Monitoring pond water quality to improve shrimp and fish production
The proper management of pond water quality plays a significant role for the success of aquaculture operations.
Each water quality parameter alone can
directly affect the animals´ health. Exposure of shrimp and fish to
improper levels of dissolved oxygen, ammonia, nitrite or hydrogen
sulfide leads to stress and disease. However, in the complex and dynamic
environment of aquaculture ponds, water quality parameters also
influence each other. Unbalanced levels of temperature and pH can
increase the toxicity of ammonia and hydrogen sulfide. Thus, maintaining
balanced levels of water quality parameters is fundamental for both the
health and growth of culture organisms. It is recommended to monitor
and assess water quality parameters on a routine basis.
In this article the most important water
quality parameters such as oxygen, pH, temperature, salinity, turbidity
and nitrogen compounds are described with insights on how these
parameters influence each other. Table 1 gives an overview of the water quality parameters with their standard values.
Table 1: Water quality parameters and their standard values
Parameter | Standard values |
(Dissolved) Oxygen | > 4.0 mg/l |
Temperature | Species dependent |
pH | 7.5 – 8.5 |
Salinity | Freshwater: <0.5 ppt Brackishwater: 0.5 - 30 ppt Saltwater: 30 - 40 ppt Optimum: 15 -25 ppt |
Carbon dioxide (CO2) | < 10 ppm |
Ammonia (NH4+/NH4-N) | 0 – 0.5 ppm |
Nitrite (NO2-) | < 1 ppm |
Hardness | 40 – 400 ppm |
Alkalinity | 50 – 300 ppm |
H2S | 0 ppm |
BOD | < 50 mg/l |
Dissolved oxygen (DO)
Dissolved oxygen (DO) is
one of the most important parameters in aquaculture. Maintaining good
levels of DO in the water is essential for successful production since
oxygen (O2) has a direct influence on feed intake, disease
resistance and metabolism. A sub-optimal level is very stressful for
fish and shrimp. It is therefore important to keep DO at optimum levels
of above 4.0 ppm.
The dynamic oxygen cycle of ponds fluctuates throughout the day due to phytoplankton photosynthesis and respiration (Figure 1).
Figure 1: The daily cycle of oxygen in a pond
As shown in Figure 1 maximum DO will occur in the late afternoon due to the buildup of O2 during the day through photosynthesis. As phytoplankton (microscopic algae) usually consumes the most O2 and since photosynthesis does not occur during
the night, DO levels decline. Critically low DO occurs in ponds
specifically when algal blooms crash. The subsequent bacterial
decomposition of the dead algae cells demands a lot of oxygen. Managing
the equilibrium of photosynthesis and respiration – as well as the algae
growth - is an important task in the daily work of a farmer.
The DO concentration in water goes up as temperature goes down, and decreases when salinity increases. |
When
feeding the fish and shrimp, oxygen demand is higher due to increased
energy expenditure (also known as specific dynamic action). To face this
higher oxygen demand, several measures can be taken:
Other sources of oxygen than photosynthesis
are diffusion or transfer from air to water. Wave action or mechanical
aeration is forcing this oxygen diffusion. Paddlewheel aerators
accomplish this by breaking water into small droplets and increasing
contact of water surface with air. Aspirator aerators compel air into
the water through a venture and a propeller. Another reason for aeration
is the circulation of aerated water through the pond.
Biochemical oxygen demand (BOD)
Biochemical oxygen demand (BOD) of the pond can affect the oxygen cycle and thus, the oxygen equilibrium. Five-day biochemical oxygen demand (BOD5)
is the amount of DO needed by aerobic biological organisms in the water
to break down organic material present at a constant temperature during
a 5-day period.
BOD5 is an important water
quality variable that may be required to demonstrate compliance with
water quality permits issued by the governments and to achieve farm
certification.
The BOD5 of pond aquaculture effluents usually ranges from 5 to 20 mg/l. The greater the BOD, the more rapidly oxygen is depleted.
Temperature
Temperature is another important water quality parameter. It
can affect fish and shrimp metabolism, feeding rates and the degree of
ammonia toxicity. Temperature also has a direct impact on biota
respiration (O2 consumption) rates and influences the solubility of O2 (warmer water holds less O2 than cooler water).
