Along the border between Norway and Finland lies the world’s greatest Atlantic salmon river. To the Norwegians, it is the Tanaelva or Tana; the Finns call it the Tenojoki or Teno. But to both countries it is known as one of the purest and cleanest rivers, passing through largely unspoiled and unpolluted regions from Finnmark, Norway’s northernmost county, into Lapland.

Now, however, the river’s waters and banks are crowded with rotting fish, their skin peeling away, and the air is thick with the odour of decaying flesh. “It’s a mess. It’s quite dramatic, says Aino Erkinaro, a doctoral researcher from the University of Oulu in Finland. “And, oh man, it smells so bad now.”

The carnage is the result of an invasion – not of Atlantic salmon but Pacific pink salmon, which has prompted both Norway and Finland declare an emergency. Also known as pukkellaks or humpback salmon, for their characteristic bulge, pink salmon were introduced into the Kola peninsula in Russia in the 1950s in an attempt to stimulate the economy. They live in oceans and swim up the same rivers every two years to lay their eggs and then die.