On Norway's vast salmon farms, a parasite is causing mass fish death. Salmon lice have gotten out of control, infesting not just farmed salmon but also the wild salmon that swim between the sea and Norway's rivers. The parasites are finding their way into open water from the open-net cages of off-shore salmon farms, where millions of fish swim.

Fishing enthusiasts, scientists and environmentalists are fighting to preserve the wild salmon as well as the culture it represents. In the autumn, aquatic biologist Helge Skoglund dives into Norway's icy rivers to help the government monitor wild salmon numbers as the animals swim from the sea to their spawning grounds upstream. Their numbers have fallen from over one million to less than 400,000, and experts fear a new low.

"I'm worried that there won't be enough salmon to ensure the stability of the population long-term," said Skoglund. The falling numbers mean many rivers are closed to fishing. On around 1,000 salmon facilities, farmers can no longer control the proliferation of salmon lice.

According to the Norwegian Seafood Council, the average fish mortality rate is around 17%. But salmon farmer Thomas Myrholt has developed an ecological alternative in the form of tanks that are closed off from the open sea. He said his stocks are free of salmon lice and hopes new regulations will be introduced for more sustainable farming.