Temperature cannot obviously be controlled
in a pond. Aquatic animals modify their body temperature to the
environment and are sensitive to rapid temperature variations. For each
species, there is a range of temperature conditions (Table 2). It is therefore important to adapt fish and shrimp progressively when transferring them from tank to pond.
Also, the O2 cycle and thus, the DO levels can be affected by changes in the environment; a cloudy day will diminish the photosynthetic O2 input to DO. Correspondingly, uncommonly high temperatures will decrease the solubility of O2 in water and hence lower DO. When a pond is in “equilibrium” DO will not change drastically.
Every 10 °C increment in temperature doubles the rate of metabolism, chemical reaction and O2 consumption. |
Table 2: Temperature (°C) conditions for aquatic species
Species | Lower lethal temperature | Preferred temperature | Upper lethal temperature |
Rainbow trout | 0 | 13 - 17 | 24 - 27 |
Nile tilapia | 8 - 12 | 31 - 36 | 42 |
Tra catfish |
9
| 23 - 27 | 33 |
Crucian carp | 0 | 25 - 32 | 38 |
Channel catfish |
9
| 22 - 29 | 37 |
Cobia | 1 | 21 - 27 | 33 |
Tiger prawn |
14
| 25 - 30 | 36 |
White shrimp | 14 | > 20 | 40 |
Carbon dioxide (CO2)
Carbon dioxide (CO2)
in ponds is primarily produced through respiration by fish/shrimp and
the microscopic plants and animals that constitute the pond biota.
Carbon dioxide levels (and toxicity) are highest when DO levels are lowest (Figure 2). Thus, dawn is a critical time for monitoring DO and CO2. High CO2 concentrations inhibit the ability of fish and shrimp to extract O2 from the water, reducing the tolerance to low O2 conditions and inducing stress comparable to suffocation.
Figure 2: The daily cycle of oxygen and carbon dioxide in a pond
An increase in CO2 may also decrease the pH, which can lead to toxicity of nitrite. If plants in the water absorb too much CO2
for photosynthesis during the day, the pH will increase, and the fish
and shrimp are subjected to higher un-ionized toxic ammonia (NH3) concentrations.
Carbon dioxide concentrations above 60 ppm may be lethal. In an emergency, CO2 can be removed by adding liming agents such as quicklime, hydrated lime or sodium carbonate to the pond water.
At higher temperatures, fish and shrimp are even more susceptible to pH variations.
|
pH
pH is a measure of acidity
(hydrogen ions) or alkalinity of the water. It is important to maintain
a stable pH at a safe range because it affects the metabolism and other
physiological processes of culture organisms. It can create stress,
enhance the susceptibility to disease, lower the production levels and
cause poor growth and even death. Signs of sub-optimal pH are besides
others increased mucus on the gill surfaces of fish, unusual swimming
behavior, fin fray, harm to the eye lens as well as poor phytoplankton
and zooplankton growth. Optimal pH levels in the pond should be in the
range of 7.5 – 8.5.
The CO2 concentration in the water also influences the pH, e. g. an increase in CO2 decreases the pH, as already mentioned above (Diagram 1). As phytoplankton in the water utilizes CO2
for photosynthesis, the pH will vary naturally throughout daylight
hours. pH is generally lowest at sunrise (due to respiration and release
of CO2 during the night) and highest in the afternoon when algae utilization of CO2 is at its greatest. Waters of moderate alkalinity are more buffered and there is a lesser degree of pH variation.
Diagram 1: CO2 and pH correlation, influencing the toxicity of NH3
Ammonia
Ammonia is a very
important parameter for good fish and shrimp production. Under
particular conditions, ammonia can easily rise (through accumulation of
overfeeding, protein rich, excess feed wastes and excreted ammonia) to
dangerously high levels.
The higher water temperature and pH, the greater the concentration of the toxic ammonia form (NH3)
|
Ammonia in water exists in two forms, as ammonium ions (NH4+), which are nontoxic, and as the un-ionized toxic ammonia (NH3). The relative proportion of the one or the other depends on water temperature and pH. If the phytoplankton absorbs too much CO2
during the day, and therefore increase the pH to a value above 8.5, the
fish and shrimp are subjected, depending on the total ammonia nitrogen
concentration, to high ammonia concentrations (NH3). As little as 0.6 ppm (mg/l) free ammonia (NH3) can be toxic to many kinds of fish and shrimp, causing gill irritation and respiratory problems.
