Grayling almost extinct Warm lakes and rivers are deadly for many fish
Water temperatures in Switzerland are also rising due to climate change. This is a deadly danger for many fish, but it is not the only reason why three quarters of native species are endangered.
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- Rising water temperatures endanger fish such as grayling and trout.
- Warmer lakes also disrupt the food chain because they produce less plant plankton and ultimately less animal plankton.
- Three quarters of Switzerland's native fish species are endangered and some are acutely threatened with extinction.
Just how bad things are for grayling in Switzerland becomes clear when they die of heat death. "Tens of thousands died in the summer of 2003, thousands in 2008 and a few hundred in 2022," summarizes David Bittner from the Swiss Fishing Association. Today, the grayling is considered critically endangered.
Like the grayling and the majority of species native to Switzerland, brown trout are also cold-dependent fish. Their preferred water temperature is a fresh 15 degrees. Bittner describes: "At 18 degrees they are stressed and eat less, although they need more energy. At 22 degrees they stop feeding, which they don't survive for long, and at 25 degrees they die directly in water that is too warm for them," describes Bittner.
Cold water contains more oxygen than warm water. The different fish species have adapted to different oxygen levels and therefore only feel comfortable in the corresponding water temperature. "What's more, as alternate-blooded animals, fish adapt to the temperature of the water. Unlike mammals and humans, they cannot keep their body temperature stable," adds Bittner.
There is no escape in heated rivers
Recently, thermometers in Lake Zurich measured a water temperature of 28 degrees at the surface. "This is no problem for lake fish such as perch and whitefish. They simply dive into deeper layers where the water is colder," explains Zurich professional fisherman Adrian Gerny. It becomes difficult in shallow rivers and streams if they have no cooler areas - shady sections or colder tributaries. If the water is too warm everywhere, mass grayling die-offs occur, for example.
In order to save fish, authorities and fishing clubs repeatedly resort to emergency fishing. The endangered fish are stunned, collected and released at a suitable location - often because streams are drying up. "However, this is an absolute emergency measure that also means extreme stress for the fish." Some do not survive the relocation.
Running waters are generally colder than lakes. But even in these, water temperatures are rising as the summers get hotter. The Federal Office for the Environment cites the Rhine near Basel as an example, which was more than two degrees warmer on average in 2021 than in 1960. "The long-term development of water temperatures in Swiss waters shows a clear trend towards higher temperatures," writes the Federal Office for the Environment (FOEN).
In addition to higher air temperatures, this is due to the discharge of heated water from nuclear power plants, industrial plants and sewage treatment plants. "Science assumes that Swiss waters will warm by 4 to 5 degrees."
Disrupted food chain
Rising temperatures also affect the food supply - especially in lakes. "They disrupt the ecological balance, the diversity and quantity of plankton decreases. This reduces the food supply for fish," explains Bittner. Water that is too warm destabilizes the ecosystem in lakes and beyond.
Fisherman Gerny mentions another element that is needed for a sufficient food supply in lakes: "Storms stir up the sediment and turn the lake water around, making the phosphate bound in it available." However, phosphate also enters Lake Zurich through agriculture and from private gardens. "It takes the circulation and the nutrient input for the phosphate supply to be sufficient," summarizes Gerny, "but the food pyramid is a complex interplay that can easily be disrupted."
According to Bittner from the Swiss Fisheries Association, the situation for fish is already critical. Three out of four native fish species are endangered, threatened with extinction or already extinct. The reasons for this are not only rising water temperatures, but also the use of hydropower, which affects fish habitats in several ways. Added to this is the input of pollutants, "not only from agriculture, but also from households", he emphasizes. "The limit values are frequently exceeded in many bodies of water," he criticizes.
Dammed lakes and canalized rivers are another reason why Swiss fish stocks are declining. According to Bittner, this leads to further problems: "Fish are food for other animals. If they are not doing well, others will lose their livelihood."
The next big fish kill is sure to come
Bittner and the fishing association are therefore in favor of all measures that slow down global warming - even if this is not the only problem facing fish and their habitats.
Together with other organizations, they are also taking measures of their own: They place deadwood in straightened rivers to create retreats and shade for fish. If the situation becomes precarious, they also discharge cold water into rivers to save endangered fish.
"Fortunately, the current summer is not so bad; in 2023 we narrowly avoided further major fish kills. The last widespread fish kills occurred in 2022. But such events will unfortunately increase in the coming years," Bittner is convinced.
