The snow on the ice killed the fish...

Back in the kayak on the ditches and canals around Woerden, after a break of 8 weeks due to the ice, I was surprised by the number of dead fish floating on the surface of the water.

Obviously this year's long (and early) ice-period has had big impact on the underwater-life.
Usually a layer of ice is no problem for the fish. In the cold water under the ice there is often even more oxygen dissolved than in warm conditions, and fish need less oxygen at a low body temperature. But it's a different story with snow on the ice. The layer of snow on the ice blocks the light. Without sunlight algae and underwater-plants can't produce oxygen, which slowly reduces the amount of dissolved oxygen in the water. After a long period of frost and snow-covered ice fish kills occur due to oxygen depletion/hypoxia.

The dead fish are mostly large Carp or Bream - it's a sad sight, but in fact, from an ecological point of view, the selective fish kill (mainly the weak and large ones) isn't too bad: it creates more room for the authentic species.

btw: The fish kill in Woerden is not as dramatic as in the video-clip with the dead fish in Leeuwarden. I assume the waters shown in the video are more shallow than the ditches and canals over here.