Warm temperatures and low oxygen levels cause fish deaths at Estonian lakes

Dead fish on the shore of Lake Peipus.
Dead fish on the shore of Lake Peipus. Source: Ivo Kask

Dead fish have been found in, and by, several of Estonia's lakes over the past month. According to experts, this is due to warmer temperatures and lower amounts of oxygen in the water.

Last month, a large number of suffocated fish were found at a lake in Põlva County. However, this is not the only lake in Estonia, where fish stocks have declined this year. Dead fish have also washed up on the shores of Lake Peipus.

Arvo Tuvikene, lead researcher and fish expert at the Tartu-based University of the Life Sciences (Maaülikool), said that fish from two different lakes had died for the same reason. "It was the same. Hot weather, lack of oxygen and both of these things combined."

According to Tuvikene, on a summer day when temperatures are as high as 30C, fish need six times more oxygen than in winter, because their metabolism works faster.

Leo Aasa, a fisher on Lake Võrtsjärv in south Estonia, has had many encounters with suffocated fish during his career. "Fish start dying both inside traps and in the water. There have also been years when the weather is very warm and the shore is full of dead fish."

For Aasa, the higher water temperature also means more work. Whereas previously the traps had to be checked once a week, now he has to inspect them twice as often.

"The shorter the time those fish are in there, the more chance there is of catching live ones," he said.

Although oxygen depletion usually occurs in shallow lakes and in the second half of summer, there are some exceptions.

"In large lakes such as Lake Võrtsjärv or Lake Peipus, even when water temperatures rise above 25 or 27 degrees Celsius, occasionally some fish species die," said Tuvikene.

Oxygen in the water is also consumed by other organisms, says Aimar Rakko, head of the Estonian Environmental Board's hunting and aquatic wildlife department.

"The lakes have simply become so full with organic pollution that biochemical processes are constantly taking place, which consume oxygen very quickly."

Tuvikene said the same, adding that the algal biomass produces large amounts of oxygen during the day, but then is all consumed at night.

The Estonian Environment Board is asking anyone who notices large amounts of dead fish in or near lakes to inform them by calling 1247.


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Editor: Michael Cole

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