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SEALS OF THE THAMES

Updated: Mar 25

In recent years, the River Thames has been in the process of recovery. Today, it is home to more than 125 species of fish, internationally important migratory birds and marine mammals such as the harbour porpoise, grey seal and harbour seal. Grey seals are frequent visitors, whilst harbour seals are in fact residents with a breeding population in the Thames Estuary.


Both grey and harbour seals have a well-developed sense of vision and hearing. Their vision is specifically adopted to see in dark and murky waters. However, both of these senses are greatly reduced when they are out of the water. Their whiskers are particularly powerful as they can detect the movement of fish under water as far away as 100 metres.


Apart from their head and flippers, their body is well insulated with a thick layer of blubber or fat. Their flippers have a dense network of blood vessels just under the skin and are used to regulate their body temperature when out of the water. To avoid getting too cold, they can shut off the blood circulation close to their skin and lift their head and flippers, taking up their signature banana-shaped pose. To avoid getting too warm, they simply dangle their head and flippers into the water and utilise blood circulation to cool their body.


Grey seals (Halichoerus grypus) are the larger of the two species with a longer, 'roman nose' profile and parallel nostrils. They spend most of their time at sea, only coming to shore along the coast during the winter months to mate, breed, nurse their younglings and moult. Nevertheless, occasional sightings can occur in the tidal Thames as far as Teddington as seals follow their favourite fish such as eels and flatfish.

Grey seal (Photo by Diana Parkhouse on Unsplash)

Harbour or common seals (Phoca vitulina) are smaller than grey seals with a cat-like face and V-shaped nostrils. They are more frequently spotted hauled out on the Thames foreshore during the spring and summer months as this is when they mate, breed, nurse and go through their annual moult. They can also be spotted in the river chasing fish such as flounder, bass, and grey mullet.

Harbour seal (Photo by Anchor Lee on Unsplash)

During the seals’ moulting process, they gradually lose their coat, subsequently increasing the blood flow just below the surface of their skin to encourage fur growth, which in turn makes them lose more heat. Therefore, spending more time out of the water, hauled out on sandbanks or on the foreshore can help them conserve heat and stay warm.


In recent years, the population of these wonderful animals is improving and there have been many more seal sightings (over 150 to date this year only). Therefore, it is important to consider that both species can be encountered along the foreshore. In most cases, when hauled out, they are simply getting warm or resting while digesting their food, thus it is best to leave them undisturbed. In fact, the best way to enjoy their company is to keep them completely unaware of human presence.

Harbour seal resting on the foreshore at Strand-on-the-Green (Photo by Wanda Bodnar)

Should you be lucky enough to have a close encounter with an individual when rowing or paddling on the Thames, always remember that their approach happens on their own terms and it is best and safest to leave them be while maintaining a safe distance. They will eventually move on. There is no exact knowledge as to why these encounters occur, but it could be speculated that they simply confuse rowing boats, kayaks and paddleboards with a natural floating object or with a protruding patch of sandbank to haul onto to rest and dry their fur.

In case of a sighting, always make sure:

Click here to find out more about the Marine Mammal Code of Conduct on the tidal Thames.


The Zoological Society of London has been monitoring the presence of marine mammals in the tidal Thames and you can help their work by submitting images or videos with other information to a dedicated website or by sharing it on social media using the hashtag #InTheThames. Photographing seals can be very useful for tracking individuals as their dark spots are unique and it can help with identification.


To find out more about the wildlife in the Thames, please visit the links below: