Thames Marine
Mammal Survey

Spotted a marine mammal?

The Thames Estuary is home to harbour seals, grey seals, harbour porpoises and sometimes even dolphins and whales! ZSL has been collecting public sightings of these marine mammals since 2004 and they are frequently sighted all the way up to Richmond.

These sightings form a crucial part of our understanding of the biodiversity in the Thames and help us conserve these charismatic top predators.

If you are lucky enough to see a marine mammal, please follow our code of conduct and report your sighting below.

Sightings reported since 2004

harbour seals
grey seals
harbour porpoises
Number of unknown seals

Marine Mammal Code of Conduct

ZSL's Seal Survey 2014

In August of each year, harbour seals spend a greater proportion of time lying out on sandbanks to grow a new fur coat for winter (the moult period). This creates the perfect opportunity for them to be counted.

In August 2013, ZSL completed the first comprehensive survey of the Thames harbour seal population using aerial, land and boat based transects. We will complete this survey every year, using the same methodology, to determine if harbour seals in the Thames Estuary are increasing, decreasing or staying the same.

Results of the 2014 survey

In 2014, we counted 938 seals in total (489 harbour seals and 449 grey seals) showing that harbour seals numbers had remained stable and grey seal numbers had more than doubled since last year. Explore the results of the survey below.

Harbour Seal Tracking

We tagged 10 harbour seals with Fastloc GPS GSM tags to provide vital information on harbour seal movements, haul out sites and foraging areas in the Thames Estuary. This information is being used to inform effective conservation and management of the population.

More information can be found here.

Explore the movements of these 10 harbour seals

The tracks displayed on the map are using raw data, so there will be occasions where seals look like they go over land. All analysis for the report was completed on ‘cleaned’ data sets.


Select a seal to track its behaviour.

What else is in the Thames?

Hidden beneath the muddy, turbid water of the Thames Estuary is an ecosystem of great ecological importance. The Thames is home to a number of fascinating species and ZSL has been working to conserve many of them since the Tidal Thames Programme was set up in 2004.

Explore the coloured areas of the map to find out more about ZSL’s work and how you can help conserve these animals by becoming a citizen scientist.

Read about ZSL's UK and Europe Projects

Bird species

  • Common tern - Nearly driven to extinction in the 19th century by hunting for the hat-making trade, the common tern made a recovery after protective legislation was introduced. The population is now largely stable and as its name suggests it can be seen around much of the UK, with significant numbers wintering on the Thames.
  • Cormorant - These fascinating ancient birds are commonly seen stood with their wings spread-out to dry their outer plumage after a dive. Although their population is currently healthy, in the Thames and further afield their reliance on fish stocks for food can put them at odds with local fishermen.
  • Dunlin - The dunlin is a long distance migrant and most common of the wading birds around the UK. Although they are currently abundant, the dunlin is susceptible to development and disturbance of their intertidal feeding grounds. New major engineering works recently completed and planned for the Thames Estuary may have an impact on local dunlin colonies.
  • Grey heron - These large unmistakeable wading birds rely on clean water to support their preferred fish based diet, as a result they disappeared from London in the 1950s due to the poor water quality of the Thames. Recently however they have been making a comeback and can be seen all along the Thames from Battersea out into the estuary.
  • Herring gull - Although still a common sight out in the Thame Estuary and around the coast of Britain, the herring gull has been in steady decline in recent years and is increasingly recognised as a conservation priority.
  • Kingfisher - The common kingfisher is a distinctive brightly coloured hunter that inhabits the banks of slow-moving water bodies. Though not currently endangered their high susceptibility to environmental fluctuations and human disturbance means they are afforded the highest level of legal protection.
  • Lesser black-backed gull - The UK plays host to 40% of all lesser black-backed gulls globally, and they can often be found nesting with their slightly larger relative- the herring gull. After an initial recovery following their 19th century collapse, they are now in decline across much of their range.
  • Oystercatcher - These large, stocky wading birds can be found all year round around the coast of the UK. Their reliance on cockle beds for food puts them at odds with fishermen and in the 1970s they were hunted as a pest. Their population is now stable across most of their range and the Thames Estuary hosts an internationally important number.
  • Red throated diver - The smallest of the UKs diver species, this migratory sea bird is most often found wintering in the islands around Scotland. However 38% of the UK’s population is thought to winter in the Thames Estuary between Kent and Essex.
  • Redshank - Instantly recognisable by their bright orange legs, the redshank can be found wintering in nationally important numbers in the saltmarshes of the Thames Estuary. These are one of many species of sea bird that has increased in number since the introduction of two Special Protected Areas in the estuary.

Aquatic species

  • Atlantic salmon - These long distance migrants were one of the first to fall to modern industrialisation and were extinct from the Thames by 1833. When they started re-appearing in recent years it was originally thought to be the result of extensive re-introduction. This has since been questioned by research that suggests an improvement in water quality has attracted salmon from elsewhere and re-introduction efforts actually had minimal effect.
  • Bullhead - This small, bottom loving fish is often found in clear freshwater streams and rivers.
  • Common smelt - These small migratory fish were once abundant around the UK and were a popular food source. Although they are now relatively rare, a population of national importance is known to breed in the Greater London area. In 2013 ZSL concluded their 7 year-long Thames Fish Monitoring Program and now has a wealth of data on how populations of fish such as smelt have changed over time.
  • Depressed river mussel - This large freshwater mollusc has declined throughout much of Europe but still retains a foothold in UK Rivers. It can be found on the north bank of the River Thames between Richmond and Twickenham, but is threatened by invasive non-native mollusc species, which ZSL monitor annually.
    Find out more.
  • European eel - Once found in unimaginably vast numbers in rivers around Europe, the European eel is now listed as Critically Endangered. Since the 1980’s ZSL has recorded a 95% decline in eels returning to the Thames. . However small elvers (juvenile eels) can still be found swimming up the Thames each summer and ZSL has a major initiative in place to ensure they don’t become a thing of the past.
    Find out more.
  • Flounder - Closely related to other flatfish such as plaice, this primarily marine fish is largely found in the Thames Estuary but can also survive further upstream.
  • River/ Sea lamprey - Until recently these primitive jawless fish were all but extinct from the Thames. Increased sightings in recent years are a welcomed sign as they are known to only breed in relatively clean water.
  • Brown trout - Similar to salmon, brown trout have been returning to the Thames as a result of habitat restoration work and can now be found throughout much of the catchment. In 2011 the Environment Agency recorded a record number of brown trout migrating up the Thames to spawn.
  • Short-snouted seahorse - Perhaps one of the most surprising inhabitants of the Thames is this rare species of seahorse, more often associated with the warmer water of the Mediterranean. It is believed that a small breeding colony exists in the River Thames and ZSL Aquarium is currently running a successful captive breeding program for the species.
    Find out more.
  • Twaite shad - Closely related to the allis shad, the twaite shad is medium sized streamlined fish that only differs from its relative in the number of gill rakes. The twaite shad has been slow to recover from the pollution levels of the first half of the 20th century and is still not thought to breed in the Thames. In 2013 ZSL concluded their 7 year-long Thames Fish Monitoring Program and now has a wealth of data on how populations of fish such as the twaite shad have changed over time.