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FISH KILL: A naturally occurring phenomenon

A large pile of dead fish on a sandy shore and more fish being pushed on the shore by waves of water.
Environment, Great Lakes, and Energy

FISH KILL: A naturally occurring phenomenon

Fish die as a result of natural and unnatural causes, including old age, starvation, injury, stress, suffocation, water pollution, disease, parasites, predation, toxic algae, and weather extremes. Most of the time, fish kills are due to natural causes over which we have no control, such as weather. Only occasionally is death directly related to pollution or improper use of herbicides or other chemicals.

Natural fish kills can potentially be of three basic seasonal types:

  1. Winter kill - occurs in late winter but may not been seen until early spring
  2. Spring kill - occurs in late May to early June
  3. Summer kill - occurs on the hottest days of mid-summer.

Non-natural Fish Kills
Only occasionally is a fish kill related to pollution or improper use of herbicides or other chemicals. When several fish species have been affected at once, the likelihood of the cause being a pollution discharge event increase. In cases where only small fish are dying the likelihood of the cause being a pollution discharge event increases due to smaller fish having a larger surface area to body size ratio. This larger surface area allows the smaller fish to absorb more of the pollutant relative to their body size. A combination of understanding the three seasonal types of natural fish kills and the patterns to recognize a fish kill caused by non-natural causes is crucial to determine when a fish kill should be reported. 

This guidance does not represent all possible scenarios for fish kills.

If you observe a fish kill and believe it occurred due to natural causes, please complete the sick or dead aquatic species form in DNR’s Eye in the Field application.

Summer Kill

Summer kill occasionally occurs in lakes and streams during extremely hot summer weather, particularly in shallow lakes with excessive amounts of aquatic vegetation. During the day, plants produce oxygen through photosynthesis, but at night oxygen production stops while other organisms continue to respire. This shortage of dissolved oxygen at night can lead to fish kills at dawn. By midday, with the increase in photosynthesis, the fish population often recovers.

Two dead sunfish floating near the water’s edge of a pond surrounded by trees and vegetation in summer.

Winter Kill

Occurs most often in ice-covered shallow lakes with productive aquatic vegetation and mucky bottoms. The low storage capacity of dissolved oxygen and high levels of decomposition in these shallow productive lakes can lead to oxygen depletion below critical levels. Fish that succumb to winter kill often go unnoticed during the initial die-off and are only noticed after the ice leaves the lake.

Two dead fish on the shore of a lake, surrounded by debris including sticks, rocks, and leaves. Patches of snow and ice are visible on the ground.

Spring Kill

Spring kill occurs in lakes and rivers when fish survive the winter but die as the water warms. Spring kill is typically the combination of multiple stressors. As fish have come through winter in a weakened state, they divert significant energy to spawning activities as the water warms leading them to be more susceptible to death. In lakes, additional stressors may result from turnover, where wave actions stirs up bottom water low in oxygen. Typical victims of spring kill are bluegills, crappie, and other fish that spawn in the spring.

The photo is of a spring kill in Bear Lake in Muskegon, Michigan.  Bear Lake has unnaturally high levels of phosphorus which promote excessive aquatic vegetation growth.  With a maximum depth of 13 inches, the relatively low storage capacity for dissolved oxygen and mucky lake bottom can lead to spring kills of spawning fish. 

A shoreline of Bear Lake with multiple dead fish, mostly yellowish in color, scattered along the shallow water and wet sand. Debris, including sticks and plant material, is visible along the edge.

Viral Hemorrhagic Septicemia (VHS)

VHS is a relatively new consideration in fish kills involving multiple species. VHS can lead to a fish kill impacting multiple species of various sizes. Most VHS outbreaks occur in the spring when water temperatures are less than 59 °F. At a low-level infection, fish might not display any noticeable symptoms. At higher levels of infection, fish will display bleeding throughout the body surface and within internal organs. Sick fish will often swim in circles and are often observed at the surface of the water. Confirming VHS infection requires laboratory testing.

 
large dead fish lying near the rocky water’s edge of a lake surrounded by scattered debris

If you observe a fish kill and believe it occurred due to non-natural causes, please report it to the State of Michigan’s Pollution Emergency Alerting System (PEAS) hotline at 800-292-4706.