Lake Geneva fish threatened by warming waters: experts March 13, 2025

Lake Geneva

By AFP - Agence France Presse

Lake Geneva fish threatened by warming waters: experts

By Agnes PEDRERO

Thonon-Les-Bains, France (AFP) March 12, 2025

The fish in Lake Geneva - the largest lake in Western Europe - are under threat as its waters warm up and become increasingly stagnant.

“Little by little, the temperature of the lake is rising,” contributing to a lack of oxygen in its depths, said Nicole Gallina, corporate secretary of the International Commission for the Protection of the Waters of Lake Geneva (CIPEL).

CIPEL is a joint Franco-Swiss organization that monitors the picturesque crescent-shaped body of water shared between the two countries.

France's National Research Institute for Agriculture, Food, and the Environment analyzes water samples collected by CIPEL.

“If there is less and less oxygen in the water, there will be less and less viable space for living organisms,” explained Viet Tran-Khac, laboratory manager at the research institute's facility in Thonon-les-Bains, on the southern shore of the lake.

Normally, in winter, surface temperatures cool to a density comparable to that of the deeper layers of the lake, facilitating the mixing of waters between the levels.

This natural cycle is essential for the maintenance of aquatic ecosystems, as it transfers oxygen to the lower levels of the lake.

However, this large-scale mixing in winter is becoming rare, as increasingly harsh winters, which scientists attribute to global warming, prevent the surface waters from cooling sufficiently.

“With climate change, we no longer have the extremely cold winters necessary for this natural mixing to occur,” Gallina told AFP.

In the current winter, the average minimum temperature measured in the upper 10 meters of the lake was 7.8 degrees Celsius (46 degrees Fahrenheit): an increase of 1.5C compared to the 1991-2020 reference period.

- New record

Data published on Wednesday by CIPEL showed that this year the waters have only mixed to a depth of 110 meters - but the lake's deepest point is 309 meters down.

Thirteen consecutive winters without complete mixing mark a “historic record”, Gallina warned, surpassing the previous longest period, established between 1987 and 1999.

“During the last complete mixing in 2012, the oxygen level in the deep waters was seven milligrams per liter,” said the biologist.

Now it has dropped to 2.4 mg per liter, below the critical limit of four mg needed for living organisms.

However, Gallina emphasized that there is still “hope” for the reoxygenation of deep waters.

Last year, a study by the University of the Swiss Federal Institute of Technology in Lausanne showed that the complete vertical mixing in 2012 also benefited from lateral mixing flows, a previously unknown phenomenon.

However, an entire ecosystem is beginning to change, warned CIPEL.

The lack of oxygen also affects the growth of phytoplankton plants, which are consumed by zooplankton organisms, which in turn serve as a food base for fish.

The future of the Arctic salmon, the fera, and other emblematic fish of Lake Geneva is under threat.

“Salmonids like the fera need cold water to spawn. Before, it spawned at a depth of three to six meters; now, it spawns at 20 to 25 meters,” said Alexandre Fayet, president of the Swiss intercantonal association of professional Lake Geneva fishermen.

“For now, we're not too worried,” but ‘we're trying to diversify and market fish that like warmer, less oxygenated waters, like carp, tench and bream,’ he told AFP.

- Transformation phase

LeXPLORE, a floating scientific platform, has been conducting research on Lake Geneva since 2019, studying 44 different parameters down to a depth of 110 meters.

Natacha Tofield-Pasche, project manager, said that in addition to the lake's rising temperature, global warming also leads to “extreme events” that drag many polluting particles into Lake Geneva, as witnessed during major floods last year in the Wallis region of Switzerland.

These events can also disable wastewater treatment plants, while the lake provides drinking water for around a million people.

CIPEL is “very concerned because it sees that Lake Geneva is going through a transformation phase”, marked by long periods without complete mixing, said Gallina.

In addition, there are other challenges, such as pollution invisible to the naked eye, like micropollutants and microplastics, or the invasion of quagga mussels.

In addition, episodes of high heat increase the risk of proliferation of cyanobacteria, which can be toxic.

Lack of mixing also promotes the accumulation of nutrients, such as phosphorus, in deep waters.

In the case of increasingly exceptional complete mixing, phosphorus can rise to the surface, causing algae blooms.

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