Animal wastes flow
Direct feeding value of pure wastes is known to be poor. Wastes act by:
These two processes are strongly interlinked, since phytoplankton is a major source of detritus for bacterial production. Also, phytoplankton, through photosynthesis, is the chief producer of dissolved oxygen in the pond used by all organisms including fish.
1. Are wastes available on-farm? If so, are the wastes already used? Should they be diverted for use in fish culture?
Livestock wastes are often important as crop fertilizers and fuel. Consider the opportunity costs.
2. Is it worth raising livestock, especially to generate wastes for aquaculture? Consider:
1. Are all wastes to be used in fish culture?
If wastes are to be used elsewhere, they should be collectible prior to entering the pond (e.g. use a sump). Also, wastes should be available in larger quantities at certain periods when their use should be reduced for fish culture (e.g. during the cool season).
2. Can all wastes be collected?
Feedlot livestock are kept confined at all times so all the wastes can be collected and used.
Small-scale farmers often allow livestock to graze or scavenge during the day and only confine these at night. This reduces feed costs considerably, often allowing only on-farm or low-cost, supplementary feeds to be given. However, collectible wastes will be less.
3. Livestock may be penned at the farmer's house for security or traditional reason; this may limit potential advantages of integration. Labour is required to collect or prepare livestock feed.
4. Ponds may be multifunctional. Large animals are usually denied access to the pond because entry to and wallowing in it can destroy the dikes and cause turbidity which reduces natural food production.
On the pond dike
Over the pond
In the layout/design aspect, consider:
Design the pond to allow limited access.
Too much manure when loaded in fishponds can cause dissolved oxygen depletion resulting in fish mortalities. When manure loading is excessively high, too much decomposition occurs; thus, the biological oxygen demand is high, using up the available dissolved oxygen.
Phytoplankton produces dissolved oxygen during the day but consumes it at night. Another source of dissolved oxygen in a static water is diffusion of atmospheric oxygen.
Indicators of low dissolved oxygen
1. When plenty of fish are on the water surface «gasping for air» (i.e. they are consuming oxygen from the thin and oxygenated top layer of water)
2. When air or gas bubbles are observed in the water
3. When the pond water is brownish or greyish
4. When the pond water smells pungent.
What to do when dissolved oxygen is low
If the water is turbid because of suspended sedimentary particles, spread over the pond surface chopped rice straw or hay, allowing them to settle at the pond bottom together with the silt. But caution: too much decomposing hay can also deplete dissolved oxygen. The pH or hydrogen ion concentration determines whether the water is acidic or alkaline. Highly acidic water (4 or below) can result in fish kills.
Methods to measure pH
Use equipment such as litmus paper, pH meter and water quality measuring kit.
Here is a practical method. Test the water: if water tastes sour, it is acidic. Know the water source; acidic water comes from swamps, bogs or stagnant areas.
What to do if water is acidic
Ways of knowing the presence of hydrogen sulphide
Hydrogen sulphide is a poisonous gas emitted from the pond bottom as a result of decaying and decomposing organic matter. Its presence is indicated by the following:
What to do when hydrogen sulphide occurs
Causes of and possible remedies for different water quality problems
Off-flavour or muddy taste of fish harvested in highly manure-loaded pond and in pellet-fed ponds can be a serious problem if fish farmers do not follow the proper harvesting procedures. People will not buy or eat the fish with off-flavor or muddy taste.
Here are some suggestions to remove the off-flavor or muddy taste:
1. Stop manure loading or delivery to the fishpond at least two days before harvesting.
2. Partially drain the pond leaving about 40-50 cm water depth.
3. Harvest fish by seining before draining the pond totally. This will minimize fish mortality and the murky odor of fish associated with muddy water.
4. Transfer fish to a net enclosure installed in a pond with clean water or in holding tanks with running water and hold the fish for at least 4-6 hours but preferably for several days.
5. Sell fish alive or fresh.
Ways to measure water transparency (or turbidity)
Use of a Secchi disk
The disk is lowered into the water from a caalibrated rope.
If it disappears within a depth < 30 cm, the water is turbid.
Using one's hand
With the hand stretched forward, cup the palm and bend it towards you.
In this position, slowly dip the hand into the water until the palm becomes invisible.
Transparency is expressed as the distance from the wrist to the end of the water mark on the arm.
Issues for further consideration
With high loading rates, the pond sediment can be raked periodically during the early afternoon period, when the dissolved oxygen concentration is at its peak, in order to aerate the bottom sediment and facilitate aerobic decomposition processes.
It is wise to apply the desired amount of manure in small doses and more frequently. Daily application gives much better results than weekly and fortnightly applications. It helps in keeping the water quality parameters under control. Frequent but smaller applications do not allow the deterioration of water quality abruptly. As soon as any symptom of algal blooming or dissolved oxygen depletion is visible, the application of manure should be suspended. Application of manure in liquid form (or sewage) keeps the particulate detritus materials suspended in the water column for longer period of time allowing for bacterial decomposition at a higher rate due to the aerobic environment. During this period, these detritus particles are enveloped with a multiplying bacterial population and are also available for fish as well as zooplankton as quality feed material.