In 1240, a whale passed from the English Channel into the mouth of the River Thames. Upon noticing the animal, people chased it through the river, past London, all the way to a town called Mortlake, where it was trapped and butchered. At nearby Battersea, a seal was shot in 1869, and a bottlenose dolphin was riddled with bullets and missiles in 1918.

The Thames has not been a good place for marine mammals in the past. Aside with overt aggression, wildlife in the river has had to battle pollution, human waste, industrial runoff and more.

But things have changed. For one thing, the river is not quite as dirty as it once was. And our attitudes towards wildlife have also improved – when a young whale became stranded near London in 2006, a two-day rescue attempt was mounted. Today, the Thames and its estuary are home not just to the occasional whale, but also a fairly permanent collection of seals and porpoises.

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Image: ZSL

Until 2004, not much was known about the marine mammals in and around the Thames, but the Zoological Society of London (ZSL) began an effort that year to document the wildlife there – and they relied on citizen scientists to help them. Now, a new report describing the findings from a decade of marine mammal monitoring in the Greater Thames Estuary has been released.

Most of the data in the study came from regular people who spotted the occasional seal or porpoise and submitted the information to the researchers online, though some of it came from folks with a bit more expertise – birders, anglers, tour boat operators and wildlife officials.

Over the decade that ZSL has run the Thames Marine Mammal Sightings Survey, they've collected 1,281 sightings, including 2,732 individual mammals. Most of the animals recorded (81%) were pinnipeds: harbour seals and grey seals. The rest were cetaceans: mostly harbour porpoises, but also the occasional dolphin or whale. And in ten years, at least three otters were spotted as well.

And the sightings weren’t limited to areas where the river meets the sea. Even in central London the water remains a fairly brackish mix, which helps explain why Canary Wharf, located in the city's leading business district, is a great place to spot seals. 

Studies like this one are important because they can help inform conservation efforts and wildlife management plans. For example, the report shows that marine mammals are around throughout the year and quite far upstream, so future plans for use of the river or its shore should be made with the resident wildlife in mind. The presence of apex predators also means that there’s enough food around, which itself means the river is becoming cleaner.

Information about wildlife hotspots like Canary Wharf is also useful for education and outreach efforts, both to reduce the chances of negative human-wildlife interactions and to encourage more people to submit their sightings to ZSL’s projects. The findings highlight the ways in which animals and people can coexist, even in some of the most developed cities in the world.

“Many people are unaware that the Thames is a biodiverse ecosystem that supports a variety of species, including top predators,” write the authors of the report.

If you've seen a marine mammal in the Thames, submit your sighting here.

Top header image: Lars Plougmann/Flickr