Almost 600 harbour and 3,000 grey seals now call the Thames Estuary home, reveals our latest seal survey highlighting the importance of the iconic river for these doe-eyed, native species.
We have been monitoring the grey and harbour seal populations in the Greater Thames Estuary since 2013, and this year’s count – completed by teaming up with military helicopter pilots based at RAF Shawbury – revealed how these two important apex species are doing.
Conservation Project Officer Hannah McCormick, who led the survey, said: “During the 2024 seal survey, we counted 431 harbour seals and 714 grey seals laying out on sandbanks along the Thames. By combining these with the number of seals estimated to stay in the water during the counts, we can estimate that there are a total of 599 harbour seals and 2,988 grey seals. These numbers are key to helping us understand how these two species are doing – alongside feeding into our understanding of how seals across the country are faring and giving us a glimpse at the important habitat the estuary provides. A flourishing ecosystem has enough food, shelter and space for all of its inhabitants, and although we still have a way to go in terms of improving the health of this important habitat, as top predators, this seal population show us that there is good habitat available to the species that call the Thames home.
Taking to the air and sea, the team conducted the surveys by both observing the seals from boats along the estuary and joining training flights with RAF Shawbury’s No 1 Flying Training School to cover the Greater Thames Estuary coastline and sandbanks between Suffolk and Kent.
How do conservationists monitor seals?
Hannah explained: “Seals are playful but shy creatures, so using a long-range lens to take photos from a distance allows us to maximise our counting accuracy while also minimising any disturbance – helping us build the strongest possible understanding of how these native species are faring in the Thames, and highlighting the importance of protecting this ecosystem.”
Supported by No 1 Flying Training School and St. Andrew’s Sea Mammal Research Unit, the count was completed over several days in August during the moulting season for the harbour seals. Over the course of these weeks, the seals will spend much of their day basking on the estuary’s sandbanks, making it easier for the survey team to spot them.
Group Captain Andy Baron, Commandant of No 1 Flying Training School at RAF Shawbury said: “The RAF and MOD are delighted and proud to have been able to contribute to a conservation project of such significant scientific importance. Supporting the ZSL project has provided a unique opportunity for military helicopter aircrew to develop core and niche skills which will further benefit Royal Navy, British Army and Royal Air Force staff and students who train at the world-leading No 1 Flying Training School.”
The information gathered by the survey will feed into St Andrew’s Sea Mammal Research Unit’s database working to understand the status of seals across the UK.
Which seal species live in the Thames?
The Thames is home to both harbour and grey seals, although harbour seals are the only one of the two to breed in the area. They live alongside a range of other wildlife that call the river home -including the the short-snouted seahorse and the critically endangered European eel – many of which our conservationists are working to monitor and recover.
This year’s survey was consistent with results from the last few surveys, with 692 harbour seals and 3134 grey seals being estimated in 2021. Hannah explained: “We’ve seen UK seals make an incredible recovery from the early 2000s, when high rates of distemper virus led to steep declines in numbers. Results since 2018 suggest there has been a decline in harbour seals, which has also been seen in other harbour seal colonies in the east of England. While the causes of these declines remain unknown, experts are investigating potential factors and by keeping a close eye on these changes; we will continue to build our knowledge of seals in the Thames while contributing to long-term regional and national data on seals.”
A ZSL-led report previously revealed that although declared ‘biologically dead’ in the 1950s, conservation efforts have led to the Thames once again becoming home to a wide variety of British wildlife including seals, seahorses and critically endangered eels, alongside evidence for an increase in the rise of important habitats such as carbon-capturing salt marsh.
Hannah added that it is key that decision-makers in the UK continue to support ongoing work to restore this important marine habitat. She said: “These surveys are just one small part of our work at ZSL to understand the state of wildlife in London’s rivers and out to the Thames Estuary. This is one of the busiest marine environments in the world, and yet it remains extremely important for wildlife. From tackling pollution to planting biodiversity-boosting seagrass, we’re working to make these waterways healthy and resilient to future change. It is vital that the health and recovery of our waterways is a key government priority.”
Members of the public can support ongoing work to understand and protect seals and other mammals living in the Greater Thames Estuary by reporting live sightings to the Thames Marine Mammal Survey, avoiding disturbing seals on land or in the water, and reporting dead sightings to the ZSL-led Cetacean Strandings Investigation Programme, which investigates stranded whales, seals, marine turtles and sharks in England and Wales.
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