What causes fish kills?

Dead fish are floating in shallow waters along the shore while others appear to be barely alive. What has happened to the lake? Why are the fish dying, and what should you do when you notice a fish kill?

One of the most common causes for fish kills is oxygen depletion during winter. As a lake is covered with ice, the oxygen contained in the water may run out over the winter, especially if the lake is small and affected by eutrophication and its water replacement rate is slow. Fish kills caused by oxygen depletion come to light as the ice melts in spring.

A fish kill may also be caused by a sudden change in the water acidity level. The risk of such changes is the greatest for lakes in areas with acid sulphate soils. Soil excavation on these areas or heavy rainfall following a period of drought may cause the pH value of downstream waters to drop, sometimes as low as close to three. While this situation does not last for very long, fish cannot survive it.

An oil spill, chemical discharge or some other sudden pollution can kill fish, especially in small water bodies. Fish kills caused by algae toxins have been observed in coastal waters.

Fish can carry parasites and diseases that may cause a fatal epidemic, which usually only affects a specific species or group of species. The disease may also weaken fish, making them more vulnerable to other diseases and stressors. A fish kill may consequently be caused by the combined effect of several factors.

If you notice a fish kill, you should always report it to your municipality’s environmental protection authorities or the region’s ELY Centre. The authorities can then start investigating its cause. If you think that the fish kill might have been caused by an oil spill or some other environmental incident, call the Emergency Response Centre (112).