty exciting.”
Tange reported the sighting on the Thames Marine Mammal Map, which is run by the Zoological Society London (ZSL).
Members of the public can record sightings on the map with a short description of the location and details of the mammal.
Anna Cucknell, 39, ZSL Conservation Project Manager, said: “People are often surprised to see seals, and the Thames Marine Mammal Map is a great tool to share how amazing these sightings are.”
According to ZSL there are usually around 400 public sightings per year in the Thames.
Since the start of 2022, the public platform has recorded 30 harbour seals, 25 grey seals, and 1 harbour porpoise, mostly in the Southbank area.
Cucknell also said the Thames is a crucial habitat for these species, with ZSL’s 2021 Seal Survey showing that there are 2,866 grey and 797 harbour seals living in the Thames estuary.
These surveys are important because they are an overall indicator of biodiversity in the estuary.
Although seals do not make their habitats in the river along residential areas such as Barnes, they often travel further upriver to chase food, and rest on the riverbank.
There has been a steady increase in the seal population since 2003, as demonstrated by ZSL’s annual population surveys.
Cucknell said: “The Thames looks brown, but it is actually thriving with life and full of species. It’s important we protect it as it is incredibly important to life’.
If you see a marine mammal on land, you should follow the ZSL code of conduct. Stay at least 50m away, keep your dog on a lead, and never approach or touch a seal.
If you see a dead or stranded marine animal, call 0800 6520 333.
To record a seal sighting, make a report on the Thames Marine Mammal Map.