Winter has been rough on fish

Most people don’t think of fish having a difficult time surviving harsh winters, but winterkill is the most common type of fish kill in Michigan and this winter has been lengthy enough to cause issues in some lakes. 

Most people don’t think of fish having a difficult time surviving harsh winters, but winterkill is the most common type of fish kill in Michigan and this winter has been lengthy enough to cause issues in some lakes. 

“This has been a pretty tough winter, especially compared to last year,” said Dave Borgeson, fisheries biologist with the Department of Natural Resources. “ The ice cover has been on the lakes for quite a while now.  You might remember, last year we had a big warm up and the ice left the lakes pretty early.” 

Ice cover is the key factor when discussing winter fish kills.  The cap of ice limits the amount of oxygen that can enter the water, thus creating a situation of diminishing oxygen levels until the ice cover leaves the lake. 

“We really see problems in the shallow lakes and lakes with lots of vegetation,” explained Borgeson. “ The greater the volume of water to pull oxygen from the less the chance of winterkill.  That same goes for vegetation.  The more vegetation in a lake, the more oxygen that gets burned up.” 

Fish species vary in their sensitivity to low oxygen.  Trout are most sensitive, followed by walleye, bass, and bluegill.  Northern pike, yellow perch, and pumpkinseed are hearty as far as oxygen is concerned, able to tolerate conditions that would be lethal for more sensitive fish. 

Water testing can be done to determine the odds of a winterkill, but the most common method of detection is the old-fashioned eyeball test. 

As the ice leaves the lakes, anglers will be quick to get their boats ready and out on the water.  These spring anglers will likely be the ones to sound the alarm on a fish kill, being the first on the water to see dead fish. 

“Seeing dead fish is really the only way you’ll notice a winter fish kill,” Borgeson said. 

Fish actually die in late winter, but may not be noticed until a month after the ice leaves the lake because the dead fish are temporarily preserved by the cold water. 

Winterkill begins with distressed fish gasping for air at holes in the ice and ends with large numbers of dead fish which bloat as the water warms in early spring. 

Dead fish may appear fuzzy because of secondary infection by fungus, but the fungus was not the cause of death. 

“Its hard to say what we will see this year,” Borgeson added. “ We won’t really know until the ice is off.  This has been a little tougher winter though.  In those shallow lakes with lots of weeds, fish could be affected.”