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Dissolved
Oxygen for Fish Production
By:
Ruth Francis-Floyd
What Is Dissolved Oxygen?
Dissolved Oxygen (DO) refers to oxygen gas that is dissolved in water.
Fish "breathe" oxygen just as land animals do. However, fish are
able to absorb oxygen directly from the water into their bloodstream using
gills, whereas land animals use lungs to absorb oxygen from the
atmosphere.
What Are the Sources of Oxygen in an Aquatic Environment?
There are three main sources of oxygen in the aquatic environment: 1)
direct diffusion from the atmosphere; 2) wind and wave action; and 3)
photosynthesis. Of these, photosynthesis by aquatic plants and
phytoplankton is the most important.
Oxygen is produced during the day when sunlight shines on the plants in
the water. Oxygen levels drop at night because of respiration by plants
and animals, including fish. These predictable changes in DO that occur
every 24 hours are called the Diurnal Oxygen Cycle ( Figure 1 ).
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| Figure 1 . |
What Is an Oxygen Depletion?
An oxygen depletion refers to low levels of DO that result in fish
mortality. A concentration of 5 parts per million (ppm) DO is recommended
for optimum fish health. When DO concentrations drop below 2 ppm, fish are
severely stressed, and when concentrations fall below 1 ppm they begin to
die. The number of fish that die during an oxygen depletion is determined
by how low the DO gets and how long it stays down. Usually large fish are
affected by low DO before small fish are.
What Causes an Oxygen Depletion?
An oxygen depletion occurs when oxygen consumption exceeds oxygen
production. Increases in oxygen consumption can be caused by an
overabundance of aquatic plants or algae in the ecosystem,
"turnover" of a body of water (see Stratification/ pond
turnover section ), increased organic waste entering the water (i.e.,
manure from feedlots, septic tank waste water, and excess fish feed),
death and decay of organic matter (i.e., plant or algae die-offs), or by
certain chemicals (i.e., formalin) that remove oxygen directly from the
water column.
Why Are Oxygen Depletions Most Troublesome in the Summer?
Although oxygen depletions can occur at anytime, they are most common, and
most likely to cause fish kills, during hot summer weather. Decreases in
oxygen production are caused by events such as cloudy weather and plant or
algae die-offs that shut down photosynthesis. These heavy populations of
plants or algae are the most important producers of oxygen in the system.
However, they are also the most important users of oxygen. There are
several reasons why oxygen depletions are more common in the summer.
High Water Temperature
Warm water is much less capable of holding oxygen gas in solution than
cool water. For example, water that is 90°F can only hold 7.4 ppm DO at
saturation, whereas water that is 45°F can hold 11.9 ppm DO at
saturation. This physical phenomenon puts the fish in double jeopardy
because at high water temperatures the metabolic rate is increased, hence
their physiologic demand for oxygen is increased.
Cloudy, Still Weather
Muggy, overcast summer days often precipitate oxygen depletions. During
cloudy weather, the intensity of light reaching surface waters is greatly
diminished, resulting in a marked decrease in oxygen production. Oxygen
consumption, however, remains unchanged. This results in a net loss of
oxygen over each 24-hour period. This loss of oxygen from decreased
production is confounded by still, muggy, humid weather common on overcast
summer days. Oxygen transfer (from the atmosphere into the water) is
minimal because there is little or no wind/wave action. The net result
over a period of several days is oxygen depletion and, often, fish kills.
Stratification/Pond Turnover
During hot weather, surface waters warm up more rapidly than deeper water.
As the difference in temperature increases between warm surface water and
cool bottom water, a thermocline develops. A thermocline is an area
of rapid temperature change that acts as a physical barrier between warm
water at the surface ( epilimnion ) and cold water at the bottom ( hypolimnion
). When a thermocline is present there is no mixing of surface and
deep layers of water. Because photosynthesis and oxygen production only
occur near the surface, water in the deep layer becomes devoid of oxygen
and develops an oxygen demand. The thermocline can be broken by heavy wind
and cold rain, common during summer thunderstorms. When the thermocline
breaks down, the oxygen-rich surface waters mix with oxygen-deficient
bottom waters. If the oxygen demand is sufficient, all DO present will be
rapidly removed from the water column, resulting in severe oxygen
depletion and a fish kill.
