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It's very common during the spring and early summer to receive phone calls from our Delavan Lake residents alerting us that they have seen a big fish die off.  Very often these spring die off events are the result of a Columnaris bacterial infection.  Columnaris bacteria thrives in warm temperatures (65 to 70F) and reduced water quality from runoff events which also cause stress in fish.  At the same time, the warming temperatures and increase in spawning hormones also cause stress in fish.

The stress reduces the immune response in fish and the result is that the fish are very susceptible to infection from the Columnaris bacteria which often results in fish kills.  Although many fish may die it is rarely a threat to the overall fish population.  Columnaris causes fatal lesions on the skin and gills of fish.  It is often just a single species and age class that is affected but it is not unusual for many species to be affected.

Humans are not at risk from Columnaris but the Wisconsin Department of Natural Resources recommends that people not eat infected fish because other bacteria is likely on the remains of the fish.