Dive into the fascinating world of Michigan's Great Lakes. Learn intriguing facts about Lake Superior, Lake Michigan, Lake Huron, and Lake Erie.
State wildlife officials say dead fish along the shore are normal in the spring and residents shouldn't be alarmed if they notice a higher-than-normal number washing ashore.
Natural fish kills are common during the winter months and may be more common this year after a return to typical winter temperatures and snowfall this season, the DNR said this week.
"Truth is, most species of fish are relatively short-lived and have a high rate of mortality, the DNR said on its website. "Even large fish, too large to be eaten by predators such as bass and pike, experience a death rate of approximately 50% per year.
"Fortunately, the deaths are usually spread-out over the year and are rarely observed or become a problem except when concentrated as a fish kill. Only a fraction of the dead fish are ever observed because many decompose on the bottom or are eaten by scavengers such as turtles and crayfish," the website said.
Frozen lakes and deep snowpacks prevent sunlight from reaching aquatic vegetation, reducing the amount of dissolved oxygen in the water over the winter, the DNR said.
The situation commonly happens in shallower lakes with soft bottoms and a large amount of aquatic vegetation.
The amount of dissolved oxygen in a body of water depends on three variables: the amount of mixing with air above the lake, the rate of oxygen production by plants and the rate of oxygen consumption, the DNR said.
Fish can become easily stressed in winter due to low energy reserves because feeding is at a minimum in winter. They are then less able to handle low oxygen and temperature swings.
That could be the case this year with the record or near-record cold temperatures and large snowfalls Michigan experienced, and potential rapid warming in the coming months, the DNR said.
Should I be worried about winter kill?
Winterkill is the most common type of naturally occurring fish kill, the DNR said.
Winter fish kill is a natural phenomenon that is "highly localized" and does not affect the overall health or fish populations.
Canals in urban areas are quite susceptible, due to the large amounts of nutrient runoff and pollution from roads, lawns and septic systems that flow into these areas, especially after large storm events, the DNR said.
"If you see dead fish as a result of winterkill, the fish may appear fuzzy. This is because of secondary infection by fungus, but the fungus was not the cause of death," said Aaron Switzer, DNR fish production program manager. "The fish actually suffocated from a lack of dissolved oxygen from decaying plants and dead aquatic animals under the ice."
How often does a winter fish kill happen?
Winterkill occurs during long, harsh winters. Fish die in late winter, but go undiscovered until lakes and ponds thaw.
February is usually a critical period and is the best time to check the oxygen content of lakes prone to winter kill, the DNR said.
"A good midwinter thaw about then often recharges the lake's oxygen supply by means of photosynthesis and melt water. Conversely, a prolonged winter, with continuous snow cover and late ice-out, increases the chance of winter kill," the DNR said.
Great northern pike, yellow-bellied perch and pumpkinseed are relatively tolerant of lower oxygen levels and are the most likely to survive, according to the DNR.