Nitrite (NO2-)
Nitrite (NO2-) is another form of nitrogenous compound that results from feeding and can be toxic to shrimp and fish. Nitrite is an intermediate product of the transformation of ammonia into nitrate by bacterial activity. The absorbed nitrites from the gut bind to hemoglobin and reduce its ability to carry oxygen.
An increase in CO2 may decrease the pH to a value below 6.5, which can lead to toxicity of nitrite through the formation of nitrous acid (HNO2). At 2 ppm (mg/l) and above, nitrites are toxic (injurious or lethal) to many fish and shrimp.
Hydrogen sulfide (H2S)
Hydrogen sulfide (H2S),
a colorless, toxic gas, is a by-product of the deterioration of organic
matter, usually under anaerobic conditions. Anaerobic soils with
moderate to high organic concentrations can be a significant source of H2S,
which is toxic to shrimp and fish even at low concentrations since it
hinders their respiration. If the bottom soil becomes black and a rotten
egg odor is recognized when sediment is disturbed, it indicates
anaerobic conditions and the presence of H2S. Hydrogen
sulfide is highly toxic in the unionized form (comparable to ammonia).
However, the unionized form is predominant at low pH (< 8) and high
temperature. At pH 7.5 approximately 14 % of the sulfide is in the toxic
H2S form and at pH 6.5 about 61 %. Therefore, sulfide concentrations should be below 0.002 ppm.
Many marine species live in close proximity to sediments that often contain H2S. Some even live in them. From the data in Table 3 it is clear, that the range of susceptibility to H2S poisoning is huge.
Table 3: Toxicity of H2S to various aquatic organisms
Common name | Species name | LC50 (ppb) |
Channel catfish | Ictalurus punctatus | 846.7 |
Indian prawn
| Penaeus indicus | 179.3 |
Oriental river shrimp
|
Macrobrachium nipponense
|
51.0
|
Crab
| Portunus trituberculatus | 31.5 |
Black tiger shrimp
|
Penaeus monodon
|
62.6
|
Pacific white shrimp
| Litopenaeus vannamei | 60.2 |
Drying
and tilling pond bottoms, in addition to maintaining thorough aeration
of ponds and frequent water exchange, are effective means in diminishing
hydrogen sulfide.
Alkalinity
Alkalinity is the
buffering capacitiy of water and represents its amount of carbonates and
bicarbonates. Alkalinity can affect the potential for primary
productivity and also the water pH. In case the water pH fluctuates
greatly during the day, lime can be used to increase alkalinity in the
water to stabilize the water pH. Values of 50 – 100 mg/l are considered
moderate and are recommended. Total alkalinity has been traditionally
expressed as milligrams per liter (ppm) of equivalent calcium carbonate
(CaCO3). Generally, alkalinity varies from site to site. In
the seawater, alkalinity is normally higher than 100 ppm but in
freshwater areas, alkalinity is often low, particularly during the rainy
season. Low alkalinity in freshwater or in low salinity areas will
affect the survival rate and molting of shrimp.
Hardness refers to the concentration of calcium and magnesium in water.
Waters can be classified by the degrees of hardness | |
0 - 75 mg/l | soft |
75 - 150 mg/l
| moderately hard |
150 - 300 mg/l
| hard |
Over 300 mg/l
| very hard |
Hard
waters have the ability to buffer the effects of heavy metals such as
zinc or copper which are toxic for fish and shrimp. Thus, hardness is a
crucial parameter in maintaining good pond “balance”.
Salinity
Salinity represents the
total concentration of dissolved inorganic ions, or salts, in water. It
plays a significant role for the growth of culture organisms through
osmoregulation of body minerals from that of the surrounding water. For
better survival and growth an optimum range of salinity should be
maintained in the pond water. If salinity is too high, fish and shrimp
will start to lose water to the environment. Younger shrimp appear to
tolerate a wider fluctuation of salinity than the adults. Drastic
changes of salinity may also alter the phytoplankton fauna and their
population densities and lead to instability of the ecosystem. Lowering
the salinity by more than 5 ppt, at each time of water exchange, is not
recommended.
Conclusion
Careful monitoring of water quality
parameters is important to understand the interactions between
parameters and effects on shrimp and fish feeding, their growth and
health. Each water parameter alone may not tell much, but several
parameters together can reveal dynamic processes taking place in the
pond. Water quality records will allow farmers to note changes and make
decisions fast so that corrective actions can be taken quickly.
References
upon request
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