Death of fish is increasingly found in lakes that suffer from a lack of oxygen in their waters. Of course, death is often caused by such a factor as algal blooms. However, a new study reports a new climate-related cause of fish death. The findings of scientists are published by the scientific journal Science of the Total Environment.

The last serious example of the mass death of fish is from a lake in Denmark. In 2018, Denmark suffered from an extreme summer drought. Temperatures in May, June, and July were 3 ° C higher than the average for the last 30 years. At the same time, on average, only 27 mm of precipitation fell compared to the normal amount of 56 mm.

Lake Filsø did not receive rainfall that year until a thunderstorm with torrential rains broke out in the lake on July 28, which is 22 hours yielded 22 mm.

A huge amount of water flowed from the catchment into the lake and brought with it a huge amount of organic material. Such a cocktail led to the mass death of fish in the lake. Such events can completely destroy the whole ecosystem, biologists explain.

A few days after heavy rain in the lake, 1203 dead pike were found. A month later, only one pike was caught in gill nets. Stocks have not recovered, and this year researchers released 8,000 pike fry and 350 kg of spawning perch biomass to restore the population.

And we will continue to breed pike in the future. The tragedy is that oxygen depletion lasted only four days. On the fifth day, oxygen levels returned to normal, but then the fish died.

Theis Krag, biologist and associate professor at the University of Southern Denmark

The reason that heavy rains can lead to mass killings of fish is as follows. When rain falls, it passes through ditches and drainage pipes and then falls into the lake. Along the way, he collects a lot of labile organic matter from the soil – leaves, dirt, and partially decomposed organic matter.

Dead fish

When all this gets into the lake, it becomes a target for hungry bacteria. Bacteria that consume organic matter use oxygen and take it from water. This causes oxygen depletion in the lake, and the fish die.

Lake Filsø is not the only lake to experience this, and it will happen to other lakes. Until now, scientists have considered extreme drought, accompanied by torrential rains, as a rare event. But climate change has become a reality now and scientists are preparing to recur such cases every 5-10 years.

Researchers are currently working on tracking organic material in Lake Filsø back to its place of origin. They are going to develop a drainage system on the waterway to reduce possible damage.