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Basic Facts
  • Life expectancy is around 30 years

  • They eat a variety of fish, crustaceans, eels and squid.

  • Breeding season is June/July for harbour seals and beginning in August for grey seals

  • Seal pups nurse for about 3 weeks

  • Adult harbour seals are between 1.3-1.7 m long while greys are 1.8-2m long.

  • The UK is home to 1/3 of the world's grey seal population and 30% of the world's harbour seal population

  • The grey seal is the largest breeding mammal in the UK.

  • Grey seals can hold their breath for around 16 minutes but normally resurface every 5-10 minutes. Harbours can hold their breath for up to 30 minutes, but normally resurface every 3-5 minutes. 

  • Around 76% of harbour seals live in Scotland.

According to the last survey done by ZSL, there are 2,866 grey seals and 797 harbour seals living in the estuary. Overall numbers have doubled since 2013 and sightings upstream of Putney are on the rise.

Which type of seal did I see?

Harbour seals, also known as common seals, are very cat like in appearance. They are smaller than grey seals and have V shaped nostrils. Our local seal Freddie was a harbour seal. Grey seals are larger and more dog like in appearance. They have a much more pronounced nose and tend to be darker in colour.  

How can you spot the difference between males and females?

Males tend to be darker in colour, while females tend to be lighter and have more spots. Males also tend to be much larger than females. 

What else lives in the Thames?

The Thames is full of life. There are over 125 species of fish, seahorses, sharks, porpoise, crabs and endangered eels also living in the river. In the past four years there have been four whale rescues conducted in the Thames as well.

Shouldn't seals live in the ocean?

Most of the seals stay in the estuary where the river meets the sea. However, much of their food can still be found in the London Thames where the river is a mix of salt and freshwater up to Teddington. 

Are seals dangerous?

Seals are very curious, playful and puppy like by nature.. But like dogs, they can and will bite if they feel threatened. They tend to be more curious in the water and more guarded when vulnerable on land. Seals can carry zoonotic parasites and mycoplasma bacteria in their mouth, so it is important for your safety and theirs to stay away. 

Image by Henrik Hansen

Did you know there are almost 4,000 seals living in the Thames?

Threats to Seals

Being a seal is hard work!


They face a multitude of threats from climate change, entanglement, chemical and physical pollution, disease, food shortage, larger predators, and habitat degradation to name a few. Also on the list is human disturbance/harassment. 

  • In a bad year the grey seal has a 75% mortality rate within its first 2 years of life

  • Scotland has seen a 40% decline in harbour seals

  • There was an increase in seal rescues linked to unusually large storms last year 

  • Regular disease outbreaks negatively impact seal populations, especially harbour seals. There is currently a mystery disease affecting them now. 

  •  The University of Exeter found microplastics in all marine mammals(including seals) that washed up on British shores.

  • By catch accounts for 30% of strandings in Cornwall

  • Disturbance incidents were at a record high in 2021.

What is Disturbance?

Seals hauls out because they are either resting, digesting, moulting, feeding their pups or are unwell. This is an extremely important time for them. When a person, dog, drone or boat get too close to seals it disrupts their ability to do these things, which are necessary for their survival. Lack of rest or escaping a threat uses up fat stores, which are crucial to increasing survivals rates and having a successful pupping season. During the busy season seals are disturbed every 15 minutes in Cornwall. 

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