WATER TREATMENT
Water needs to be treated before it can be used in the hatchery. Usually, it is necessary to pass incoming water through some form of physical (gravel/sand bed) filter (see Figure 30).
If the incoming water is not filtered, or is still turbid, it may be necessary to allow the solids to settle in one tank before transferring it to another tank for treatment. Mix the seawater or brine with freshwater to form 12 ppt brackishwater (see Table 4). Then treat the water as in Box 6.
Water drawn from underground sources may not need to be settled However, the removal of protozoa and bacteria by chlorination, as indicated in Box 6, is still essential.
Other forms of water treatment may be helpful. Some hatcheries filter the brackishwater before use through 5 μm filters but most commercial hatcheries omit this step. Some add 10 ppm of the chelating agent EDTA to larval rearing water to improve performance.
Others use water which has a potentially unsuitable iron content (see Box 1).
Water quality remains important, not only in the incoming supply but also within the hatchery itself. Monitor the water in your larval tanks frequently to see that its quality is being maintained (Box 7). Simple field kits can be purchased to check the parameters listed in Box 7 but they are not specified in this manual because they are standard water quality items. For small flow-through hatcheries it is not practical to install facilities for the other types of analytical work, especially where they involve the analysis of seawater or brackishwater. Samples of water for the analysis of other parameters, such as hardness, metals, pesticide residues, etc., should be sent to governments, universities or private laboratories who have the facilities and staff to deal with them.
Further reading on water quality and analysis is available in Boyd (1979).
If you are using a recirculation system, adjust the salinity and temperature to 5-7 ppt and 28 to 31°C, allowing the system to stabilize before stocking.
This salinity allows larvae to be stocked directly from the hatching tank without acclimation. The salinity in the culture tank should then be increased to 12 ppt. Neither artificial brackishwater nor freshwater should be added through the biofilter tank once the biofilter substrate has been added. Its bacteria are sensitive to sudden changes in tempera-
BOX 6
Treatment of brackishwater
LLOW IT TO STAND so that any sediments will settle. It is essential to remove as much of the suspended solids as possible, otherwise the chlorination that follows may be partially or totally ineffective
Chlorinate the brackishwater with 5 ppm of active chlorine7.
Allow the water to stand for one day.
Remove the residual chlorine by vigorous aeration for 6 hours before use (note: sodium thiosulphate can be used to remove the residual chlorine more quickly but its use is not recommended because it also may prove toxic to larvae. Vigorous aeration for 6 hours is adequate).
7 Both dry and liquid sources of chlorine vary considerably in their chlorine content from product to product and from batch to batch. It is best to determine the level of chlorine in each batch before use, to ensure that you are actually using the quantity of chlorine that you think you are adding. The methods for determining the level of chlorine in commercial bleaching powder or liquid bleach are contained in Annex 4, Table 6.
BOX 7
Regular monitoring of larval water quality
HE FOLLOWING PARAMETERS should be measured in flow-through hatchery tanks:
TEMPERATURE
pH
SALINITY
DISSOLVED OXYGEN
NOTE: dissolved oxygen meters are very expensive in some countries. Monitoring this parameter is ideal but can be omitted if you are sure that your aeration system is working perfectly
The following additional parameters should be measured in recirculation systems:
AMMONIA
NITRITE
ture and salinity. New water should be disinfected and filtered prior to introduction into the system. The use of ultraviolet light (UV) to treat the water in recirculation systems has been recorded in experimental hatcheries for M. rosenbergii but is not regarded as being essential for commercial hatcheries.
The pH of the water used in recirculation systems does not usually vary much during the larval cycle but occasionally, especially if the biofilter medium is artificial (non-calcareous), periodical buffering with sodium bicarbonate (NaHCO3) or sodium carbonate (Na2CO3) may be necessary, as noted earlier.
Temperature should be kept steady, not only for the sake of the larvae but also because the biofilters do not operate efficiently if it fluctuates. The biofilters should maintain levels of unionized ammonia, nitrites and nitrates at acceptable levels.