FEEDING

A wide variety of feeds are employed by different hatcheries, including the nauplii of brine shrimp (Artemia spp.), a freshwater cladoceran (Moina spp.), fish eggs, squid flesh, frozen adult Artemia, flaked adult Artemia, fish flesh, egg custard, worms and commercial feeds. This freshwater prawn manual describes only one feeding regime in detail, which has been found to be effective.

However, many alternative feeding systems exist and the readers of this manual may wish to experiment with locally available feeds. Those who wish to consider

FIGURE35
Many of the larvae that go missing during the larval rearing cycles are not mortalities but are lost through operator error.
For example, it is easy to lose larvae during water changing and tank cleaning. Losses can be minimized by filtering the water removed from tanks and returning the live larvae, if healthy

SOURCE: EMANUELA D’ANTONI, AFTER NEW AND SINGHOLKA (1985)

the use of alternative live foods are recommended to obtain another FAO manual (Lavens and Sorgeloos, 1996), which includes sections on the culture and use of rotifers (e.g.
Brachionus plicatilis) and cladocerans (e.g. Moina spp.), as well as Artemia, in aquaculture hatcheries. Useful information on live food production is also contained in another FAO publication (Moretti, Pedini Fernandez-Criado, Cittolin and Guidastri, 1999), which describes the use of rotifers and Artemia in marine finfish hatcheries.
Two feeds are used in the feeding system described in this manual, namely brine shrimp nauplii (Artemia nauplii, referred to subsequently as BSN) and prepared egg custard feed (hereafter called EC). Methods for preparing these feeds before use are given in Annex 4 (BSN) and Annex 5 (EC). BSN are small crustacean nauplii hatched from cysts which can be bought in vacuum packed bags and cans. An example of a feeding schedule is given in Table 8.

BOX 10

Maintenance schedule for recirculation systems
IN THE MORNING:

MONITOR the system, checking water temperature, water level and water flow.

CLEAN the screens and check if they need changing.

REPLACE water losses with water that has been properly processed and stored (this avoids ‘shocking’ the larvae or the bacteria of the biofilter by adding water of different composition, salinity or temperature).

MONITOR the mechanical filters and clean if necessary.

FEED the larvae according to the normal schedule and then stage (see Annex 1) and monitor their progress and health.

HARVEST your Artemia nauplii and store (see Annex 4, Table 4) excess quantities for future use.

PREPARE a new batch of Artemia cysts for hatching.
IN THE AFTERNOON:

SCRUB the bottom and sides of the tanks to remove all algae and organic debris. Keep the aeration going while you do this so that larvae do not become trapped between the mop and sides or bottom of the tank.

TURN the aeration and water flow off and siphon the visible waste from the larval tanks. Make sure you turn the aeration and water back on!

CHECK the siphoned waste (see Figure 35) for dead larvae. Count the number of dead larvae and subtract this from the number stocked, or the number estimated the previous day. Remember that this will be an underestimate because of cannibalism on dead or weak larvae. Your estimates will not be accurate but making them helps you to look out for increasing levels of mortality or acute problems. In well-managed recirculation hatcheries, survival is normally better than in flow-through hatcheries (partly, perhaps, because fewer animals are lost but mainly because the water quality is more stable).

MEASURE ammonia and nitrite levels (the frequency of these tests may be reduced to two or three times per week once the system is stable).

SEE if there is any decrease in food consumption (this would be an indicator of bacterial or water quality problems). If microbiological facilities are available the bacterial concentration of the water could also be monitored.
EVERY SECOND DAY:

DISINFECT all small equipment, such as beakers, porous air stones, hoses, buckets, etc., with 5 ppm/L active chlorine solution, rinse thoroughly with freshwater, dry, and store.

Most freshwater prawn larvae do not feed on the first day (hatching day). However, you are recommended to provide some BSN in the late afternoon of the first day because some larvae begin to eat early. From day 2 until day 4, feed BSN five times per day, with the last and main feed in the evening. After that, you can gradually reduce the number of BSN feeds per day until, by day 10, you are only giving BSN at the evening feeding time.
The evening meal should be given as late as possible (18.00-19.00). The amount of BSN you give at each feeding time depends on your visual examination of the larval water.
Freshwater prawn larvae do not actively search for food, which is why BSN (which swim actively in the same part of the water column as the larvae) are such a valuable feed type.
The ideal is therefore to have BSN always present in the tanks in sufficient numbers for the larvae to ‘bump’ against. The amount of BSN required at any one time depends pri-

TABLE 8 Hatchery feeding schedule

NOTE: BSN = BRINE SHRIMP NAUPLII; EC = EGG CUSTARD BASED DIET. THE QUANTITIES TO BE FED ARE DISCUSSED IN THE RELEVANT SECTION OF THE MANUAL.

marily on the tank volume, not on the number of larvae present, although the latter of course controls the rate at which BSN are consumed. This concept is clearly illustrated in Box 11.

