Mid hard times brought on by the novel coronavirus pandemic, one sight is putting a smile on people's faces in Portugal - dolphins splashing around near Lisbon's shores.

"I have loved dolphins forever," wrote a social media user left in awe at a video shared online of a group of dolphins jumping out of the water. "It is great to see them up close, and in our very own Tagus river!"

Though dolphins have been sighted in the Tagus since Roman times, the mammals are no longer seen often, according to a 2015 report by the Sea School and the Marine Science Association in Lisbon.

But over the last two months, social media channels have been alive with videos and images of dolphins shared by those lucky enough to catch sight of them leaping out of the waves.

"With the improvement in water quality, the river has been gaining new life and a friendly family of dolphins has been seen several times during the last month," Lisbon's mayor's office wrote on Facebook.The pandemic has halted the cruise ship industry and fewer commuter ferries have crossed the river.

"With the improvement in water quality, the river has been gaining new life and a friendly family of dolphins has been seen several times during the last month," Lisbon's mayor's office wrote on Facebook.

The pandemic has halted the cruise ship industry and fewer commuter ferries have crossed the river.

But marine biologist Francisco Martinho, who specialises in dolphins, said there was more to the story.

"It's not because the river has become more peaceful that dolphins are spending more time there," Martinho said. "It's because there are more fish than usual for them to eat."

Martinho said it was not clear why more fish were being found in the waters and said the dolphins were likely to leave in a few months if the fish run out.