Good water quality is essential to maintain optimum marron growth. To manage water quality, regular testing needs to be conducted. Relatively inexpensive testing kits are available through analytical supply stores. The following parameters should be monitored on a regular basis.
Water Temperature:
The optimum growth rates for marron is achieved between 170 C and 250 C. Growth will decline and eventually cease when temperatures hit below 12.50 C. Mortalities will start to occur when temperatures reach 300 C.
Ponds can also undergo thermal stratification and occurs when the pond divides into two separate layers. This will depend on the amount of light that penetrates the water column and is mainly due water depth and the turbidity in the pond.
Thermal stratification is caused by the upper surface layer being warmed by the sun and therefore will remain on the surface as it is less dense than the bottom cooler layers of water in the pond. Unless the stratification is broken down by some sort of mechanical aeration (ie wind action, airlift systems etc), its effect can be detrimental to pond life.
Thermal stratification will also cause oxygen stratification. This occurs when oxygen rich waters on the surface can not mix with the cooler bottom waters where oxygen is being continually consumed by plant, organic and animal matter. If the stratification persists, oxygen levels on the bottom will drop to levels that would be lethal to bottom dwelling marron.
Dissolved Oxygen:
Oxygen levels in pond systems depend on water temperatures, stocking rates, and the amount of aquatic vegetation growing in the pond. Oxygen levels experience day and night fluctuations where levels are high during the day and then drop at night. This is due to aquatic plants within the pond producing oxygen during the day via photosynthesis (which requires sunlight) and respiring (consume oxygen) at night.
It is recommended that oxygen levels within a pond be maintained above 6ppm. However marron can withstand oxygen levels as low as 3ppm although not for extended periods.
pH Levels:
pH is the measure of the concentration of Hydrogen ions (H+ ) in the water. Water with a pH of above 7 is considered to be alkaline (low concentration of H+ ) whereas a pH level of below 7 is acidic (high concentration of H+ ). pH levels in freshwater ponds depend on factors such as nature of the catchment area (eg: limestone areas would have a high pH) and the amount of respiration and photosynthesis occurring in the pond (ie the amount of carbon dioxide present – carbon dioxide dissolves in water to form carbonic acid).
Water with a pH between 7.5 and 8.5 are recommended however marron can tolerate a pH of 7.0 and 9.0. A pH of below 7.0 will increase the toxicity of dissolved metals within the water column and soften the exoskeleton of the marron. A pH of above 9.0 will greatly increase the toxicity of ammonia within the ponds.
Alkalinity and Water Hardness:
Alkalinity refers to amount of carbonates and bicarbonates in the water and water hardness refers to the concentration of calcium and magnesium. As calcium and magnesium bond with carbonates and bicarbonates, alkalinity and water hardness are closely interrelated and produce similar measured levels.
It is recommended that alkalinity and hardness levels are maintained around 50 to 300 mg/l which provides a good buffering (stabilising) effect to pH swings that occur in ponds due to the respiration of aquatic flora and fauna.
A lack of calcium in the water will also result in soft shelled marron as they rely on the intake of calcium from the water column to harden their shells after moulting. Water alkalinity and hardness can be increased by liming ponds which involves adding a measured amount of lime to the pond.
Salinity:
Salinity is a measure of the saltiness of the water. Marron can withstand salinities up to 17 ppt however their growth rates start decreasing when the salinity reaches 4ppt.
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 marron ponds.
Ammonia:
Ammonia in ponds is produced from the decomposition of organic wastes resulting in the breakdown of decaying organic matter such as algae, plants, animals and uneaten food. Ammonia is also produced by the marron as an excretory product.
Ammonia is present in two forms in water – as a gas NH3 or as the ammonium ion (NH4 + ). Ammonia is toxic to crayfish in the gaseous form and can cause gill irritation and respiratory problems.
Ammonia levels will depend on the temperature of the pond’s water and its pH. For example at a higher temperature and pH, a greater number of ammonium ions are converted into ammonia gas thus causes an increase in toxic ammonia levels within the marron pond.
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