The Importance of Water Quality in Aquaculture
Phytoplankton are microscopic photosynthetic organisms found suspended in water Phytoplankton are found in most bodies of fresh and salt water, including channel catfish ponds. Soon after a fish pond is dug and filled, samples of different species of phytoplankton can be found.
These new ponds can be seeded with phytoplankton through such sources as wading birds, turtles, and wind-blown spores. Many species of phytoplankton will reach a new pond, but only certain ones will survive and flourish.
The species that colonises the new pond depends upon the suitability of the environment for growth and, to some respect, chance.
It is not uncommon for ponds that are constructed side-by-side with similar soil types and the same water sources to contain very different populations of phytoplankton.
Eutrophication is the aging process, ending in “death” that takes place in a pond as it begins to be overgrown with the characteristics of a bog or lagoon. A dead pond is unable to support aquatic life and so should be guided against for a successful fish farming business.
The following descriptions summarise the rate and level of eutrophication attained by a particular water body, based on the physical and chemical characteristics.
(i) Oligotrophic (Very Healthy)
This indicates a very healthy pond or water body. Such is stable with good clarity water quality and is aging at a slower than normal rate. The ecosystem is balanced an able to convert all nutrients to all levels of the food chain.
This situation supports fish life and production is enhanced through maximum utilization of the entire food chain.
(ii) Mesotrophic (Moderately Healthy)
This pond condition may need aeration and possibly enzyme too. This indicates that the pond is aging progressively at a normal rate with good water quality. Benthic organisms are converting and oxidising the organics falling to the bottom on demand, or slightly slower than
demand.
(iii) Eutrophic (Unhealthy)
This shows that aeration and enzymes are necessary. Implication of this is that the water quality is eroding at an unhealthy accelerated pace. Efforts must be made to reverse the continued aging process.
Organic matter is accumulating, undecomposed and unconverted to the benthic layer, which is strangling the lake and all organisms that inhabit it. The low oxygen condition will result in periodic fish kill if un-aerated.
(iv) Hyper Eutrophic (Severely Degraded)
It is a must to have aeration and enzyme in this situation. Aging is accelerated beyond normal due to pollution and other influences. The pond will not be able to sustain a balanced ecosystem of life for very long.
Immediate and long-term plans must be made to retard the eutrophication process. Stratification is clearly evident and bottom dwelling organisms and bacteria are unable to convert and oxidize the rapid deposition of organic sludge.
Also evident are excessive weed and algae growth, low dissolved oxygen and fish kill or asphyxiation as a result of high level of nutrients in the water body. In this regard, phosphorus measured in open water is the most accurate indicator of primary productivity.
Phytoplankton Management
To date there is no effective way to manage a phytoplankton bloom other than reducing nutrient input in the form of feeds. This will limit the nutrients (phosphorus, nitrogen) available for growth and reproduction of the phytoplankton community and will result in a moderate phytoplankton density. In a catfish pond, this is around 40 to 50 pounds of feed/acre/day. (This is not an economical option for most catfish producers.)
Other methods have been used to try to control phytoplankton densities but have been completely ineffective. Studies have shown that the use of copper sulfate (CuSO4) to selectively thin the bloom does much more harm than good. Thinning the phytoplankton bloom in this manner results in very poor water quality and overall reduced fish production.
The use of dyes to shade out dense phytoplankton blooms has been studied. Dyes probably result in poorer water quality throughout the growing season. Dyes may also select for
undesirable blue-green algae in treated ponds.
Beneficial Practices Which Improve Water Quality
Beneficial (or best) management practice (BMPs) is the term used to describe a practice, or system of practices, designed to minimise the impact of agricultural activities on natural resources while at the same time maintain economic viability of the agricultural industry.
To ensure a supply of good quality water for aquaculture and agricultural purposes and domestic use it is necessary to consider BMPs at three levels:
• Watershed management; to protect both surface and ground water sources;
• Water source management; (such as a dugout, river or ground water aquifer) and
• Water treatment for use.
1. Watershed Management
Watershed management techniques offer the first water treatment strategy for maintaining good water quality. Watershed management includes beneficial management practices (BMPs) which protect surface water supplies (dugouts, small reservoir, rivers or lakes) and ground
water aquifers (shallow or deep water supplies below ground) by reducing the potential for contaminants to enter the water source.
Some examples of substances which could contaminate a water source include sediment, fertiliser, pesticides, animal waste, animal pharmaceuticals,
fuel, oil and hazardous products used in farming, or by-products used in agricultural processing.
There are three general types of beneficial practices which help reduce the risks of water quality degradation.
1a) Managing Agricultural Inputs
Managing agricultural inputs is an important element of pollution prevention. Proper handling and use of fertilizers, manures and pesticides will prevent or limit the impact on the environment.
– Nutrient management is the practice of applying fertilisers and manures only in the amounts that can be taken up by the crop. Over-applications increase the risk of contaminating surface and groundwater supplies.
– The use of pesticides can be minimised through “Integrated Pest Management”. This refers to a management strategy that includes an understanding of the target pest and use of a combination of physical, chemical, biological and cultural controls. Proper storage, mixing and handling are also essential in minimizing risk to the environment.
– Another practice includes livestock watering at sites well away from natural water courses (including wells or sensitive aquifers) in order to keep manure out of the water and building fences to prevent cattle access to water courses. Read more about this practice in Controlling erosion and runoff.
1b) Controlling Erosion and Runoff
It is an important beneficial management strategy. Runoff from fields to which pesticides, fertilizer and manure are applied, as well as runoff from livestock operations can contaminate water.
Practices such as strip-cropping, shelterbelts and use of cover crops prevent erosion and reduce the movement of nutrients and pesticides from agricultural land.
Residue management through conservation tillage and continuous cropping is also effective at controlling erosion, but requires higher inputs of fertiliser and herbicides.
1c) Barriers and Buffers
Barriers and buffers can be planted to intercept potential contaminants from agricultural lands. In most cases, these are strips of vegetation that slow the velocity of runoff water enough for sediment to settle out, water to infiltrate into the ground and nutrients to be taken up by plants
in the vegetated buffer zone. Grassed waterways, vegetative strips and field borders are examples of buffers that can be used in annually cropped fields.
Where buffer zones surround a stream or lake, they are usually referred to as riparian buffers. These strips capture sediment and nutrients from water that is moving into the waterway from surrounding agricultural lands. The vegetation also stabilises the banks and shores from the erosive action of the waterway itself.
2. Management of Water Sources
Management of the water source includes best management practices (BMPs) to sustain the best possible water quality at the source.
BMPs for Surface Water Sources include:
• Proper citing and design of river or dugout intakes
• Selective and limited use of algicides in farm ponds
• Control of surface water inflow to the reservoir or dugout
• Use of biological, chemical or physical techniques such as aeration, coagulation or reservoir covering BMPs for Groundwater Sources include:
• Diverting surface water runoff away from well pits
• Protecting sensitive aquifers,
• Preventive maintenance such as shock chlorination of wells when necessary.
3. Water Treatment for Use
Watershed management techniques offer the first barrier for maintaining good water quality. Management of the water source is the second barrier, aimed at maintaining and improving water quality. Recirculating system is the most recent technology in treating waste water from fish
pond while at the same time re-using it for the purpose of producing fish.
Although both strategies will improve the quality of the source water, they can not drastically improve water quality when pure water is necessary, such as for boilers used in agricultural industry.
Watershed management and water source protection strategies must be combined with other treatment processes, such as in-house treatment and disinfection, to ensure that the water is safe for drinking and aesthetically-pleasing for general household use.
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