LOGIN
Pond / Lake Health:
Fish
Kills in Ponds and Lakes
Fish kills in ponds and small lakes usually occur during summer
and winter, but they may happen any time during the year. Fish
kills are usually related to low oxygen levels; however,
pollution, fish diseases and a phenomenon known as turnover can
also kill fish.
Unfortunately, once fish begin dying, it is usually too late to
do anything. The best method to prevent fish kills is good pond
management, which begins with proper pond construction and
watershed maintenance.
LOW OXYGEN
- SUMMER
Like most creatures, fish require oxygen to live. The amount
of oxygen in a body of water will vary with water temperature,
aquatic plant densities and amount of sunlight.
As water temperatures increase, water loses its capacity to
"hold" oxygen. Unfortunately, as water warms, the cold-blooded
fish become more active and require more oxygen.
Larger fish of any species are usually the first to die when
oxygen concentrations become too low. Desirable fish species,
such as grass carp, bass, channel catfish and bluegill, are more
susceptible to low oxygen levels than bullheads, carp and other
less desirable species.
Aquatic plants are generally considered beneficial to bodies of
water because they produce oxygen. As the amount of sunlight
decreases, however, plants use more oxygen than they produce. At
night and during periods of low light, vegetation actually
competes with fish for oxygen. Decomposing vegetation also
requires oxygen, making less available to fish. Studies show
that summer oxygen levels are usually lowest just before sunrise
and after extended periods of cloudy weather.
Ponds and small lakes are vulnerable to summer fish kills when
vegetation covers 20 percent or more of the water surface. Ponds
that turn a pea-soup color from microscopic algae are also
subject to fish kills.
Fish swimming near the surface of the water and appearing to be
gulping air indicate a low oxygen problem. A quick response can
be the difference between losing or saving the fish in your pond
or lake.
Because oxygen is in short supply, you must provide it by
aerating the water or by adding large quantities of fresh,
oxygenated water.
Aeration is usually easiest and on small ponds can
be accomplished with a pump and nozzle or a boat motor, using
water from the pond itself. Water molecules will pick up oxygen
from the air, so redepositing the water in a fine misty spray
works best. Well water can also be used but, because it is low
in oxygen, must be sprayed onto the pond's surface to be
effective.
Commercial surface aerators may be necessary to increase oxygen
levels in larger ponds and lakes.
Keep in mind that aeration is only a remedy for low oxygen
levels, not a cure for them. Proper watershed, pond and aquatic
plant management practices (refer to other Aquaguides for
information about these practices) will help you control
excessive vegetation and prevent fish kills from occuring.
Should chemical control of excessive aquatic vegetation become
necessary, avoid treatment when water temperatures are warm,
since decaying vegetation will lower oxygen levels even further.
Apply chemicals to no more than 1/3 of the vegetation at one
time and only when water temperatures are below 80 degrees.
During extremely warm weather, when water temperatures exceed 85
degrees, it's a good idea to reduce or stop feeding fish
supplemental foods, because uneaten feed sinks to the bottom and
decomposes, further lowering oxygen levels.
Most ponds produce an ample supply of natural foods, so there is
no need to worry about the fish going hungry.
LOW OXYGEN - WINTER
Excessive vegetation can be just as detrimental under the
ice as during summer. Sunlight can penetrate several inches of
clear ice, but a small amount of snow atop the ice can block
nearly all light from the water.
Without light, plants cannot produce oxygen and will eventually
die. When dead plant material decomposes it consumes oxygen.
Large amounts of decaying plant materials can reduce oxygen in
water to levels lethal to fish.
To prevent winterkill, try to keep a portion of the ice free of
snow to permit sunlight to reach plants and result in more
oxygen production. This is especially important in shallow ponds
with extensive areas less than eight feet deep and an abundance
of aquatic vegetation. Create "windows" by shoveling snow off in
blocks about 10 feet square in various parts of the pond; the
more of them the better.
Make sure the ice is safe before walking onto the pond. As a
general rule, four inches or more of clear ice is enough to
support anyone.
POLLUTION
During the spring and fall rainy seasons, runoff from feed
lots, crop land and fertilized lawns can lead to fish kills.
Heavy rains can wash large amounts of nutrients and fertilizers
into ponds, accelerating plant growth which can lead to oxygen
depletion. Proper watershed management includes diverting or
eliminating the sources of excessive nutrients (phosphorous,
nitrogen) so they do not threaten fish life in ponds.
Chemicals, including herbicides and pesticides, entering a pond
can directly lead to fish kills. Ammonia, which comes from
animal wastes, is highly toxic to fish. Toxic chemicals usually
affect all species and all sizes of fish.
The majority of chemical fish kills can be avoided by carefully
following all label directions, especially when applying
chemicals near water. If you suspect that a fish kill was caused
by chemicals, look for possible sources within the pond's
watershed.
DISEASES
Early spring can be a dangerous time for fish. As water
temperatures rise, populations of disease-causing organisms,
such as bacteria and parasites, increase. These organisms can
infest fish weakened by spawning actitivies and the stresses of
winter and, if abundant, may kill them.
Diseases in ponds seldom kill all the fish and are likely to
affect only one or two species. In most cases, it is impractical
to treat the pond or the diseased fish. Fish populations usually
rebound quickly.
TURNOVER
Turnover occurs in the spring and fall when surface water mixes
with water from near the bottom that may contain little or no
oxygen (See "Pond and Lake Turnover" Aquaguide). Gasses
potentially lethal to fish that have built up in the pond's
deepest water are slowly circulated throughout the pond and
eventually released into the atmosphere.
Turnover is usually gradual and not harmful to fish. However, if
strong winds, cold fall rains or rapid temperature changes
accelerate the turnover process, fish can die as a result of
being suddenly exposed to low quality water.
CONCLUSION
Fish kills can be caused by natural or manmade events, but
your pond does not have to be one of the victims. Maintaining a
well- vegetated watershed, adequate pond water depth (especially
around shoreline areas) and moderate aquatic plant densities
will help prevent most problems. If you have any questions about
fish kills or other fisheries related topics, contact the
nearest Department of Conservation Fisheries District office.
Back to Lake and Pond Health
Index
