The history of London is intimately tied with the River Thames that runs through its center. A major transportation route for all of the city’s history, it has also doubled as a sewer for much of the past. In fact, pollution in the river got so bad that in 1858 the government in Parliament had to suspend work due to the horrific smell, and by 1957, the river was declared “biologically extinct.” The amount of oxygen was so low that it could support no life.

Yet in a recent report from the Zoological Society of London (ZSL), based on public sightings over the past ten years, the Thames and its estuary are now thriving with marine mammals. In fact, there have been an astonishing 2,732 mammals spotted swimming the waters. Seals were the most common species seen, but porpoises, dolphins, and even whales have been recorded. This news highlights just how far conservation and the clean-up of the Thames has come, and demonstrates that even some of the busiest urban centers can be havens for wildlife.

^{Grey seal spotted swimming in the Thames. Credit: nmahieu/Flickr (CC BY-ND 2.0).}

“People are often surprised to hear that marine mammals are regularly spotted in central London,” explained Joanna Barker, European Conservation Projects Manager at the ZSL, and coauthor of the new report. “As a top predator, their presence is a good sign that the Thames is getting cleaner and supporting many fish species. The presence of these animals is also a great example of how urban environments are important for wildlife.”

Most people look at the Thames and see a brown, dirty river, but fish stocks in the waterway have slowly been recovering, and as these are getting healthier, the predators are following. In fact, seals have been spotted as far up stream as Hampton Court Palace, porpoises and bottlenose dolphins as far as Teddington Lock and Kew in west London, and whales at Gravesend in Kent (although one did famously make it as far as Battersea in 2006, it unfortunately died of dehydration).

The researchers have even created a map of all the recordings in the Thames, and amazingly the greatest density of sightings occurred in Canary Warf, central London, which averaged out at 138 sightings per square kilometer. These were mainly seals, both harbor and grey, and the authors suspect that the high number of people living and working in the area in tall buildings overlooking the river contributed to this being such a hot spot for marine mammal viewings.   

“We were pleased to see that harbour seals were some of the most commonly spotted mammals,” said Barker. “Their numbers have dramatically declined in some parts of Scotland, so the fact that they are frequently sighted in the Thames estuary confirms that the South East is an important area for their conservation.”

The stretch of river between the Houses of Parliament and the O2 Arena, one of the most visited and populated regions of the city, also had a huge density of sightings, though this is probably linked to the high number of pedestrians and bridges crossing the Thames at this point. Many people are unaware of just how biodiverse and important an ecosystem the Thames is, and if anyone happens to be lucky enough to spot a marine mammal splashing in the waterways of London, then they can report the sighting on the website here.