Most hatchery managers incubate 1 to 3 grams of cysts per liter of water. It is inadvisable to incubate more than 5 grams of cysts per liter of water because a density higher than this could cause foaming.
For this example, the 2.56 grams of disinfected cysts should be transferred to a 1-liter Imhoff cone (settling column) filled with seawater of 5 to 32 ppt salinity and at a temperature of 25 o C (77 o F) for 15 to 20 hours.
Some hatcheries use diluted seawater (as low as 5 ppt) because less energy is required for the nauplii to emerge from the cysts at the lower salinity. Constant florescent light is supplied at an intensity of approximately 2,000- lux.
One or two 40-watt tubes should be sufficient. The hatching container is vigorously aerated to keep the cysts in suspension and exposed to the light.
Although this example uses a small hatching container, they can range from 1 to 10,000 liters, depending on the needs of the hatchery. The container should have a conical or cone-shaped bottom to keep cysts syspended. If cysts are allowed to settle, hatch rates may be poor.
Separate cysts from shells:
After 15 to 20 hours of incubation, most of the cysts will be hatched and there will be a noticeable color change in the culture from brown to orange. Stop the aeration at this time. The pinkish orange nauplii will be seen swimming in “clouds.”
Any empty or undissolved shells tend to float, while the full, unhatched cysts and some debris sink. If left undisturbed for 5 minutes, nauplii will concentrate toward the bottom of the container. If the container is clear, a black cloth can be placed on top of the container to speed up the concentration of nauplii toward the bottom.
Open the valve at the bottom of the container, first to remove debris and then to catch freshly hatched nauplii. A siphon also can be used to remove first the debris and then the nauplii from the bottom. The nauplii should be collected on a 100- to 120-micrometer screen, washed with clean seawater, and placed in a small volume of water.
Washing removes contaminants and hatching metabolites. It is sometimes very difficult, especially with certain brands of Artemia, to separate the nauplii from the empty shells and debris. Adding salt to the incubation container may help this separation and will not harm the Instar I nauplii.
Try adding a small quantity of salt first to see the results, then add more if necessary until separation occurs. Any unhatched cysts should be saved to use in the next culture, because some of them might hatch with the next batch.
Count the hatched Artemia:
Hatch rates vary, so it is important to quantify the number of Artemia that hatched (Fig. 3). To do this, bring the level of seawater in the beaker holding the concentrated freshly hatched nauplii to some easy-to-use volume, such as 100 ml (3.38 fl oz).
The nauplii should be mixed continuously while a 1-ml sample is drawn off with a pipet and placed in another beaker. The contents of this beaker should be increased to 100 ml with distilled water (1:100 dilution). Add full strength Lugol’s stain or iodine to the beaker to slow down or kill the nauplii if high concentrations are present.
Mix the contents of the beaker well and draw a 1-ml sample with a pipet for counting. If the numbers are high, count the nauplii in 0.1 ml. This count gives the concentration of nauplii in 0.1 ml of the dilution (H).
Multiply the number counted by ten to get the number of Artemia in 1 ml, then multiply by the dilution factor (100) to calculate the number of Artemia per ml in the original beaker. To get the total number of Artemia in the original beaker, multiply the concentration per ml by the volume of the beaker in ml. This results in the concentration of Artemia per ml in the original 100-ml beaker.
In the form of a formula:
Number of Artemia/ml concentrated in 100 ml beaker is (G) = H x 1,000. H = number in 0.1 ml
Calculate the number of Artemia in the rearing tank:
Follow a similar procedure to determine the number of Artemia remaining in the rearing tank from previous feedings. Scoop a sample of approximately 100 ml from the rearing tank into a beaker. Add Lugol’s stain or iodine to kill the Artemia. Use the same counting procedure and calculation as above, but do not dilute the sample.
The formula is:
Concentration of Artemia per ml (I) = number per 0.1 ml (J) x 10
Subsamples also can be taken with a Hensen-Stemple pipet (Fig. 4) or an automatic pipet with a large intake so as not to limit the uptake of animals.
Calculate the number of Artemia to be fed:
Because Artemia are quite costly and an excess in the culture system is not desirable, it is critical not to overfeed. The quantity of Artemia added to the larval system must be adjusted on the basis of the number remaining from the previous feeding. Each day, the stage of the fish larvae is assessed and used to determine the total quantity of Artemia required per ml of the rearing tank (K).
Subtract the number of Artemia per ml in the tank from the total you need to find the quantity per ml to be added. The quantity to be added (L) equals (K) minus (J). Multiply this number by the volume of the tank in ml to find the total number you require in the tank (M). Divide this number by the number of nauplii per ml in the fresh batch of Artemia (G) to determine the number of ml to be transferred to the fish tank.
It may be easier to calculate with the following formula:
Author:
Granvil D. Treece
