Two dolphins have tragically been reported dead on the banks of the River Thames in London after one was spotted swimming in the river late last week.

The unexpected guest to the capital’s waterways was spotted multiple times from late morning on Thursday August 1, and according to witnesses appeared to be “struggling with the tide” as it made its way through the water.

Yet just days later it was confirmed that a dolphin had washed up on the bank of the river near Chelsea Harbour Pier, after having sadly died.

In a further blow, another dolphin was also found dead on the riverbank at Greenwich, but it is not yet known when it died, reports the BBC.

Discussing the tragedy, a Port of London authority spokesperson said: “We are aware of reports of two dolphins that sadly died. We are working with the Cetaceans Strandings Investigation Programme and will recover the animals.”

In the run up to the dolphin’s death, the British Divers Marine Life Rescue (BDMLR) had been tracking its movements and noted that it was still active in the area on Friday and added that “its behaviour wasn’t really changing.”

However, it was also noted: “It’s too far really for a dolphin to be up the river.”

Mary Tester, a Thames area coordinator with the BDMLR, told BBC London that there were a “lot of different theories” on why dolphins were “coming down the river”, with one suggestion being the improved water quality of the river increasing the level of fish, a vital source of food for the animals.

While dolphins in the Thames are still a relatively uncommon occurrence, sightings are becoming more common and members of the public who spot a dolphin swimming in the Thames, or any inland waterway, have been urged to alert the BDMLR and avoid following it in boats or canoes.