With water temperatures exceeding 27 degree Celsius (80.6 Fahrenheit), large numbers of fish have died in the Rhine. The cold-loving grayling - a member of the salmon family - has been particularly hard hit in the Schaffhausen region in northern Switzerland.
More than a ton of dead graylings has already been recovered from the Rhine, the fisheries administration of the canton of Schaffhausen told the media on Monday. A short distance away, at Lindli, lifeless fish drift on the surface of the river.
The spike in fish fatalities suggests the rescue measures taken so far are proving insufficient. The temperatures are simply too high. The section between the lower part of Lake Constance and the Rhine Falls is the most affected. Authorities across riparian cantons hope that the record temperatures of 2003 will not be repeated.
The Swiss Fishing Federation is also concerned about the fate of fish in the waterways of the Central Plateau, another habitat for the graylings.
Water temperatures above 20 degrees Celcius (68 Fahrenheit) are a stress factor for this species. From 23 degrees on, the situation becomes critical. Temperatures 25 degrees and above, especially for extended periods, amount to a death sentence.
The waters of the Rhine currently reach up to 27.6 degrees in some aeras. To avoid unnecessary disturbance of the river fauna, a wooden footbridge overlooking the Rhine Falls, below which fish have found refuge, is currently closed to visitors.
Elsewhere, in Ellikon am Rhein (ZH), cold water basins have been set up in cow water troughs to accommodate fish. But some local species have not been spared. Dead eels were found in the Zurich municipality of Rheinau.
Several of these measures had been developed following the hot summer of 2003, when nearly 90% of the grayling population had perished. "We have done everything in our power," said Samuel Gründler of the Swiss Fishing Federation.
The fisheries organisation has criticised German fishermen for targeting the fish as catch rather than contributing to save them. In many cantons of the Swissl Plateau, rescue missions have already been carried out. Fishermen save fish from drying streams and "relocate" them elsewhere.
Further downstream, at Basel in particular, no massive fish deaths were observed. Fishing "patrols" are in place to monitor the situation. Last week, entire sections of the river were closed to protect stressed fish.