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Marine Mammals in the Thames

With cleaner waters we are fortunate to welcome more marine mammals to the Thames. Marine mammals that visit the Thames regularly are grey seals, harbour seals and porpoises. Less regularly we see dolphins and whales.

If you spot a marine mammal in the upper reaches of the tidal Thames or you see a marine mammal that you think is in distress, please contact VTS on 0208 855 0315. They will direct craft to investigate and take appropriate action if necessary.

ZSL Marine Mammal Sighting Survey

The PLA has joined with the Zoological Society of London (ZSL) to assist in a sighting survey of marine mammals in the tidal Thames. Seals have been spotted in the Thames as far up as Richmond Lock. They feed on a wide range of fish in the estuary and can sometimes be seen resting on the sandbanks in the outer Thames at low tide. There has been a dramatic global decline in the numbers of common harbour seals; populations have decreased by more than 50% since 2000. Marine mammals are known to visit the Thames estuary but have only been recorded by this survey.

The Zoological Society of London (ZSL) launched the Thames Marine Mammal Sightings Survey in 2004 and during the first ten years, over 1,300 marine mammal sightings were reported by members of the public. If you are fortunate enough to spot dolphins, porpoises or seals in the Thames please participate in the survey. PLA staff members have been issued with guidance on the different types of marine mammals that may visit the Thames and are asked to inform the PLA's Environment team detailing the details of date, time, species and number of individuals. This information is also sent on to ZSL periodically.

The data collected will help to develop patterns and trends in species distribution, behaviour and habitat use. Findings will also be fed into national databases.

See https://www.zsl.org/conservation/regions/uk-europe/thames-marine-mammal-conservation for more information.

You can report your sighting at http://sites.zsl.org/inthethames/main/ or using Twitter #inthethames.

ZSL have published a Code of Conduct relating to marine mammals.

The table below helps explain the differences between grey and harbour seals:

Common Name The Grey Seal The Common or Harbour Seal
Latin Name Halichoerus grypus Phoca vitulina
Nostrils Parallel nostrils
V-shaped nostrils
Head Flat head profile
Concave (curved) head profile
Muzzle Elongated muzzle Short muzzle
Male Size & Weight 2.5m, 300kg 1.8m, 100kg
Female Size & Weight 1.8m, 175kg 1.5m, 80kg

Harbour seal photos with kind permission of Dai Taylor (photo@london.com)

ZSL Annual Seal Survey

ZSL have been undertaking annual seal population surveys in the Thames Estuary every August since 2013. These surveys have recorded an increasing total population of seals in the Estuary – further details can be found at https://www.zsl.org/conservation/regions/uk-europe/thames-marine-mammal-conservation.

Cetaceans in the Thames

Cetaceans are whales, dolphins and porpoises. Porpoises are regular visitors to the Thames, less frequently we also see dolphins and whales. If you are lucky enough to spot a cetacean in the Thames, please contact the PLA VTS Officers on 0208 855 0315 (available 24 hours a day). If the animal appears to be in distress the PLA will send a vessel to investigate and contact the relevant authorities to arrange assistance if required.

Code of Conduct

The British Divers Marine Life Rescue has produced a Code of Conduct for watching cetaceans (dolphins, porpoises etc). Click here to view the code (pdf file - 44kb).

Recreational users are reminded to keep clear of animals and not to disturb or harass them by the PLA in various user guides. More specific information can also be found in the Code of Conduct for Personal Watercraft (PWC) (Jetskis).


Seals do haul out as part of their natural behaviour and can be seen on sandbanks and pontoons. They often adopt a banana shape with their head and tail in the air. It is now common to see them hauled out throughout London as well as the outer estuary. You may even spot them on pontoons or small boats sunbathing.

If you spot what you think is a stranded seal or another marine mammal on the banks of the Thames, please report it to the PLA VTS Officers on 0208 855 0315 (available 24 hours a day). The PLA will send a vessel to investigate and contact the relevant authorities to arrange assistance if required.

Greater Thames Seal Working Group

Set up in 2013, the GTSWG is a platform for collaborative work on the major issues facing seal populations in the Region. The PLA is a member of this group.

Further information can be found at https://www.zsl.org/greater-thames-seal-working-group.

Thames Marine Mammal Working Group

A new Group has been set up to raise awareness of mammals in the Thames, how to behave around them, how to determine if they are in distress and to coordinate responses for distressed or injured animals.

Further information can be found here.

Other Useful Links

Page last updated August 2017.