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What
comes first, the water or the fish? Often, aquaculture projects come
about from one of two situations. Either someone is knowledgeable about
a species that they are hoping to culture and wants to find a site with
a source of water to fit the species or has a site with an available
source of water and is looking for a suitable species to rear. Water
source and species selection are closely linked. If large quantities of
water are involved, such as is the case with pond or flow-through
culture, then it is very difficult and expensive to treat water beyond
using simple screens and aeration. Pumping from deep wells, heating, or
chilling large volumes of water is energy intensive, expensive, and
unlikely to be economically or environmentally sustainable over the long
haul. For all these reasons, water quality from the source should be
well matched to the environmental optima of the species to be cultured
if the project is to be a success. Temperature
is often one of the most important factors when matching a water source
and a culture species. Except for small water volumes used for egg or
broodstock holding or for re-circulation systems, attempting to
significantly change the temperature of the culture water by heating or
cooling is costly. Exceptions to this rule are geothermal or waste heat
from a power plant to heat water, or possibly the use of deep, cold
marine water for cooling warm water. The amount of water needed for a
fish production system will depend on the intensity of culture. In
extensive systems, water requirements may be based upon management
concerns, such as the time to fill a pond, or the amount water needed to
makeup for evaporation or leakage. It is recommended that enough water
be available for aquaculture ponds to fill in two weeks. Amounts needed
for evaporation or leakage will depend on the area where the ponds are
located. For
intensive systems, water requirements will be dictated by the first
limiting water-quality parameter. Typically, oxygen is first limiting,
followed by carbon dioxide and ammonia. A mass-balance approach can be
used to determine water requirements for a given species and level of
production. Conversely, this approach can also be used to determine the
potential production levels of a given species for a known water source.
The mass balance approach is very powerful and can be used to examine
and identify which water-quality parameter is limiting for a given set
of circumstances, and which water treatments will have the greatest
impact on production. The
mass balance approach accounts for all inputs/ outputs and
production/consumption of a compound of interest in a defined system.
Compounds can get into or out of the system either predissolved in the
water flow or they can transfer into the water once it is already in the
system (form air, feed, light, and so on). Flow (Q, in volume/time,
m3/hr) in a system times concentration (C, in mass/volume, g/m3) equals
the rate of mass flowing (g/hr) into or out of a system pre-dissolved in
the water. Transfers (T), production (P), and consumption (R) are also
rates in mass/time (g/hr). Transfers of mass into or out of the system
come from or go outside of the system. For example, oxygen may be
transferred into a system and carbon dioxide out of a system due to
aeration. Compounds can also be taken up or released by the fish,
bacteria, or some other internally generated activity of the system, to
or from the water. Typically this relates to the biomass (B) of
organisms contained in the system. For example, the consumption of
oxygen by fish depends on the size and number of fish. Accounting for
all the ins and outs and conversions that occur can be expressed
mathematically. Mass is conserved, so the mass of a given compound into
and out of a system is in balance, and this can be expressed in the
equation below. The same approach can be used for energy, which is also
conserved. QinCin
+ T + (P - R)B = QoutCout
where
Qin is the influent water flow (e.g., Lpm), Cin is
the influent concentration of the compound in water (e.g., mg/L), T =
transfer of the compound into (or out of, if negative) the tank (e.g.,
mg/hr), P = production of the compound in the tank (e.g., mg/hr/kg of
organism), R = consumption of the compound in the tank (e.g., mg/hr/kg
of organism), B = biomass of the organisms in the tank (e.g., Kg), Qout
= effluent water flow (e.g., Lpm), and Cout = effluent
concentration of the compound in water (e.g., mg/L). For
example, if we have a spring that produces 100 m3/hr of water at 15'C
(59 °F), and it contains 10 mg/L of oxygen, and we want to know the
biomass of rainbow trout (Oncorhynchus mykiss) that can be
maintained with this water, we can use the above equation to figure it
out. We need to know what the trout will do to the oxygen in the water
