Thames Seal Programme

Harbour Seal in the Thames

The Thames Estuary is home to harbour seals, also known as ‘common seals’, and grey seals, but they are some of the least understood seal populations in the country. ZSL is using seal telemetry (tagging), annual population surveys and a public marine mammal sighting survey to gather data to better inform conservation and management. This information will form an integral part of the newly formed Greater Thames Seal Working Group.

Why we are there

Research conducted by the Sea Mammal Research Unit (SMRU), has shown dramatic declines in some Scottish harbour seal populations. The reason for these declines is unknown and may be as a result of shifting prey species, disease, disturbance or competition with the grey seal. The Thames Estuary harbour seal population is becoming increasingly important at a national level and represents 12% of the English harbour seal population, but is one of the least understood seal populations in the country.

Annual Harbour Seal Population Survey

In August of each year, ZSL carry out a harbour seal population survey in the Greater Thames Estuary using a combination of aerial, boat and land-based transects. The survey covers from Felixstowe (Suffolk) to Deal (Kent) and is timed to coincide with the harbour seal moult period, where seals spend a greater proportion of their time hauled out on sand banks.

The annual survey is essential to estimate the size of the Thames Estuary harbour seal population and to detect any changes in population size over years. Watch the 2013 population survey video.

Harbour Seal Tagging Project

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 Thames Estuary. The data has provided us with crucial information on the most important habitats for harbour seals in the Thames Estuary, which we can use to better inform conservation and management.false

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. The Greater Thames Seal Action Plan (GTSAP) was written by the GTSWG to identify and address the major conservation and management priorities for seal populations in the Greater Thames Estuary. Find out more about GTSWG.

Marine Mammal Code of Conduct

The marine mammal code of conduct for the Thames Estuary was jointly developed by ZSL and GTSWG, to provide advice on what to do when you see a marine mammal in the water or on land.

View the full code of conduct

Public Marine Mammal Sighting Survey

The Thames Estuary is home to harbour seals, grey seals, harbour porpoises and sometimes even dolphins and whales. Since 2004, ZSL has been collecting public sightings of these marine mammals, to help us understand how they use the Thames Estuary. If you are lucky enough to see a marine mammal, please report a sighting.



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)

People Involved

Joanna Barker is the project manager of ZSL's Thames Seal Programme

Partners and sponsors

  • Kent Mammal Group
  • Sea Mammal Research Unit

​Kindly funded by:

  • SITA Trust
  • BBC Wildlife Fund
  • Sea Changers



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. 

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