Thames Marine Mammal Conservation

Seals spotted during the 2016 seal survey

ZSL is monitoring Thames marine mammals to better conserve these top predators.

Why we work to monitor marine mammals

The Tidal Thames is home to a number of marine mammals, including harbour seals, grey seals, harbour porpoises and pilot whales. These species are important top predators in a complex estuarine environment and can be found throughout the region, including in central London. ZSL has been monitoring marine mammals in the Tidal Thames since 2004, to better understand and conserve these species.

Distant seals spotted during the 2016 survey
Public marine mammal sightings

ZSL relies on the public to submit sightings of marine mammals to better understand their distribution in the Tidal Thames.

ZSL has been running the Thames Marine Mammal Sighting Survey since 2004, so if you spot a seal, porpoise, dolphin or whale please report your sighting to help our conservation work. If you are lucky enough to see a marine mammal, please follow our code of conduct during your sighting.

In the first 10 years of the survey, over 1,300 marine mammal sightings were reported to ZSL, find out more about our results here .

If you see a dead or stranded marine mammal, please inform the Cetacean Strandings Investigation Programme.

Seal survey 2016

ZSL’s focus on the Thames seal populations

There are two species of seal in the UK, the harbour seal and the grey seal; over the last 15 years we have seen substantial changes in their numbers. Harbour seal population growth has been punctuated by two epizootic disease outbreaks (Phocine Distemper Virus (PDV)) in 1988 and 2002.

Since then, some harbour seal populations have shown further worrying declines, the cause of which is currently unknown. Grey seal numbers, however, have rapidly increased over the last 15 years, especially on the East coast of England. It is thought that this rapid expansion is due to grey seals colonising new coastal breeding sites as they are no longer hunted by humans for their fur.

Seal survey 2016

Annual seal population surveys

ZSL initiated annual seal population surveys in 2013, so that any changes in the number of seals using the Tidal Thames could be identified. Prior to these surveys, seals in the Tidal Thames were some of the least studied in the country.

The surveys are completed every August using aerial, boat and land based transects. This coincides with the harbour seal moult period, where harbour seals spend a greater proportion of their time hauled out on sand banks, allowing us to estimate the population size of this species. Find out more about ZSL’s latest survey results through our report and video.

Harbour seal tagging

In 2012, ZSL tagged ten harbour seals in the Thames Estuary with GPS GSM tags, to gather information on harbour seal movements, haul out sites and foraging areas in the Tidal Thames Estuary. The data has provided us with crucial information on the most important habitats for harbour seals in the region, to inform conservation and management. Read about all the results here

Greater Thames Seal Working Group

The Greater Thames Seal Working Group (GTSWG) was set up in November 2013, to provide a platform for collaborative work on the major issues facing harbour and grey seal populations in the region. GTSWG members work together to achieve the actions set out in the Greater Thames Seal Action Plan (GTSAP). Find out more about the GTSWG here

Interactive Seal/Marine Mammal Map Screenshot

Marine Mammal Sighting Survey

Record seal sightings on our interactive marine mammal map

Project information

Key Species

  • Harbour seal (Phoca vitulina)
  • Grey seal (Halichoerus grypus)
  • Harbour porpoise (Phocoena phocoena)
  • Bottlenose dolphin (Tursiops truncatus)
  • Common dolphin (Delphinus delphis)
  • Pilot whale (Globicephala macrorhynchus)

People Involved

Joanna Barker manages ZSL’s work on marine mammals in the Tidal Thames


  • Agence des Aires Marines Protégées
  • Bramley Associates
  • Cetacean Strandings Investigation Programme
  • Crown Estate
  • Environment Agency
  • Essex Wildlife Trust
  • Harwich Fishermen’s Association
  • Kent and Essex IFCA 
  • Kent County Council
  • Kent Wildlife Trust
  • Kent Mammal Group
  • London Array 
  • Maldon District Council
  • Marine Management Organisation
  • Metropolitan Police Thames Unit
  • Natural England
  • Nature Break Tours
  • New Under Ten Fishermen's Association
  • Port of London Authority
  • Sea Mammal Research Unit
  • Southend Council
  • Thames Estuary Partnership
  • Thanet District Council
  • Thanet Fisherman's Association

Map of the Greater Thames Estuary


The Greater Thames Estuary is home to a population of harbour seals, also known as ‘common seals’, and grey seals, which utilise the marine and coastal environment.  In light of the significant amount of economic activity in the region (fishing, energy production, shipping, tourism, and recreation), but also recognising the conservation status of seals, it was agreed to establish a working group comprised of stakeholder representatives from all sectors. The Greater Thames Seal Working Group (GTSWG) aims to better understand and manage the interactions between the seal populations and human activities in the Greater Thames Estuary.

Purpose of the GTSWG:

  • Facilitate dialogue and information sharing on all matters concerning seals in the Thames, with a focus on the status of harbour and grey seal populations; interaction with industry; future plans for development; and conservation designations.

  • Identify priorities for research and monitoring, to better understand the Thames seal populations and interactions between seals and human activities.

  • Ensure that the seal populations are appropriately considered when drafting and implementing Government legislation.

  • Work with DEFRA to look at reviewing current policy and legislation with regard to seals. 

 Greater Thames Seal Action Plan (GTSAP):

The Greater Thames Seal Action Plan (GTSAP) was drawn together after the inaugural meeting of the GTSWG in November 2013. It outlines the main outputs GTSWG want to achieve between 2014 and 2019, dependent on funds being secured. File Greater Thames Seal Action Plan (188.96 KB)

How to join the GTSWG:

Membership of the working group is open to all interested stakeholders with the majority agreement of the founding members. Please email for more information