How to Determine If Low DO Is the Cause of a Fish Kill
Unfortunately, the only way to know for sure if oxygen depletion has
caused a fish kill is to measure oxygen in the affected water while the
fish kill is in progress. Indications of oxygen depletion as a probable
cause of a fish kill include:
- All fish die at approximately the same time (often during the night
or in the pre-dawn hours).
- Large fish may be affected more than small fish.
- Moribund fish may be seen at the surface "gasping" for
oxygen (this is called "piping").
- Some species may die with their back arched, gills flared, and mouth
open. This is most commonly seen in hybrid striped bass, and
occasionally in catfish.
- The weather immediately prior to the fish kill may have been hot,
still, and overcast. A severe thunderstorm may have occurred
immediately prior to the fish kill.
- An oxygen depletion severe enough to result in significant fish
mortality is often observed in water with heavy populations of algae
or aquatic plants.
What Should I Do If I Suspect a Fish Kill Has Been Caused by Low DO?
The most important thing to do if fish are dying from low DO is to turn on
an aerator. If emergency aeration is not available, little can be done to
help the fish. To confirm the problem, oxygen levels should be tested
while the fish kill is in progress. Many county extension agents are
equipped with water testing equipment.
Preventing an Oxygen Depletion
An oxygen depletion can be predicted and therefore prevented, by
monitoring dissolved oxygen levels in a pond. The most efficient tool for
measuring DO is an electronic oxygen meter. These instruments are
available through most aquaculture supply companies at a variety of
prices. Chemical test kits are also available. These are more troublesome
to run, but are accurate and do not require as great an investment by pond
owners.
Commercial catfish farms often hire night oxygen crews to monitor DO
concentration in each pond at 2-hour intervals through the night. This is
the surest way of avoiding a fish kill caused by low DO. Aeration systems
can be turned on if oxygen levels drop below a certain concentration
(usually 2 to 4 mg/L).
Monitoring oxygen throughout the night is impractical for recreational
pond owners and part-time fish farmers. For these people it is easier to
"predict" an oxygen depletion by measuring DO levels in the late
afternoon (5 to 6 PM) and late evening (8 to 10 PM). The decline in DO
during the night can be predicted by graphing DO concentration against
time on standard graph paper ( Figure 2 ). If the projected concentration
of DO is below 4 mg/L before 7 AM, emergency aeration is recommended.
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| Figure 2 . |
If equipment to test DO concentration (meter or test kit) is not
available, the following observations and conditions can be used to
anticipate oxygen depletion:
- Fish swim at or near the surface gulping air (piping).
- Fish stop feeding suddenly.
- There is a rapid change in water color to brown, black, or gray.
- A putrid odor arises from the water.
- There is a loss of algae bloom.
- There has been an extended period of hot cloudy weather.
- There is a heavy summer wind and rainstorm.
Emergency aeration should be applied whenever fish show signs of oxygen
depletion or when dissolved oxygen drops below 4 mg/L.
Summary
Dissolved oxygen (DO) is oxygen gas (0 2 ) that is dissolved in
water. Most DO in ponds is produced during photosynthesis by aquatic
plants and algae. For this reason DO increases during daylight hours,
declines during the night, and is lowest just before daybreak. Dissolved
oxygen concentrations below 5 mg/L may be harmful to fish, and piping
(gulping air at the surface) may be observed when DO falls below 2 mg/L.
Low levels of DO are most frequently associated with hot, cloudy weather,
algae die-offs, or heavy thunderstorms. Dissolved oxygen can be monitored
using an electronic oxygen meter or chemical test kit. Emergency aeration
should be supplied whenever DO falls below 4 mg/L or environmental
conditions favor an oxygen depletion.
Ruth Francis-Floyd, IFAS Extension
Veterinarian, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville, 32611.
Copyright:
This document is copyrighted by the University of Florida, Institute
of Food and Agricultural Sciences (UF/IFAS). Published: September
1992. Reviewed May 1997. |