BOX 11

Feeding BSN depends on tank volume, not the number of larvae in it

Suppose that each freshwater prawn larva consumes 50 BSN/day.

Suppose that you have 150 000 larvae in one of your tanks.

You would therefore need 50 x 150 000 = 7.5 million BSN/day to provide sufficient food.

However, suppose you only have one larva in another of your tanks.

Would you only put 50 BSN into the tank and would the larva find them? No!

This demonstrates that it is the density of the BSN that matters, not the total quantity.

As a guide, there should be about 3-6 BSN/ml directly after feeding, depending on the age of the prawn larvae, and 1 BSN/ml left in the water just before the next BSN feeding time. If there is more than 1 BSN/ml at the latter time then you have been overfeeding or the larvae are not feeding well. If there is less than 1 BSN/ml, you should add more this time than last time. A density of 3-6 BSN/ml in a tank with 5 m3 of water means that 15 to 30 million BSN have to be added.
The quantity of brine shrimp cysts (‘eggs’) necessary to produce 1 million BSN depends on the source and quality of brine shrimp cysts used and the preparative treatment they are given; it is usually stated on label of the cans. As a rough guide, however, you can assume that 75 to 150 g of Artemia cysts will be required to produce the 15 to 30 million BSN required for the daily feeding of a 5 m3 larval tank initially stocked with 50 larvae/L and expected to provide about 25 PL/L. Normally one larval cycle in this size of tank will consume 1.25-2.5 kg of brine shrimp eggs.
By day three, you can start feeding tiny quantities of EC, gradually increasing the feeding frequency to five times per day, spread out evenly throughout the day. Give the last feed of EC about 15.00. Do not give EC for the final late afternoon feeding because the quantity necessary to supply the requirement throughout the night in one feeding would foul the water; use BSN only. From day 5 you are starting to reduce the frequency of feeding BSN and by day 6 you should be feeding EC about 5 times per day. Continue feeding at this frequency throughout the rest of the larval cycle. After day 10, you need only give BSN at the evening feeding, to ensure the presence of food during the night. By this time you should be using very much greater quantities of EC at each feeding time.
The exact quantity of food to be given at each meal cannot be prescribed because it depends on the utilization of the feed by the larvae. You must judge this visually. The quantity of EC feed consumed will increase as the larvae grow. The basic rule is that each larva should be seen to be carrying a particle of EC immediately after every EC feeding. Use EC particles of about 0.3 mm in size up to larval day ten; from then until metamorphosis use 0.3-1.0 mm particles. The particles of EC must be kept close to the larvae; this is an additional reason for ensuring vigorous aeration in larval tanks. Underfeeding will lead to starvation, cannibalism and slow growth; overfeeding (especially if large quantities of EC are obvious before the next feeding time commences) will cause water pollution. Pollution through overfeeding is obvious through the presence of EC particles before the next feeding or if there is a lot of ‘foam’ or ‘scum’ on the water surface. Should water pollution occur by error, the water must be immediately exchanged, as explained earlier in this manual.
As a very approximate example, you should expect to use about 7.5 kg of EC for every larval cycle in a 5 m3 tank initially stocked with 50 larvae/L. Initial quantities of EC at day five for this size of tank and stocking density would be about 25 g/tank at each feed and will rise to around 100 g/tank/feed.
An alternative feeding regime is presented in Table 9. In this system, the BSN and EC feeds are supplemented with a commercially available inert feed. The use of supplemental feed not only tends to reduce feeding costs but is thought to compensate for nutritional deficiencies in Artemia nauplii. However, the nutritional quality of the Artemia can be increased by enrichment (Annex 4).
The general recommendations in this section of the manual apply also to recirculation systems but all hatcheries have their own feeding regime variations. For example, some hatcheries that use recirculation systems turn off the water flow system during feed-

TABLE 9 Alternative hatchery feeding schedule 9

NOTE: BSN = BRINE SHRIMP NAUPLII; ID = INERT DIET; EC = EGG CUSTARD BASED DIET. THE INERT FEED USED IN THIS FEEDING REGIME WAS LANSY MB (INVE
AQUACULTURE NV., B-9080 LOCHRISTI, BELGIUM). THE MANUFACTURERS CLAIM THAT ITS USE ENABLES 40% OF THE BSN AND 70% OF THE EC TO BE REPLACED.
LANSY MB IS THE FIRST STEP IN THE DEVELOPMENT OF A COMPLETE ARTEMIA REPLACEMENT DIET.
SOURCE: DERIVED FROM CORREIA, SUWANNATOUS AND NEW (2000)

ing to avoid BSN leaving the tank. Others use small-mesh screens (90 to 150 μm) to avoid losses of BSN in the biofilter.