Drought in summer: Fish kills in the Rhine – good news at last

Science drought in summer

Fish deaths in the Rhine – good news at last

ARCHIVE - August 18th, 2022, Rhineland-Palatinate, Bingen: A children's bicycle covered with dried mussels and algae lies on the rocks of the largely dried-up riverbed of the Rhine.  (to dpa

The Rhine near Bingen in August. And where are the fish?

Source: dpa

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In the extremely dry summer of 2022, the water in many rivers had receded. Shipping on the Rhine also had to be partially suspended. Scientists have now investigated the consequences for fish stocks.

Dhe extremely low water levels in the Rhine during the long, dry and hot summer of 2022 severely affected shipping – but according to experts, did not cause any massive fish kills. The river has apparently largely recovered ecologically.

“This year it wasn’t as dramatic as in the summer of 2003, when we died Shellfish fell and fish carcasses swam past,” said Jörg Schneider from the Office for Fish and Water Ecological Studies in Frankfurt.

In the summer of 2022, the low water level of the Rhine warmed up to around 26 degrees – salmon and trout for example, would often just have survived this. Fish would also have sought “refuges” in somewhat less hot water zones. However, barges could load significantly less without risking grounding.

ARCHIVE - July 19, 2022, Rhineland-Palatinate, Oberwesel: A cargo ship passed a sandbank on the Rhine in July 2022 (aerial photo with a drone).  According to experts, despite the long-lasting summer heat and extremely low water, there have not been any major fish kills.  - (to dpa

Restricted voyage: A cargo ship bypasses a sandbank in July

Source: dpa

Zoology professor Jochen Koop from the Koblenz Federal Institute for Hydrology explained that, according to a rule of thumb, for a widespread fish die-off in the Rhine, a water temperature of 25 degrees would have to be exceeded for at least 40 days in a row – that was not the case this year.

The head of the secretariat of the International Commission for the Protection of the Rhine in Koblenz, Marc Daniel Heintz, said: “We were relieved that after the long drought it rained a lot in September.” In several years the water level was lowest in autumn – this time, however, it did not fall further, but rose again significantly.

At low tide, the fairway for ships becomes narrower, fish have less space and, according to experts, can probably be more easily pulled into the ship’s propellers by the strong suction of large freighters.

Fish kills in Germany

The biologist Schneider explained that in tributaries such as the Moselle and Main, the water temperatures in the summer were about two to four degrees above that of the Middle Rhine. Typical for very warm waters without much current is the formation of blue-green algae. These are actually bacteria (cyanobacteria).

In parts of the Moselle, they spread more strongly than ever before in the summer of 2022. They can produce toxins that endanger human and animal health. Blue-green algae have only appeared in the Moselle since 2017.

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