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Marine Mammals in the Tidal Thames

Marine mammals have all the common features of terrestrial mammals, including being warm-blooded, having hair for at least part of their life cycle, the ability to produce milk to nurse their young and breathe air through their lungs. Marine mammals have special adaptations that allow them to thrive in the marine environment where they may experience extreme temperatures, pressure and darkness.

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 0203 260 7711. They will direct craft to investigate and take appropriate action if necessary.


All cetaceans are protected under Section 9 of the Wildlife and Countryside Act 1981 (more information on this legislation can be found in chapter 2 of this document) as a schedule 5 species. This legislation makes it an offence to:

  • Intentionally kill, injure or take any cetacean,
  • Intentionally or recklessly disturb any cetacean,
  • Have in someone’s possession or control any live or dead animal, or any part of, or anything derived from a cetacean.

All cetaceans are protected under part 3 of the Conservation of Habitats and Species Regulations 2017. They are termed European Protected Species and any potential effects on these species must be fully assessed.

Harbour Porpoises

The harbour porpoise (Phocoena phocoena) are the most common cetacean seen in the tidal Thames. At approximately 2m in length they can be identified by the small and dark dorsal fin and their dark grey colouring with a lighter underside. It is common for harbour porpoises to be sighted alone or in small groups of two to three individuals.

A harbour porpoise (Phocoena phocoena) in the tidal Thames showing the characteristic dorsal fin.


Although sightings of these species are much rarer, common dolphins (Delphinus delphis) or short-beaked dolphins can be spotted in the outer estuary towards the North Sea. With tall and triangular dorsal fins which curve slightly backwards these species can be found travelling at high speed in groups. From above the water, the common dolphin appears dark grey in colour but underneath the water the white underside and yellow-cream hourglass pattern on their sides becomes visible.

Other Cetaceans

Several cetacean species migrate past the Thames Estuary in the North Sea, occasionally one of these migrating individuals will enter the Thames. The following species have all been visitors to the River Thames in the past 10 years:

  • Northern minke whale (Balaenoptera acutorostrata), a small baleen whale ranging between seven and nine metres in length with a dark grey body and white bands on their pectoral fins
  • Humpback whale (Megaptera novaeangliae), a large baleen whale approximately 15m in length with long pectoral fins (front flippers) and black bodies with white undersides. These species have characteristic knobs on the top of their head
  • Sei whale (Balaenoptera borealis), ranging between 12 and 14 metres in length the sei whale is usually blue-grey, dark grey or black in colour with a white underside. The dorsal fin of this species is slender and found towards the tail flukes, the dorsal fin and blowhole will both be visible when the animal is at the surface of the water
  • Beluga whale (Delphinapterus leucas), normally found in the cold Arctic and sub-Arctic waters the beluga whale is an uncommon visitor to the Thames. Reaching a maximum of 6m in length, the beluga is pure white as an adult, and blue brown as a juvenile, and often found in large social groups.


Seals are protected under the Conservation of Seals Act 1970, section 1 of this act makes it an offence to intentionally or recklessly kills, injury or take a seal.

Grey Seals

The grey seal (Halichoerus grypus) is the largest of the two species found in the tidal Thames. Grey seals range from approximately 1.8m in length for females and 2.5m for males, their fur can vary from dark grey to a creamy white with lighter colouring on their stomach. This species can be identified by their flat head and long muzzle with parallel nostrils, their eyes are located midway between their nose and the backs of their heads.

In the tidal Thames, grey seals will feed on fish such as eels and can be found as far upstream as Richmond.

Harbour or Common Seals

The harbour, or common, seal (Phoca vitulina) is smaller than the grey seal and their fur ranges from dark brown to pale grey and is often with mottled. Unlike the grey seal, the harbour seal has a round head, short muzzle with eyes positioned towards the front of the head.

In addition to feeding on fish such as sandeels and cod, they also eat squid, whelks, crabs and mussels. When hauled out on the mudflats and sandbanks they lie in a banana shape, with their head and tails lifted off the surface of the ground and groups are often more spread out further away from the water’s edge than grey seals.

  Grey seal Harbour seal
  Halichoerus grypus Phoca vitulina
Head Shape Flat with long muzzle
Rounded with short muzzle
Nostrils Parallel nostrils
V-shaped nostrils
Male Length & Weight 2.5m, 300kg 1.8m, 100kg
Female Length & Weight 1.8m, 175kg 1.5m, 80kg


Related Information

Page updated January 2022