Many fish were found dead in some parts of the country, especially after heavy rains on Thursday. Water officials have received dozens of reports of dead fish since Friday. With the heat already returning, officials are working hard to remove the dead fish as soon as possible, said Hoogimradschapp Holland Norderkwartier, water authority.
The water department says the fish are dying from lack of oxygen. The sewers flooded because of the rain. As a result, dirty water flows into ditches, ponds and rivers. The rain also washed road dirt and dog droppings into the water. Water pollution rapidly reduces oxygen levels in water. After that, the fish literally gasps for air and dies if there is no oxygen supply in the water.
Larger fish, which need more oxygen, are especially hungry for air, according to the water department. Sometimes fish stay on the surface in a kind of floating state. In this state, we put ourselves in a passive state in order to require as little oxygen as possible. Then they swim again. Therefore, not all fish that stop moving automatically die. Despite this, the water authority reported that unfortunately hundreds of fish did not survive.
Blue-green algae and botulism also develop rapidly in low-oxygen, warm water. Therefore, the Rhine en IJssel Water Board installed two aerators at Ahauser Aa on the Twente-German border to pump additional oxygen into the water. Also in the Netherlands, the Ahauser Aa is called Sipbeek and specifically passes through Deventer.
The Water Board is asking anyone who sees dead fish floating in the water to report it. The water board stressed that it was not intended to lift the fish out of the water themselves and that this should be done by a professional.
https://nltimes.nl/2023/06/25/mass-fish-deaths-due-heavy-rainfall Heavy rain kills fish