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With a commercial spawning of species such as Barramundi, Snapper or Silver Perch, especially where outdoor larval ponds are used to on-feed, the breeding process has to be orchestrated to within a few days to take advantage of the size and nature of the phytoplankton/zooplankton bloom and how that relates to larval survival.
An induced spawning may take several days of preparation with tanks and broodstock and ponds and staff. Hypophysation can induce spawning time to within a few hours, which allows much greater control for hatchery management and productivity than to try to duplicate natural spawning conditions for these species. (400um dia. egg 1000-2000iu LHRH-a barramundi/42hour ovulation/280C), (300iu HCG/kg/36 hour silver perch ovulation/26 degrees C). See below for an example of hatchery protocol with silver perch and how significant hypophysation is to that procedure.
Broodstock were maintained in several ponds within 50 km of the hatchery. Broodstock totalled 400-500 with 2 distinct body shape variations. One short fattish body and the other being longish and slender in appearance Generally the shorter fish grew better and were used for commercial farm orders. Records were kept on the transfer and identification of different batches so as to maintain a healthy genetic pool.
Broodstock were usually naturally in spawning condition from November until the end of February although they could be conditioned in the hatchery in mid winter. Two seasonal spawning runs were possible
Using 120mm mesh gill net usually between 3.30 and 7pm.
Fish were immediately cut from net and sedated with cove oil at 7 drops per 60 litres with aeration. Visual examination confirmed spawning potential of both males and females.
Fish were removed to the transporter tank in a sedated state.
Back at the hatchery fish were again heavily sedated (4ml/1000litre) and a sample of eggs and sperm taken by glass pipet.
If eggs were opaque and not clumped (stage II maturation) the fish was weighed and injected at 300iu/kg.
Sperm was examined for motility (about 1 per season is non-motile)
Injection time was usually at approximately 9pm. (9pm + 36hrs = 9am)
Males were given a 50% dose injection. But usually males express quite freely.
1 male and 1 female were placed in 1000 litre oval tanks with high aeration and no water exchange.
Spawning occurred 36-42 hours later at 24-260C.
Spawning time was noted and eggs removed 1 hour later (water hard) by dip-net scooping method.
The eggs were rinsed and volumetrically counted. Average count was 9-13 eggs per 0.2mls, which averaged 55,000 per litre. With 3-3.5 litres per 2kg female.
Fertilisation was usually better than 95% assessed 12 hours after spawning. Commence filling ponds at confirmation of fertilisation.
Hatching and larval tanks consisted of 200litre plastic barrels with 1 litre of eggs maximum and vigorous fine bubble aeration. (but not too aggressive) No water exchange over 3-5 day pre-feeding larval period.
At jaw function/yolk sack absorption, larvae were siphoned and or scooped and concentrated into outdoor ponds (25,000 larvae/0.25Ha/25-40% survival) or transferred to 1000litre recirculating tanks for 3-4 weeks of nursery rearing. They immediately feed on fine dust formulation. However growth was retarded significantly but once released into ponds 4 weeks later survival percentage was usually better than 70-80%. Like most larvae they are extremely sensitive to being handled and dip netting will kill them instantly.
Spent broodstock again sedated with clove oil and injected with 1 Chloromycetin dose (3mg/500g) if damaged. Broodstock were kept in tanks for 3-4 days and then again sedated and returned to broodstock ponds. Spawning mortality was variable and undefined.
In contrast NSW Fisheries Research at Port Stephens Snapper breeding program utilises temperature and photoperiod manipulation only. The hatchery-reared snapper are 5 years old and weigh between 5 and 20 kg. By adjusting the photoperiod and increasing the temperature to 190C the fish spawn naturally, but “temperature is the key” (Pers com S. Fielder 2002). “Snapper are asynchronous spawners and can be induced with this method every few weeks as you can turn them off and on with a few degrees Celsius”.
Such a system would require continual broodstock maintenance for the year where they are only needed for a few days. It is a costly system to set up and maintain but very efficient in spawning terms. Broodstock maintenance would be a major cost factor for a commercial hatchery, which would probably seek cheaper alternatives unless a continuous batch culture system was being used.
Interestingly, Stuart Fielder stresses that the technique appears simple but has been developed firstly by hypophysation techniques and then over time by assessing where modifications and simplifications could be made. It appears as though once domesticated mature stocks are available, the need for hypophysation endocrine control is reduced. Nevertheless, for an asynchronous spawner with only a portion of vitellogenic cells processed to oocyte maturation at any time, perhaps hypophysation was the necessary way to develop the most efficient method. Which is an important point in the process of developing either hatchery reared or domesticated broodstock.
In comparison, the same hatchery tanks were used to spawn and rear Australian Bass Macquaria novemaculeate. This species appears very difficult to hold and maturate (pers. com G Allen 1987) so wild stocks are still caught, sedated, injected immediately and returned to the hatchery. Wild bass are still injected immediately (HCG at 500iu/kg) due to catch-stress factors, potentially, affecting egg viability.
Once the eggs are fertilised the hatchery processes, eg egg and larval density, types of foods and tank function are basically the same for both bass and snapper (and barramundi as well). Interestingly, due of the biological and environmental similarity, multi-species hatcheries are becoming a trend where such species can be propagated. A recent example is the newly commissioned multi species hatchery in Broome WA for tropical species (2000).