Ukrainian President Volodymyr Zelensky on Wednesday accused Russia of "ecocide" for the devastation he said its invasion has wrought on Ukraine's wildlife.
Thousands of dead marine mammals had washed up on the shore of the Black Sea, he said, including tens of thousands of dolphins.
"And this is only a small part of the devastating consequences," he added.
"The Russian war has a huge impact on the wildlife of our country," Zelensky added, denouncing Russia's "ecocide in Ukraine".
The president posted a report by United24 Media, a government initiative to raise donations, about a national park in the Black Sea region of Odessa.
"We estimate that at least 50,000 dolphins have died as a result of the war," said Ivan Rusev, research director at the Tuzly Estuaries National Nature Park.
This would amount to 20 percent of the overall Black Sea dolphin population, he said.
Many scientists, including Rusev, have blamed military sonar used by Russian warships for the disaster.
In August, Rusev, speaking to AFP, estimated that 5,000 cetaceans had been killed -- about 2 percent of the total dolphin population in the Black Sea.
The Black Sea was home to an estimated 2 million dolphins during the 20th century, but fishing and pollution contributed to their decline.
A survey found there were about 250,000 dolphins left in 2020.
