For grayling, trout and whitefish in Lake Constance and the Rhine river, the current temperatures already pose an existential threat.
The unusually warm summer – June was the hottest for a century in Switzerland – has set alarm bells ringing among conservationists and administrative bodies. The specter of 2003, when dead fish lined the banks of Lake Constance and the smell of rotting fish became a mainstay of lakes in Swiss villages, looms once again because of the unusually hot summer temperatures.
"We are not far off experiencing the same catastrophe again," Philipp Sicher, managing director of the Swiss Fisheries Association, told Swiss daily Blick.
The water temperature in Lake Constance is currently 25 degrees Celsius and a mere two degree increase would mean that most fish species would no longer be able to survive. In some areas, fish can be relocated, but in others only heavy rainfall can determine their fate.
"Unlike in the midlands, where we have the opportunity in rivers and smaller rivers to fish out threatened species and relocate them to cooler waters, we lack this opportunity in the Rhine and Lake Constance," added Sicher.
While bream, roach and carp are at home in such tropical temperatures, most commercial fish specimen cannot survive above 27 Celsius.
The scorching summer is only the latest woe for Swiss fish. Thousands of fish die every year in Swiss lakes because of more than 1,000 obstacles created by the country's hydroelectric plants' turbines, reports local daily Solothurner Zeitung.
The hydroelectric plants have been ordered to remodel their infrastructure by 2030 and nearly 1 billion Swiss francs, alongside taxes generated through electricity, have been set aside for the task.
"I fear that the renovations will be postponed endlessly because the money is missing," Sicher told Solothurner Zeitung, adding that the government is neglecting its conservation obligations towards fish species.
A petition by Aqua Viva, the WWF and the Swiss Fisheries Association aims to pressure the government and has already collected 25,000 signatures.
Migratory fish such as eel and trout are particularly affected. As a result of more than 1,000 artificial obstacles created by the hydrolelectric industry, "the fish are cut off from their seasonal spawning and feeding grounds," states the fishing association's petition.
Approximately 70 species of fish live in Switzerland's water bodies, of which 54 are indigenous, according to fishfinder.ch.
"We are running out of time. At some point it will be too late for our already highly-endangered Swiss fish," said Sicher.
Only a dramatic increase in rainfall can remedy the situation, added Sicher.