In fish farming oxygen level in pond water is very important. 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 blood stream using gills,
whereas land animals use lungs to absorb oxygen from the atmosphere.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, derived from photosynthesis, 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 (dissolved oxygen) that occur every 24 hours are called the diurnal oxygen cycle
. Dissolved oxygen concentration in ponds fluctuates on a 24-hour basis. This fluctuation is called a diurnal oxygen cycle. Dissolved oxygen increases during day light hours when photosynthesis is occurring and decreases at night when respiration continues but photosynthesis does not. What is Oxygen Depletion? Oxygen depletion refers to low levels of DO
and may result in fish mortality. A
concentration of 5 mg/L DO is recommended
for optimum fish health. Sensitivity to low
levels of dissolved oxygen is species specific,
however, most species of fish are distressed when DO falls to 2-4 mg/L. Mortality usually
occurs at concentrations less than 2 mg/L.
The number of fish that die during an oxygen
depletion event is determined by how low
the DO gets and how long it stays down.
Usually larger fish are affected by low DO before smaller fish are. What Causes Oxygen Depletion? Oxygen depletion occurs when oxygen
consumption exceeds oxygen production.
Increases in oxygen consumption can be
caused by an over-abundance 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
feed lots, 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 Depletion Events Most
Troublesome in the Summer? In out door ponds, oxygen depletion events
can occur at anytime, however, they are
most likely to cause fish kills during hot
summer weather. A decrease in oxygen
production is caused by incidents such as
cloudy weather and plant or algae die-offs that shut down photosynthesis. 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 depletion events are more common in the summer and
they are discussed below. 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 mg/L DO at saturation, whereas water
that is 45° F can hold 11.9 mg/L DO at
saturation. This physical phenomenon puts the fish in double jeopardy because at high
water temperatures their metabolic rates
increase, hence their physiologic demand for
oxygen increases. 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 from photosynthesis. 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 waters. 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 rapidly be 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 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 event severe enough to result in significant fish mortality is often
observed in water with heavy populations of
algae or aquatic plants. What To Do if Low DO is Suspected as the
Cause of a Fish Kill 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. Some county extension agents are equipped with water
testing equipment. In addition, biologists with
the Florida Game and Freshwater Fish
Commission or an IFAS Aquaculture Extension
Specialist may be available to assist. Preventing Oxygen Depletion An oxygen depletion event 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 trouble some to run, but are
accurate and do not require as great
investment by pond owners. Commercial catfish farms often hire night
oxygen crews to monitor the DO
concentration in each pond at two-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 2-4 mg/L) depending on the fish
species. 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
(5-6 p.m.) and late evening (8-10 p.m.). The decline in DO during the night can be
predicted by graphing DO concentration
against time on. If the projected concentration of DO is below 4 mg/L before 7 a.m. emergency
aeration is recommended. Estimation of potential for dissolved oxygen depletion. 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 suddenly stop feeding. There is a rapid change in water color to brown, black or gray, signifying loss of an
algal bloom. A putrid odor arises from the water. There has been an extended period of hot cloudy weather. There is a heavy summer wind and a rain storm. Emergency aeration should be applied
whenever fish show signs of oxygen
depletion or when dissolved oxygen drops
below 4 mg/L. Many recreational pond owners purchase
aerators and place them on electric timers.
Proper use of the timer should have the
aerator turn on during the late evening (10
p.m. to midnight) and turn off after daylight
(7-8 a.m.). Using an aerator is not a complete substitute for monitoring DO concentrations
and an oxygen depletion event resulting in a
fish kill may still occur. However, use of an
aerator is recommended and will prevent
many problems. Summary Dissolved oxygen (DO) is oxygen gas (O 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 event. Call 08032861326 for help and consultancy.
