Water quality
The production from
fish farms is strongly influenced by the quality of the water in which the
animals are grown. Poor water quality can have a variety of effects on fish and
crayfish. These effects range from killing all the animals in a pond, to
reducing their growth and increasing their susceptibility to disease and stress.
In operating a commercial farm it is essential to understand the factors that
influence water quality and to ensure it is correctly monitored and managed.
Water quality will vary
greatly from place to place and can change dramatically through time, even
within a day. The practices used on a particular fish farm affect how water
quality changes. A farm that has many fish in each pond, for example, is likely
to have more water quality problems than a farm with few fish per pond. Ponds
with many fish need to have food added to the pond as there will not be enough
natural food to feed the fish. Adding food to ponds increases the chances of
having problems with the water quality. The more food added the greater the
risks.
Some of the features of
water quality that should be considered by fish farmers are listed below:
Dissolved oxygen is one
of the most important aspects of water quality for fish farmers. The amount of
oxygen in the water can change rapidly and low oxygen levels can kill all the
animals in the pond.
Stocking a large number
of animals into a pond will mean that there is a greater chance of having
problems with oxygen. The animals will consume more oxygen themselves, also the
farmer will have to give them extra food which, again uses up oxygen as it
decomposes. If large numbers of animals are going to be grown in a pond, the
farmer needs to add extra oxygen by aeration.
This is a complex
aspect of water quality but it is also critical for fish farmers to understand.
A pond that is too acid will cause many secondary problems, as well as directly
affecting the animals. Most animals will not grow or reproduce properly in acid
water. Additionally, the stress created by acidity can lead to secondary
diseases. pH is the measure of acidity of the water and ranges from 0 to 14, a
pH greater than 7 is alkaline and less than 7 is acid.
The alkalinity of the
water is a measure of the stability of the pH of the water. Alkalinity is
expressed as the amount of carbonate (generally from dissolved lime) in the
water. The more carbonate in the water the higher the alkalinity and the more
stable the pH.
Water temperature is a
factor that is difficult for farmers to control, but it is important to
understand the impact it has on the animals being grown.
All animals have a
range of temperatures in which they can survive and a smaller range of
temperatures at which they will thrive and grow well.
The wastes passed by
fish and crayfish contain nitrogen compounds, mainly ammonia, which can build up
and pollute the pond water. Ammonia is converted to nitrite in the pond by
bacteria, both ammonia and nitrite may be poisonous to fish and crayfish. Other
bacteria in the pond can convert the nitrite to nitrate which is less harmful.
As with the other aspects of water quality there are complex factors which
determine how poisonous these compounds are at any particular time.
Most animals have a
limited range of salinity at which they will grow properly and reproduce.
There are other aspects
of water quality that may be important to a fish farmer, these include:
pesticides; herbicides; hydrogen sulfide; carbon dioxide; phosphorous; minerals.
It is critical that a
fish farmer monitors the water quality in his ponds and keeps clear records so
that he can identify problems and take remedial action before it is too late.
Water quality can change very rapidly, if the changes are not detected the crop
can be lost in a matter of hours.
References:
http://www.natfish.tafensw.edu.au/perspectives/Section2-7.html
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