and what the minimum level of oxygen should be to keep our trout
healthy. For this example, we will assume that the trout removes oxygen
at the rate of 200 mg O2/kg of fish/hr and requires a minimum
of 5 mg/L to stay healthy. The concentration to keep the fish healthy
will be equal to the outflow concentration since when it gets to that
level, we want to get rid of it. For
our example the equation has the following values: Flow
(Qin and out) = 100 m3/hr or 100,000 L/hr both
in and out Transfer
of oxygen (T) = 0 mg 02/hr (we have no aeration in this
example) Production
of oxygen (P) = 0 mg 02/kg of biomass/hr (if there were
plants or algae in the system then this might be a nonzero number) Consumption
of oxygen (R) = 200 mg 02/kg of fish/hr Concentration
of oxygen in the inflow (Cin) = 10 mg/L (this is saturation
at 15 °C) Concentration
of oxygen in the outflow (Cons) = 5 mg/L (set as the minimum acceptable
level) Plugging
in the values: QinCin
+ T + (P - R)B = QoutCout (100, 000 L/hr • 10 mg/L) + 0 mg/L/hr + (200 mg/kg hr • B kg) = (100,000 Lfhr • 5 mg/L) Solving
for B, (kg of trout) B
= 2500 kg (5500 lbs) The
same exercise, repeated with each water-quality factor, can be used to
determine the first limiting water quality parameter. This approach can
also be used to determine which water treatments will be necessary to
increase production. In the foregoing example, the effect of aeration
can be addressed by adding in a value other than zero for the production
term. The calculations become more difficult when re-circulation and
multiple treatments are considered. Surface
water sources, especially those from established ecosystems, will
contain pathogens and potential predators. Pollution may also be a
concern for rain, surface, and groundwater sources that are in proximity
to a pollution source. Various screens and sterilizers are available to
reduce predators and pathogens, but treatments for pollution will depend
on the nature of the pollution, and may not be treatable. Regulations
for the removal of water from either a groundwater or surface-water
source exist at multiple levels in almost all countries. This is true
for marine or freshwater sources. Additional regulations govern
discharge of water from aquaculture facilities. Property owners should
check with local authorities and government agencies regarding rights
and permits for development of the water source. Compiled
after: |
|
Table
1: |
|
Parameter
|
Acceptable range
|
|
pH |
6.0
- 8.0 |
|
Alkalinity
(measured as CaCO3) |
100
- 400 ppm |
|
Total
hardness |
100
- 400 |
|
Carbon
dioxide |
0
- 15 |
|
Iron
(Fe) - ferrous |
Less
than 0.1 ppm |
|
Ammonia
(NH3) |
Less
than 0.02 ppm |
|
Nitrate
(NO3) |
0
- 3.0 ppm |
|
Nitrite
(NO2) |
0.1
ppm |
| Table 2, Part 1: Water Sources and Some Associated Water-Quality Parameters | |||||
| Source |
Quantity |
Pathogens |
Salinity |
Temperature |
Suspended |
|
Generally |
Not
usually |
Fresh
to full |
Stable
over |
Low |
|
|
Streams
and |
Common |
Fresh |
Variable
short |
Varies,
can |
|
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Stable, |
Common |
Brackish
to |
Variable |
Varies,
can |
|
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Limited
and |
Reduced, |
Fresh |
Variable |
Low |
|
|
Highly |
Low
pro- |
Fresh |
Variable |
Very
low |
|
|
Can |
Requires |
Fresh
to full |
Set
by |
Depends
on |
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| Table 2, Part 2: Water Sources and Some Associated Water-Quality Parameters | ||||||
| Source |
Dissolved Oxygen |
Other |
Total
Gas |
Metals |
Oxidizers |
Buffer |
|
Low |
Carbon |
Can
be |
Iron
and |
None |
Depends
on |
|
|
Low
to high |
Generally
low, |
Same
as |
Depends
on |
None |
Depends
on |
|
|
Low
to high, |
Same
as above |
Generally |
Same
as above |
None |
Generally
well |
|
|
Low
to high |
Low |
Variable. |
Should
not be |
Chlorine |
Depends
on |
|
|
High |
Low |
Saturated |
Depends
on |
None |
Poorly |
|
|
Depends
on |
Carbon |
Generally |
May
build up |
Ozone
is |
Highly |
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Table 3: Recommendations for the safe reuse of wastewater in aquaculture: source, condensed from Pullin (1993). |
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The term ‘wastewater’ is used here in a generic sense to mean human excreta: whether fresh (‘nightsoil’); in the form of sewage or wastewaters in the narrow sense (excreta, with added water to facilitate waterborne transportation); or other partially treated forms such as septage. |
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| Pullin RSV, Compiler. 1993. Discussion and recommendations on aquaculture and the environment in developing countries. In: Pullin RSV, Rosenthal H, Maclean JL, editors. Environment and aquaculture in developing countries. ICLARM Conference Proceedings. 31. p. 312-338. | |
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Created by B. Ueberschär, 25.01.2003