Several areas needed to be looked at. We investigated the systems in place both overseas and in Australia , and, after some initial trials, decided to follow the methods developed here in New South Wales by Stuart Rowland and his team at Grafton for silver perch.
We split the research into three distinct areas:
- Weaning
- Nursery
- Grow out.
- Weaning.
Weaning is a process where fingerlings are transferred from a plankton diet to an artificial diet. It fits in with current fingerling production methods, where fingerlings may be shifted from one pond to another half way through the cycle to maintain growth rates once one pond has been eaten out of plankton.
The method we use takes about 3-4 weeks. Initially, we feed the fingerlings frozen plankton, at the same place in the tank, after turning off aeration and water inflow. The fingerlings become habituated to this in a couple of days. We generally feed them frozen plankton for a week or so to allow recovery from purging and transport. During this time we also treat them with formalin and salt baths to reduce parasite loads and as a quarantine treatment.
Plankton is sieved (250 m m-1mm), concentrated into a sludge , then frozen in ice blocks. The volume of plankton needed is determined by observation. The ratio of plankton sludge to crumble diet is reduced by 10% each day. Plankton ice blocks are replaced by mix each day until they are 100% crumble. Once the plankton-crumble blend is finished, the crumble is spread over the water surface above the basket.
Fish which do not wean successfully die off after 10-20 days. Weaning success is usually 90-95%, although this can be variable depending on the batch of fish. Small fish (0.15g, 19mm ) wean as well as larger fish (0.5g, 32mm ), but grow quicker during the weaning period.
Weaning is best done in dim light conditions, although fish will wean in bright light, but are stressed. We have weaned up to 4000 fingerlings in a 2t tank. Up to 50000 fingerlings have been weaned in a raceway (5m 3 ).
Nursery:
To start with, we introduced the weaned fingerlings into cages near shelter in the pond. A proportion revert back to natural food, which is an issue as these fish do not grow afterwards. The introduction into the pond is a critical period.
Some fish do not appear to grow when stocked into ponds. In the first three months the difference between growers and non-growers becomes obvious, so this is a good time to grade out the non-performers. There was no distinct difference between high and low densities. ( see Figure)
Nursery Management :
Sinking food essential as golden perch are bottom feeders. Because they are bottom feeders, feeding must be monitored using feed trays. Sample the fish regularly to determine size of pellet being fed, and always have a transition period when increasing the pellet size. We found that significant numbers of fish chocked if this was not observed.
Broadcast feeding is best, even if providing shelter (ie feed over the hole pond area, not just at one spot). Grade out after three months, although it does not appear to be strictly necessary. In our experience, about 50% of fish will not perform in aquaculture. These fish do not eat pellet food but do cost in oxygen and ammonia etc.
Growout:
After about two to three months of nursery, grading is advisable. It is more for management purposes than production per se . The lower 50% of fingerlings do grow, but not quickly enough to be viable for aquaculture.
Grow out from 200g to market size is relatively quick. With good water quality and temperatures >15 ° 100g per month is achieved, sometimes more. When an average size of 600g+ is achieved, the pond is ready to harvest. Fish sizes will range from 350g to 1kg+ (90%>450g).This takes from 14-18 months.
Grading:
Sydney fish market does not have grades for aquaculture golden perch.
•We sold in five size classes-
- 250-350g
- 350-450g
- 450-700g
- 700-1000g
- 1kg+
All size classes sold well. The smaller fish occasionally got a better price than the larger fish, but we only sold small numbers as a trial to see if people would buy them and they did. However, as the market preference is for larger fish we would still recommend aiming for the 400-700 g size range.
Market factors varied, but all size classes sold well and all for >$14/kg if market conditions were normal. Lower prices were sometimes obtained if the market was chaotic due to equipment breakdowns or if fish were held over to another day when buyers of golden perch were not there. Generally the price was about $2 per kg less than wild caught fish.
Conclusion:
- Feed a sinking pellet, 40% protein.
- Monitor feeding closely-size, quality, regularity .
- Maintain water quality.
- Realise that only 50% of fish may grow, bank on 30%.
- Taste a sample of product before harvest!
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