Dissolved oxygen:
Dissolved oxygen is probably the most critical water quality variable in freshwater aquaculture ponds. Oxygen levels in ponds systems depend on water temperatures, stocking rates of aquaculture species, salinity, and the amount of aquatic vegetation and number of aquatic animals in the ponds.
Dissolved oxygen concentrations will vary throughout the day. Dissolved oxygen in the water is obtained through diffusion from air into water, mechanical aeration by wind or aeration systems, and via photosynthesis by aquatic plants (Fig 4).
Oxygen is also lost from the system via respiration where oxygen is consumed by aquatic organisms (both plants and animals), and by decaying organic matter on the pond floor. Declining oxygen levels can be caused by a number of factors.
This includes large blooms of phytoplankton and zooplankton, high stocking rates, excessive turbidity that will limit the amount of photosynthesis occurring and high water temperatures.
Levels of dissolved oxygen will also decrease after a series of warm, cloudy, windless days. Low dissolved oxygen can be lethal to our aquaculture species. Some effects include stress, increased susceptibility to disease, poor feed conversion efficiency, poor growth and even death.
A number of measures can be put in place to help alleviate low oxygen problems. There are a number of different types of aeration systems that when installed in ponds will help circulate and oxygenate the water.
The majority of existing growers use Airlift pumps however other systems include Paddle wheels, Aspirator pumps and Diffused air systems. Flushing ponds with fresh water and reducing feeding rates will also help increase oxygen levels within the ponds.
When taking measurements of dissolved oxygen within an aquaculture ponds it is important to note that readings will alter depending on the time of day, the amount of plant growth within in the pond, and the position in the pond from where the measurement was taken. This is due to the following reasons.
Aquatic plant life, algae or phytoplankton are present in large numbers will produce high oxygen levels within a pond during the day due to photosynthesis occurring. However at night without the presence of sunlight, these organisms will be consuming oxygen rather than producing it via photosynthesis which may result in dangerously low oxygen levels (Fig 5). Therefore if the pond has extremely high oxygen levels during the day there may be a good chance they will drop considerably at night.
Another factor which may affect the reliability of a dissolved oxygen reading is the location of the measurement taken within the pond. An aquaculture pond can undergo stratification where the pond’s water column will split into two separate layers (Fig 6). The top layer will heat up however oxygen levels will still remain high due to oxygen diffusing from the air into the water.
The bottom layer of the pond will remain colder and oxygen levels will deplete due to decaying organic matter on the bottom of the pond essentially sucking oxygen from the water. The top and bottom layers which are separated by a barrier known as a thermocline, will not mix due to the lack of mechanical movement within the pond’s water column.
Pond stratification may occur during periods of hot, still weather and is usually more prevalent in deeper, highly turbid ponds. Pond stratification is more likely to be avoided by installing aeration systems within the ponds to ensure that water is circulated during the critical periods of time, and making sure that pond depth is no greater than around two metres.
Water temperature:
Fish and crayfish are ectotherms as heat is obtained from their external environment. Therefore the body temperature of culture animals is usually the same as that of the water temperature. Temperature will affect all chemical and biological processes.
Temperature therefore has a direct effect on important factors such as growth, oxygen demand, food requirements and food conversion efficiency. The higher the temperature, the greater the requirement for oxygen and food and the faster the growth rate.
Optimum temperature conditions will depend on the species of fish that is cultured. These conditions will need to be met to ensure optimal growth and reproduction success. It is therefore important to select a culture species that is best suited to the climate in your area for pond culture, or in the case of tank culture, be prepared to heat or cool the water supply.
pH levels:
The pH is the measure of the hydrogen ion (H+ ) concentration in soil or water. The pH scale ranges from 0 to 14 with a pH of 7 being neutral. A pH below 7 is acidic and an pH of above 7 is basic. An optimal pH range is between 6.5 and 9 however this will alter slightly depending on the culture species. pH will vary depending on a number of factors.
Firstly pH levels of the pond water will change depending on the aquatic life within the pond. Carbon dioxide produced by aquatic organisms when they respire has an acidic reaction in the water. The pH in ponds will rise during the day as phytoplankton and other aquatic plants remove CO2 from the water during photosynthesis.
The pH decreases at night because of respiration and production of CO2 by all organisms. The fluctuation of pH levels will depend on algae levels within the pond (Fig 7).
Sub-optimal pH has a number of adverse affects on culture animals. It can cause stress, increase susceptibility to disease, low production levels and poor growth. Signs of sub-optimal pH include increase mucus on the gill surfaces of fish, damage to the eye lens, abnormal swimming behaviour, fin fray, poor phytoplankton and zooplankton growth and can even cause death.
In the case of freshwater crayfish low pH levels will cause the shell to become soft. This is due to the shell of the crayfish being composed of calcium carbonate which reacts with acid. Sub-optimal pH levels are usually caused by acidic water and soils, poorly buffered water (will be discussed further on) and increased CO2 production.
Treatment methods will depend on whether there is a high pH problem or a low pH problem. To treat a pond with low pH, a pond can be limed with agricultural limestone or fertilised to promote plant growth. To decrease a high pH, the pond can be flushed with fresh water, feeding rates can be reduced to decrease nutrient input into the pond, gypsum (CaSO4) can be added to increase the calcium concentration, or alum (AlSO4) can be added in extreme cases.
Salinity:
The term salinity refers to the total concentration of all dissolved ions in the water, it is not, as many people think, the concentration of sodium chloride in the water. Measurements of salinity are referred to as mg/l or ppm. When salinity is high, it is common to report it as ppt. For a point of reference, seawater is approximately 35ppt.
If salinity is too high, the fish will start to lose water to the environment. As freshwater fish are not physiologically adapted to osmoregulate within a saline water source, decreased growth and survival can occur under these conditions.
The salinity of the water source that is to be used for aquaculture should be tested before a project commences. Salinity tolerances will vary amongst species therefore it is important to chose an aquaculture species that is best suited to the salinity of the water source.
Turbidity:
Water turbidity in freshwater ponds is caused by phytoplankton and zooplankton (microscopic plants and animals) and suspended solids such as clay and silt particles in the water column. Water turbidity is important as it determines the amount of light penetration that occurs in the water column of a pond.
This in turn will have an affect on the temperature of the water and the amount of vegetation and algae that will grow in the pond itself. For example a highly turbid pond will allow less light penetration therefore the temperature of the water will be lower.
A combination of less sunlight and lower temperatures will result in a decreased amount of vegetation present with in the ponds which depend on sunlight and warmth to grow. A low turbid pond will of course have the opposite affect.
Turbidity is measured in centimetres using a sechii disk which consists of a round plate divided into alternate black and white “pie” sections. This disk is attached to a graduated rope or a metal handle divided into measuring units (usually at 2 cm intervals). The disk is lowered into the water until it can not be seen and then raised until it re-appears. Sechii depths between 20cm and 60cm are recommended for optimal management of freshwater ponds.
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