It might still have a reputation for being mucky and unpleasant, but the multiple conservation efforts concentrated on the Thames Estuary appear to be bearing fruit. The Zoological Society of London (ZSL) reports that in the last ten years, citizens have notified them of more than 1,000 marine mammals sightings across the Greater Thames Estuary.

The Thames might not yet be in perfect health, but it’s had a remarkable revival from its former state of misery. Damage to London’s sewage system caused by World War II, expanding industrial effluent, and rising water temperatures in the first half of the twentieth century choked the river to death. By 1949, there was a 26-mile (42km) oxygen-free, borderline-dead stretch of the river, writes marine ecologist Martin J. Attrill in his book A Rehabilitated Estuarine Ecosystem.

After decades of concerted effort, things are looking much better, especially if by “better” you mean “cute seal faces at Canary Wharf.” According to the ZSL, this is one of the marine mammal-watching hotspots, although we can probably attribute this to there being more people at Canary Wharf to report sightings, artificially inflating the numbers.

Since 2004, when the ZSL started its public survey, 1,281 “complete and valid” sightings of marine mammals in the Greater Thames Estuary have been submitted. Pinnipeds (seals) made up slightly less than eighty percent of the sightings, with most of the rest being harbour porpoises, dolphins, and whales. The remaining sightings were of otters.

The 2,732 marine mammals reported across the sightings weren't necessarily 2,732 individual animals—many individuals are likely to have been spotted multiple times, so actual population numbers are still unknown.

Obviously, the public might not be the most reliable zoological source; people also report seeing Nessie. Still, public data has its uses, most notably by helping researchers to pinpoint the regions where they should conduct more robust surveys. Cleaner data on seal and porpoise populations is currently being gathered by various conservation efforts.