Fish die-offs common in spring (2006-06-13)

In May and June of each year, the Minnesota Department of Natural Resources (DNR) receives reports of dead crappies, sunfish and bullheads washing up on the shore of some lakes. This year is no different.

The die-offs are frequently caused by rapidly warming water temperatures, which trigger an outbreak of columnaris disease, a naturally occurring fish disease caused by a bacteria that poses no threat to humans.

"Minnesotans get very concerned when they see dead fish on the shore," said Ling Shen, DNR fish health specialist. "Rightfully, they call us. Most times we’re able to reassure them that it’s a natural occurrence that doesn’t affect the overall fish population."

Columnaris frequently occurs when water temperatures rise above 55 degrees. When fish move into shallows in spring, they are more concentrated and more susceptible to contract the disease from other fish. Shen said it is not clear why some lakes have outbreaks of the disease and others don't.

Columnaris is not known to cause any harm to humans. It usually affects the skin and gills of fish. The flesh is not affected and is safe for human consumption.

People who notice large numbers of dead fish in a lake or stream are asked to report the event to the DNR at (651) 296-6157 or toll free 1-888-MINNDNR (646-6367).