French animal health authorities are concerned that the Beluga whale which appears to have strayed into the River Seine is underweight and has suffered 'skin changes'. Belugas can survive in fresh water, but not indefinitely.
Beluga whales are usually found in Arctic waters. This individual was spotted in the Seine, swimming up-river earlier this week, and has reached a river traffic control lock about 70 kms from the French capital.
The whale was first sighted on Tuesday in the river that flows through Paris to the English Channel.
French rescue services as well as firefighters and biodiversity officials mobilised swiftly and kept a close eye on the whale throughout the day to evaluate the "worrying" health of the mammal, the local police service said.
Officials added that the whale seemed to have suffered "skin changes and to be underweight".
It is "currently between the Poses dam and that of Saint-Pierre-la-Garenne", around 70 kilometres northwest of Paris.
'A very noisy river'
Gerard Mauger, deputy head of French Marine Mammal Research Group GEEC, said the mammal spent "very little time on the surface" and appeared to have "good" lung capacity.
But Mauger said rescuers were struggling to guide the whale to the mouth of the Seine.
An adult beluga can reach up to four metres in length.
Authorities in Normandy's Eure department have urged people to keep their distance to avoid distressing the animal.
Lamya Essemlali, head of the non-profit marine conservation organisation Sea Shepherd, said some of her team would arrive with drones in the evening to help locate the whale.
"The environment is not very welcoming for the beluga, the Seine is very polluted and cetaceans are extremely sensitive to noise," she said, adding that the Seine is "very noisy".
Food the most urgent necessity
In late May, a killer whale -- also known as an orca, but technically part of the dolphin family -- was found dead in the Seine between the port of Le Havre and Rouen.
Killer whale lost in France's River Seine found dead
The most urgent need is to feed the beluga to prevent it from suffering the same fate as the orca which seems to have starved to death.
The Eure authorities said lone belugas do sometimes swim further south than usual, and are able to temporarily survive in fresh water.
While they migrate away from the Arctic in the autumn to feed as ice forms, they rarely venture so far south.