A dolphin swam right past Putney Bridge this week. It never made it back to sea.

A dolphin died on the foreshore at Mortlake this week, days after it was seen swimming as far upriver as Putney and Hammersmith Bridges. It is the second time in two years that a dolphin swimming this deep into London has ended in a death, and new research suggests it will not be the last.

The animal, an adult male short-beaked common dolphin, was first seen in the tideway between Hammersmith Bridge and Putney Bridge on Tuesday 30 June. It was spotted again the next day off Strand-on-the-Green, near Chiswick. By Thursday 2 July it was found dead on the foreshore at Mortlake.

A Port of London Authority spokesperson said sightings this far upriver “can often indicate they are already in distress.” Teams from the Cetacean Strandings Investigation Programme (CSIP) and British Divers Marine Life Rescue (BDMLR) attended the scene. CSIP investigates every whale, dolphin and turtle death around the UK coast. CSIP’s Rob Deaville recovered the body and posted photographs from the foreshore, saying the dolphin had travelled further west up the Thames than any common dolphin on record. A post-mortem was scheduled for Friday; the result has not yet been published.

Not the first time

In the same post confirming the species, the organisation noted that this week’s death moves from one sad story into a pattern: “the last strandings recorded in the Thames were a pair of individuals in August 2024.”

Putney.news covered that earlier sighting live, when a dolphin was seen swimming past Putney Bridge in August 2024. Days later, it was found dead in Battersea, with a second found in Greenwich. Before that, it was back in 2017 that a dolphin died in the Thames.

A pattern in the literature

Why is a warm-water hunter dying in a cold London river? There’s a simple explanation: it’s no longer as cold as it used to be. A 2025 study of 836 UK-stranded common dolphins found infectious disease deaths rising sharply as sea temperatures climb, with every extra 1°C linked to a 14% jump in that risk. A build-up of banned industrial chemicals in their blubber adds a smaller risk on top.

Warm-water dolphins made up 82% of all UK strandings between 1990 and 2018, against just 18% for cold-water species, and that gap keeps widening as the water they can survive in moves north. CSIP’s Rob Deaville puts it plainly: the common dolphin is now, for the first time, the most commonly reported dolphin species stranding on UK shores.

Set against that backdrop, three Thames deaths in nine years, in 2017, 2024 and now, raise an obvious question: is this an early, local echo of a bigger shift, something Putney residents might see more of as summers grow hotter and lengthen?

A Thames dolphin sighting remain a rare event because of where dolphins live – out past Ireland toward the Atlantic. The result is a map of strandings with a heavy concentration in south-west England and west Scotland. The Thames entry is on the other side of the UK. But while three events cannot prove a Thames-specific trend, the sudden frequency taken alongside the increase in sea temperature and a notable increase in the number of dolphins in our waters with infectious diseases does suggest we may be seeing more of this.

What other countries do differently

The United States treats a rise in strandings very differently. Since 1992, US law has required a formal, government-coordinated response network, run by the National Oceanic and Atmospheric Administration with more than 120 partner organisations. When deaths spike unusually, the agency can declare an “Unusual Mortality Event,” a legal designation triggering dedicated federal investigation and funding, used more than 72 times since 1991.

The UK has nothing equivalent. CSIP is funded by the government to collate and report strandings data, but there is no statutory trigger for a “significant die-off.” Researchers say there is no globally recognised coordination of stranding responses anywhere in the world, a gap illustrated by a bird-flu outbreak that killed tens of thousands of seals in South America.

If you see a dolphin in the Thames

There is no dramatic rescue waiting to happen. As Julia Cable of BDMLR explained after the 2024 sightings, “there’s no way we can actually physically move it unless it were to strand.” All anyone can do is monitor the animal, keep other river users away and hope it finds its own way back out.

The advice for anyone who spots a dolphin, or any other Thames wildlife in distress, is simple: report it to BDMLR, keep your distance and keep noise down, since noise appears to make a disorientated animal’s situation worse.