LARVAL REARING 1. Controlled environment

The number of eggs released from a full spawning ranges from 300,000 to 500,000.  Partial spawnings with low hatch viability are discarded.  Egg diameter is 0.3 mm.  Aeration in the bowls is stopped allowing the eggs to settle to the base.  Water exchange is given at a level of 50%.  Ovarian tissue is cleaned from the bowl sides and a 400 micron mesh net is used to remove faeces from the water.  Strong aeration is given in the bowls to keep the eggs in suspension. 

Figure 19 200 Litre Spawning Tanks

Figure 20 1350 Litre Transport Tank

Figure 21 Maturation Tanks

Figure 22 Nauplius 6

 Number of eggs spawned is calculated by taking ten random samples at different places in the spawning bowl with a 100 ml beaker.  Nauplii hatch 12-14 hours after spawning, approximately 3 pm.  Similarly, hatch rate is calculated.  Healthy nauplii exhibit a strong phototactic response. The aeration is ceased and a grey lid is placed over the bowl with a light bulb suspended over a one centimetre hole in the top of the lid to concentrate the nauplii at the surface of the water.   After a period of ten minutes a hose is inserted through the hole and the nauplii are subsequently siphoned into the nauplii catcher bucket (Figure 22).  A secondary hose provides a constant flow of saltwater to the catcher simultaneously.  A 250 micron screen in the centre of the catcher bucket enables effective washing to occur as the nauplii are collected.  This acts to prevent the vertical transmission of viral, bacterial (Vibrio spp.), fungal, microsporidean and other diseases from the broodstock.  Siphoning is ceased before reaching the base of the bowl to leave the weaker nauplii and egg mass together which are then discarded and the bowls chlorinated.  The five tonne larval rearing tanks have been prepared with 250 micron filter screens and one tonne of seawater.  If the ambient water is < 29⁰C the one kilowatt heaters are positioned in the tanks.  The nauplii are inoculated into the tanks at a density of 150 nauplii per litre. 

Figure 23 Zoea 1

There are six nauplii stages in which no feeding occurs as nauplii are absorbing yolk supplies (Wikipedia, 2010) (Figure 23).  Chaetoceros muelleri is pumped into the larval tanks at nauplii 6 at a minimum density of 50,000 cells per mL.  Larvae metamorphose to zoea 1 the following morning and commence feeding (Figure 24).  Zoeal stages 1 to 3 are fed with algae in the ratio of 80% to 20% C.muelleri (at 80,000 cells per ml) to S.tropicum.  C.muelleri must be given at a greater concentration to zoeal 1 and 2 stages as this algae is the appropriate size for ingestion.  The larvae are given feed at 6 intervals throughout the day.  Supplementary feed for zoea 1 to 3 is Inve microencapsulated diet Car #1, at a size 5 to 30 micron.   At zoea 3 stage the larval tanks are at full capacity.  Mysis 1 commences on day 5 and at this stage Artemia salina at 1 individual per ml is added to the tanks (Figure 25). Initially water exchange is at 20%, and then increased to 30-40%.  Inve microencapsulated feed CD#2 (30-90 micron) is added when required.  The algae proportion is inversed for C.muelleri and S.costatum, 20% to 80% at mysis 3 stage (Figure 26).  Postlarvae 1 stage is reached at day 10 when Artemia is provided at 5 individuals per ml.  Bacteriological testing for luminescent Vibrio and Pseudomonas spp. is routinely undertaken by the streaking method using TCBS agar plates.  Probiotic bacteria are used to control the levels of toxic metabolites, strengthen the immune system of the cultured animal and repress the growth of pathogenic micro-organisms. The production of inhibitory compounds by the probiotic bacteria suppresses the metabolism of the pathogens, in addition to inducing competition for nutrients and other resources (Jobling, 2010). Feeding regimes are based on the specific requirements of the various larval stages validated by frequent and detailed examination of the feeding activity of the larvae in each tank.

Figure 24 Nauplii Catcher

Larvae are transferred to the Nursery Section at Postlarvae 1 stage.  The transfer is accomplished by first lowering the tank levels to the line of the conical or in the case of the 5 tonne parabolics, to the one tonne level.  The ball valve outlets of the larval tanks are connected by a section of hose to the inlet of the 10 tonne parabolic nursery tanks which have been filled with seawater to the level of entry (Figure 27).  The process is a moderated gravity flow which affords minimal stress on the larvae.  Postlarvae 1 enter a clean tank environment for the duration of their development to postlarvae 15 (Figure 28).  The feeding schedule consists of Artemia maintained at 5 individuals per ml, by four feeds daily, and supplementary nutrition of Inve Frippak PL+150 (150-200 micron) for postlarvae 1-5, and PL+300 (200 -400 micron) for postlarvae 5-15, supplied twice daily.  

Figure 25 Mysis 1

Figure 26 Mysis 3