Underwater heat stress Too warm lakes put pressure on fish Fishermen are alarmed

Rising water temperatures in Swiss lakes are creating significant stress for fish populations, prompting concerns among professional fishermen. The phenomenon, linked to increasingly frequent heatwaves, has led to noticeable changes in aquatic ecosystems. Water temperatures exceeding 26 degrees Celsius are placing pressure on fish species and other aquatic life, affecting their survival and behavior. This situation has become particularly critical earlier than usual in the year, according to experts. Alexander Roschi from the Swiss Fishermen's Association stated that the current conditions are alarming, emphasizing that this is the most serious situation for many bodies of water and their inhabitants early in the year. Prolonged high water temperatures have begun to influence catch numbers significantly. Andrin Krähenbühl, head of the Fibish Advisory Service, noted that during extended heatwaves, fish tend to retreat into deeper, cooler areas of lakes. If these conditions persist over time, the composition of catches will change as well. Shallow waters are seeing an increase in warm-water loving species while cold-water fish move to deeper parts of the lake where they are more difficult for fishermen to locate and catch. Species such as chub and whitefish, typical of Swiss lakes, are reacting sensitively to rising temperatures. The trouts, considered one of the most heat-sensitive fish species in Switzerland, survive mainly by retreating into the colder depths of lakes. The decline in fish populations is evident, with some species showing a dramatic decrease compared to previous years. Reto Leuch, president of the Professional Fishermen’s Association and a professional fisherman on Lake Constance, mentioned that five years ago there were already fewer chubs, but today, before the spawning season, they catch about a third of what they did back then. He described the decline as massive and attributed it partly to climate change, which affects even winter conditions when water temperatures do not drop sufficiently. Lake environments that are not very deep, such as the Murten Lake or the Greifensee, are especially vulnerable. In lakes where oxygen levels are low in the depths, the living conditions for cold-water fish are severely threatened. Krähenbühl explained that these fish are limited both by the warmth from above and the availability of oxygen from below. Fishermen are adapting to these new challenges by adjusting their fishing methods to accommodate the changing conditions. Some species, such as catfish, carp, and roach, are less affected by higher temperatures, and their populations have grown significantly over the past 25 years. Krähenbühl highlighted that professional fishermen are noticing these changes and must adapt accordingly. The impact of these environmental shifts extends beyond just the fish population. It influences the entire ecosystem of the lakes, altering the balance between different species and affecting the livelihoods of those who depend on fishing. As the climate continues to change, the need for adaptive strategies becomes ever more pressing, requiring collaboration between scientists, policymakers, and local communities to ensure sustainable practices and protect aquatic biodiversity.