Fish Kill





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Dead and dying fish are an ugly sight. Truth is, most species of fish
are relatively short-lived and have a high rate of mortality. 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.

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 are of three basic seasonal types: winterkill, which
occurs in late winter but may not be seen until early spring; spring
kill, which is occurs in late May to early June; and summer kill, which
occurs on the hottest days of mid summer.


Winterkill is the most common type of fish kill. When severe, it has
devastating effects on fish populations and fishing quality. Winterkill
occurs during especially long, harsh winters, such as occurred in
northern Michigan during the winter of 1995-96. Shallow lakes with
excess amounts of aquatic vegetation and mucky bottoms are prone to this
problem. 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.

Actually, the fish suffocated from lack of dissolved oxygen. Trace
amounts of dissolved oxygen (measured in parts per million, ppm) are
required by fish and all other forms of aquatic life. Even living plants
and the bacteria that decompose organic materials on the bottom of the
lake require oxygen. As a rule of thumb, the critical level of oxygen is
about 2 ppm for most game fish native to warmwater lakes, and levels
below 1 ppm for extended periods of time are lethal.

But species of fish vary in their tolerance of low oxygen. Trout are
most sensitive; walleye, bass, and bluegill have intermediate
sensitivity; and northern pike, yellow perch, and pumpkinseed are
relatively tolerant. Bullheads and certain minnows are very tolerant.
Lakes prone to periodic winterkill can often be detected from the
composition of their fish populations – tolerant species predominate,
sensitive species are rare, and prey greatly outnumber predators.
Fortunately, usually enough fish survive, either in the lake or in
connecting waters, to repopulate the lake in a couple of years. Only for
extreme die-offs is fish restocking necessary.

The dissolved oxygen content of water depends primarily on three
variables. These are the amount of mixing with the air above the lake,
the rate of oxygen production by plants, and the rate of oxygen
consumption (respiration) by living aquatic organisms. During periods of
prolonged ice cover, the lake is sealed off from the atmosphere and
cannot be recharged with oxygenated air. Furthermore, ice and snow
reduce the amount of sunlight reaching aquatic plants, thereby reducing
photosynthesis and oxygen production. (During photosynthesis, living
plants use sunlight energy and carbon dioxide to make plant tissue and
dissolved oxygen). Meanwhile, on-going consumption of oxygen depletes
the supply of oxygen stored in the lake when the lake froze over.
Shallow, productive lakes are at a disadvantage because they have a low
storage capacity and high rates of oxygen-consuming decomposition.

February is usually a critical period and is the best time to check the
oxygen content of lakes prone to winterkill. 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

A short-term solution to impending winterkill, suitable for ponds and
small lakes, is to aerate with commercial devices or outboard motors. A
significant improvement can be made in the oxygen content of about 1
acre of water by running a small outboard motor for about 4 hours.
Select a relatively warm day to use the outboard method. Mount the
outboard on a dock, frame, or small boat and lower the shaft into a
large hole in the ice. Tilt and run the motor so as to push water on top
of the ice. Then, at the edge of the flooded area, chop more holes so
the water can return. Beware of weakened ice! Move to another location
before the outboard hole becomes dangerously enlarged or water is no
longer pushed onto the ice. Run the motor over relatively deep water so
that bottom mud is not stirred up along with the water.

The only long-term solution for winterkill lakes is to reverse the
natural process of filling and enrichment (eutrophication). Dredging or
sucking bottom sediments can increase the volume of water, reduce the
nutrient-rich sediment, and reduce the growth of nuisance plants.
However, such projects are extremely costly, require a site for
disposing of the bottom material, and may require a permit. Lake
residents can help slow down the rate of eutrophication by keeping all
types of plant fertilizers out of the lake.


Spring kill occurs in lakes and rivers when fish survive the winter but
die as the water warms rapidly in May and June. It rarely claims many
fish and is usually over in a couple of weeks. Spring kill is almost
always due to natural causes beyond our influence. The usual victims are
large bluegills and crappies, and other fish which spawn in the spring
such as perch, bass, pike and suckers.

A combination of stresses is usually responsible. Fish come through the
winter in a weakened condition because they’ve been eating at a reduced
rate. As the water warms, their metabolism increases and they divert
much energy to strenuous spawning activities. In lakes, additional
stress may be added during “turnover”, which is when wave action stirs
up bottom water low in oxygen and high in noxious gases. Diseases and
parasites also become more active and on a few occasions have been
implicated in fish kills. An example is the spring salmon mortality in
Lake Michigan caused by bacteria kidney disease (BKD).


Summer kill occasionally occurs in lakes and streams during extremely
hot summer weather. High temperature and low dissolved oxygen combine to
stress the fish. Most prone to summer kills are pike, perch, suckers,
bass, and bluegill living in shallow, productive lakes or bays with
excessive amounts of algae or rooted aquatic vegetation. The plants
consume large amounts of oxygen at night, causing a temporary shortage
of the vital gas just before dawn. A cloudy, calm day extends the
critical period by reducing re-oxygenation from photosynthesis and wave
action. Apparently, fish in the oxygen-depleted areas do not sense the
danger and swim to safety in time.

Summer kill may also occur in deep, unproductive lakes containing trout
or cisco. These fish require both cold and well-oxygenated water. During
summer they seek refuge in the cold bottom layers where temperatures
are less than 72 degrees F. Death results if the oxygen level there
declines below about 4 ppm. Trout will also die in streams if they are
unable to find cold spring water. Several stream trout mortalities were
reported during the hot summer of 1995.

A very unique type of fish kill is caused by a lightning strike on
water. Death occurs immediately. Large fish, which draw more electricity
than small fish, may be killed selectively.

In conclusion, the risk of some types of fish kills can be reduced by
keeping as many nutrients out of the water as possible. Sources of
nutrients include septic fields, fertilized lawns and farm fields, and
wastes from livestock and waterfowl (including tame geese). Reducing
nutrient input starts the following favorable chain reaction: production
by aquatic plants is reduced, less decomposition is required, and
oxygen will not become depressed to critical levels.

Natural fish kills are obnoxious, and may affect fishing and
predator-prey “balance” for years. However, they are often not serious
in the long run because lakes contain thousands of fish per acre. They
may be thought of as nature’s way of thinning out fish populations.
Usually, fish kills indicate that the habitat is of marginal quality for
certain species because of the broad range of weather conditions we
experience in Michigan.

Infrequently, fish kills indicate habitat or pollution problems we may
be able to correct. And sometimes, fish kills beneficially reduce
over-populated, slow-growing panfish and actually increase growth rates
and improve fishing.