ZSL is monitoring Thames marine mammals to better understand and conserve these top predators.
Why we work to monitor marine mammals
The Tidal Thames is home to a number of easily recognisable and charismatic marine mammals, including harbour seals, grey seals, harbour porpoises and the occasional bottlenose dolphins. These species are important top predators in a complex estuarine environment and can be found throughout the estuary and river, including in central London. ZSL has been monitoring marine mammals in the Tidal Thames in a number of ways since the early 2000s, to better understand and conserve these species.
Annual seal population surveys
There are two species of seal in the UK, the harbour seal and the grey seal.
Harbour seal population growth has been punctuated by two local disease outbreaks of Phocine Distemper Virus (PDV) in 1988 and 2002. Since then, some harbour seal populations in Scotland 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.
When ZSL initiated annual seal population surveys in 2013, the seal colonies in the Thames were some of the least studied in the country.
The surveys are completed in August using aerial transects. This coincides with the harbour seal moult period, when harbour seals spend a greater proportion of their time hauled out on sand banks, allowing us to estimate the population size of this species and of grey seals in the Thames. By surveying year on year, we have been able to map critical habitat for the seals and follow trends in the seal population numbers.
In 2018, ZSL conducted the first harbour seal breeding survey in the Thames since the 2011 survey by the Sea Mammal Research Unit (University of St Andrews) and now conducts breeding surveys every other year. These breeding surveys take place in early July and provide an estimate of the number of harbour seal pups born in each year and their distribution across the Thames coastline and sandbanks.
Harbour Porpoise surveys
In 2015 ZSL partnered with Marine Conservation Research International to conduct the first harbour porpoise survey of the Greater Thames Estuary (read the published paper here). The survey monitored the Thames Estuary between Suffolk and Kent using visual and acoustic techniques. In 2022 our partnership conducted a follow up survey. These surveys have shown that the Thames Estuary is an important habitat with significant densities of porpoises present in the area, over mulitple years.
Since 2004, ZSL has encouraged members of the public to submit their sightings of marine mammals from the Thames and its tributaries in order to help us better understand their distribution.
Over 3,000 marine mammal sightings have been reported to ZSL from the Thames to date. These sightings have demonstrated that seals and porpoises are regular visitors in the Thames all the way up to Teddington lock, and are present in the estuary year around.
If you are lucky enough to spot a marine mammal in the Thames – enjoy it! Please report your sighting to help us learn more about, and conserve, Thames marine mammals and remember to keep your distance, both for your safety and so as not to cause the animal any stress. For more information on how to act around marine mammals please follow our code of conduct. If you are worried about the animal and / or want to understand what is normal and abnormal behaviour please follow our action plan.
Greater Thames Seal Working Group
The Greater Thames Seal Working Group (GTSWG) was set up in 2013, to provide a platform for collaborative work on the major issues facing harbour and grey seal populations in the region. The members of the group are all stakeholders in the Thames, coming from varied backgrounds from commercial fisheries, development, government agencies and environmental charities. To find our more about the working group please email Marineandfreshwater@zsl.org.
Marine Mammal Sighting Survey
Record seal sightings on our interactive marine mammal map
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.
Individual showing signs of an injury in the water or foreshore
Observable skin lesions
Individual tangled in net or plastic
Individual showing signs of malnutrition (i.e.: concave flanks, indented stomachs)
Individuals showing signs of laboured breathing (could be a sign of Phocine distemper virus)
Porpoises and dolphins
Individual on the foreshore
Individuals far upstream in the river
Floating (not actively swimming)
Individual showing signs of an injury
Individual tangled in net or plastic
Based on the information above, if you think the animal may be injured, ill or in distress, please phone 0208 855 0315 (24 hours). This will take you through to the Vessel Traffic Services of the Port of London Authority (PLA). They will send a nearby vessel to investigate and will coordinate an emergency response if necessary. The emergency response will be by British Divers Marine Life Rescue (BDMLR) or the RSPCA and may involve support from the Metropolitan Police marine unit, the London Fire Brigade river unit, the RNLI or the PLA. If the animal is sadly dead the Cetacean Stranding Investigation Programme will be called.
How you should behave around marine mammals
Please appreciate these intriguing animals from a distance. If you get too close you may cause them distress, or to panic and bite. When hauled out onto land or a pontoon seals can feel vulnerable and be easily startled. Noise and being approached by people, boats, kayaks or dogs etc. can cause seals to panic and move back into the water – this can cause them injury and exhaustion.
Please do not attempt to rescue a mammal. You may harm the animal or you may be harmed in the process. Also be aware that the Thames is a tidal river (as far as Teddington) with a tidal range of up to 7m. Waves from passing vessels can catch you unawares and the river has very strong and fast currents.
If you see a marine mammal in the water, many of the same rules apply. Please do not surround the animal with large numbers of people/watercraft and never approach marine mammals head on so that the animal has room to escape. Approach the marine mammals at low speeds and remain more than 100 metres away, avoiding driving through groups, never attempt to feed them and give mothers and calves extra room.
ZSL has an easy Code of Conduct to follow if you see marine mammals in the Thames.
Information for responsible wildlife watching operators is available here.
If you are worried about any wildlife crime, -the police have Wildlife Crime Officers that can deal with cases of crime against wildlife, including the killing, injuring and intentional or excessive disturbance to them. If you witness an incident then call the Police incident phone number (101) for advice and assistance or check out the wildlife crime website.
Tidal Thames Marine Mammal Partnership
This page has been produced by the Tidal Thames Marine Mammal Partnership which is a coalition of first responders, wildlife organisations and the harbour authority which aims to raise awareness of how to act around marine mammals in the Thames. The partners are :