Water quality includes all physical, chemical and biological factors that influence the beneficial use of water. There are many water quality variables in pond fish culture. All other things being equal, a pond with good water quality will produce more and healthier fish than a pond with poor quality.
Fish and crayfish perform all bodily functions in water which include eating, breathing, excreting wastes, reproducing and taking in or removing salts. Water quality within aquaculture ponds can affect these functions and therefore will determine the health of the fish and consequently the success or failure of a fish farming operation.
Water quality within an aquaculture pond is continuously changing depending on certain conditions. These behavioural changes are referred to as “Pond Dynamics” and must be understood if a pond system is to be managed effectively.
Water quality is divided up into physical, biological and chemical characteristics. Each one will be discussed separately.
Physical characteristics of an aquaculture pond refers to its structure or “zones” (Fig 1). A pond is divided up into different zones, each with their own biological and chemical characteristics The pond wall is known as the “littoral zone”.
Within this region aquatic plants thrive as they are able to access sunlight. Aquatic plants can be important in aquaculture ponds as they provide oxygen to the pond water, and food and shelter for some aquaculture species. However it is important not to allow aquatic plants to overrun your pond.
The “limnetic zone” is commonly referred to as the water column of the pond. Aquatic organisms such as phytoplankton, zooplankton and fish will inhabit this area. It is within this zone that the majority of water quality measurements are taken. The “benthic zone” or pond bottom is perhaps the most forgotten area within aquaculture ponds.
However this area is extremely important particularly for freshwater crayfish culture. Pond soil must be kept in good condition otherwise a build up of decaying organic matter can lead to oxygen problems within a pond. This will be discussed in more detail further on.
Biological characteristics:
Biological characteristics of an aquaculture pond refers to the aquatic organisms that live within the pond. This includes both plants and animals. The relationship between aquatic plants and animals is known as a trophic level or a food chain (ie who consumes who). This relationship is a continuous cycle as shown in Fig 2.
Bacteria form the base of the food chain within an aquaculture pond. Bacteria break down organic matter to produce nutrients such as phosphorus and nitrogen, and carbon dioxide (CO2).
These products are then utilised by phytoplankton, microscopic algae, to produce oxygen via photosynthesis. Oxygen and phytoplankton are then consumed by zooplankton which are tiny aquatic organisms.
Fish and crayfish feed on zooplankton as well as larger aquatic plants and supplementary feed that may be added to the aquaculture ponds.
Uneaten supplementary feed, dead aquatic organisms (including planktonic organisms and aquaculture species) and animal wastes will settle on the pond floor. Bacteria will feed on this decaying organic matter and the cycle will commence again.
Inorganic or organic fertilisers consisting of nitrogen and phosphorus can be added to aquaculture ponds to stimulate the growth of phytoplankton. This in turn will promote zooplankton growth. Before fish fry can be stocked within a pond, there must be a sufficient amount of zooplankton for them to feed on if the fry are to survive and grow.
The individual biomass of aquatic organisms within an aquaculture pond will vary over time (Fig 3). It is therefore important to have a good understanding of the population dynamics within your pond to stabilise population numbers of aquatic organisms and to ensure that the system will not crash.
Chemical characteristics:
Chemical characteristics refer to the water quality parameters that are measured within an aquaculture pond. Water quality in ponds change continuously and are affected by each other along with the physical and biological characteristics that have been mentioned previously. With this in mind water quality should be monitored regularly. This can be achieved by recording simple visible water characteristics such as water colour, clarity, plant and animal life.
Alternatively relatively inexpensive testing kits and recording probes (more expensive) can be purchased from analytical supply stores. The following water quality parameters are considered to be the most important in aquaculture.
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