The species is migratory and their range stretches up to the Arctic in summer and down to the tropics in winter, but this whale, which is approximately 3-4m (10-13ft) long, is far off course and the outlook is not good.
It is thought the minke whale’s trip up the Thames is the furthest a whale has swum up the river – it is currently well over 50 miles from the sea.
Whales have previously entered the river – the second longest in the UK after the Severn – with mixed results.
In 2018, a beluga whale, nicknamed Benny, was repeatedly sighted in the Thames over three months, however he did not swim far up the estuary and was mainly seen feeding in the Kent stretch during November and December. It is believed he eventually left the river of his own volition and went back out to sea.
In September 2009, a young humpback whale was found dead in the Thames at Dartford in Kent.
And in 2006 a juvenile northern bottlenose whale, who became known as Willie, swam up the Thames to central London, attracting thousands of people who gathered to witness the spectacle. Unfortunately the whale died during a rescue attempt. Its skeleton was given to the Natural History Museum.
But why do whales, which feed in the open ocean, swim into river systems?
Dr Peter Evans, the director of marine environmental charity Sea Watch Foundation, told The Independent it was partly a mystery.
“We don’t exactly know why whales sometimes do this and there are probably different reasons in different cases.
“Sometimes whales and dolphins are almost certainly following prey – usually shoals of small-sized fish – but they may also enter shallower waters if they are in difficulties, for example, poor health which may force them to keep near the surface, or navigation error.”
He said it’s not common behaviour but it does occur, and pointed out that there was another minke whale a few weeks ago which was found dead in the river Trent, which flows into the Humber.
“We’ve had cases of humpback whales and belugas entering the Thames Estuary,” he said.
“The humpbacks usually die but to our knowledge the beluga whale simply moved away after spending many weeks in the area.
“Some species are clearly a long distance from their normal habitat and range. That applied to the beluga whale but also to the northern bottlenose whales and to cases of sperm whales that have stranded in the southern North Sea.
“The two latter species normally live far offshore in deep waters, whilst belugas normally inhabit Arctic waters. A few years ago, a narwhal (a high Arctic species) was found in the river Scheldt in Belgium.
In those cases, he said, the mammals likely “first entered the North Sea following prey but then as they travelled further south they lost their way.”
One of the problems facing these species in this region, he added, “is that not only is it very shallow but there are not many good navigational cues so they can become ‘trapped’ in the area and possibly swim upriver in an attempt to find a way out.”
Danny Groves from Whale and Dolphin Conservation, told The Independent that while the behaviour isn’t normal, it is far from uncommon around the world.
He said: “Unfortunately, thousands of whales and dolphins and porpoises get into difficulty across the globe every year, some after swimming into estuaries and then upstream into rivers.
“Many later die on shorelines and the reasons for this are varied. For some this may be due to natural causes, some undoubtedly because of human activity.”
“Many whales and dolphins get into difficulty because they may have been struck by a vessel at sea, injured in fishing nets, driven off course by loud underwater noise from seismic surveys for oil or gas, or loud underwater sonar from military exercises.”
Speaking about the minke whale currently in the Thames, he said: “It is rare and very worrying to see a whale like this minke get into difficulty in a river. This whale could have become lost whilst following prey, or could be ill or injured.
“This poor whale is way off course and will still face a battle to get back down river and out into the sea.”
“Once back out, this young whale will still face numerous threats at sea. It is a sad irony that as we try to save this minke, only days ago Norwegian whalers announced that they had killed nearly 50 minke whales in just 13 days as their annual hunts got underway.”
Asked if human activity could have caused the whale to enter the river, Mr Groves said: “Whales live in a world of sound, navigating, communicating and feeding using echolocation. Any human interference with this world can have terrible consequences for them and once stranded they can die very quickly unless helped back out to sea.
He added: “What is certain is that whales play an important role in keeping the ocean healthy and in mitigating the effects of climate change. As our allies in that fight we need to save each and every one of them.”