One speaks of fish mortality when the fish population in a body of water dies on a massive scale . This can affect individual species within the population or the entire population.
Possible causes for fish death are: fish diseases , lack of oxygen or intoxication . In rare cases, strong fluctuations in water temperature are also the cause of fish deaths. Even hydroelectric power plants provide numerous dead fish; Eels are particularly badly affected due to their size. In the case of brown trout , there have been inexplicable fish deaths time and again in recent years, such as B. in the pre-alpine rivers Amper , Iller , Isar , Mangfall and Thur . In 2019, all dead animals from the Thur showed inflammation of the liver and heart, and ultimately died of heart failure . The cause for this has not yet been found.
Lack of oxygen
Fish need oxygen to live. In unpolluted or less polluted water, there is usually sufficient to abundant oxygen dissolved in the water so that fish find enough oxygen to breathe gills. If the nutrient input is too high it can happen that the oxygen concentration is reduced so much ( hypoxia ) that the fish suffocate; Colloquially one speaks of the water tipping over . Likewise, the entry of fire-fighting foam can lead to a lack of oxygen and thereby cause fish to die.
Nutrients are continuously introduced into a stagnant body of water ( lake ) or a very slowly flowing body of water, e.g. B. phosphates , silicates . When the growing season starts, more algae can grow than before. They use photosynthesis to generate energy from light and generate oxygen in the process. However, at night they consume oxygen. It can happen that so much oxygen is consumed that the algae die themselves because the water becomes completely oxygen-free. Then the dead algae sink to the bottom, rot there and fermentation gases form and rise.
This process is accelerated when organic, oxygen-consuming substances (e.g. insufficiently treated wastewater , liquid manure ) enter the body of water .
However, oxygen deprivation can also have natural causes, for example if the oxygen balance of the water is interrupted when the water body freezes over for a long time or the low -oxygen water layer in a meromictic water body is carried to the surface by the onset of circulation. Overstocking in the context of fishery management of a body of water can also cause a lack of oxygen.
Fish can try to swim just below the surface in oxygen-poor water and benefit from the fact that oxygen from the air dissolves in the water there. But if the oxygen concentration drops too much, that doesn't help either. The fish suffocate and drift dead on the surface of the water.
The entry of inorganic water-polluting substances can lead to poisoning, which damage the fish population or lead to the death of the fish population if they enter the water directly or indirectly (e.g. dissolved in sewage). This is e.g. B. the case with the entry of heavy metal-containing industrial wastewater or the highly concentrated entry of acids or alkalis. During the major fire in Schweizerhalle in 1986, large fish deaths in the Rhine were triggered by the insecticides carried in with the extinguishing water .
The intoxication of the fish carcass can be proven in the context of a toxicological examination based on irritated and damaged organs and gills or by direct detection of the toxic substance in the carcass.
- NABU: Mysterious fish deaths at the Federsee , article from April 30, 2008
- Detroit Free Press (May 1, 2006): Rise in dead fish is linked to weather
- srf.ch / kassensturz (June 11, 2019): Toxins in bodies of water - The suffering of fish: These are the most problematic pesticides
- ↑ Thomas Anlauf: Brown trout die in the Isar and nobody knows why. In: sueddeutsche.de . September 4, 2016, accessed May 20, 2019 .
- ↑ Fish deaths in the Thur - mystery remains unsolved. In: srf.ch . May 18, 2019, accessed May 20, 2019 .
- ↑ Lara Wüest: Fish deaths in the Thur near Bütschwil may remain unsolved - fishermen are disillusioned. In: tagblatt.ch . September 20, 2019, accessed September 21, 2019 .
- ↑ Fish deaths after major fire: water samples evaluated. In: owl24.de. November 15, 2019, accessed November 18, 2019 .