Rural Industries Research & Development Corporation

The New Rural Industries
A handbook for Farmers and Investors

Silver perch

Introduction

The silver perch, Bidyanus bidyanus (Mitchell), is an Australian native freshwater fish that is endemic to the Murray-Darling river system. It was commonly used as food by Aborigines, but is now uncommon in the wild.

Hatchery techniques for silver perch were established by the early 1980s. Research at the NSW Fisheries' Grafton Research Centre since 1990, has demonstrated that silver perch is an excellent species for culture in earthen ponds. High survival (> 90%), fast growth rates
(2-3 g/fish/day) to market-size (caµ 500 g) at high stocking densities (20,000 fish/ha) lead to high production rates (10 t/ha/year), with relatively low feed costs ($1.00/kg of fish). Silver perch has the potential to form a large industry (> 10,000 t/year) based on high-volume, low-cost production.

The major challenges for the industry are to reduce feeding and production costs, to develop efficient farms using good aquaculture practices, to develop and implement a quality control program and to establish a processing component leading to the supply of silver perch in a consumer-friendly form with an effective marketing campaign.

Markets and marketing issues

Silver perch is a premium quality fish, which has a mild flavour, moist, white flesh, with very few bones and a high meat recovery of 40%. Like all freshwater fish, it can develop a `muddy' taste or `off' flavour from the pond environment, and so needs to be purged after harvest.

Silver perch are marketed in the live-fish trade which serves mainly Asian restaurants in Sydney and Brisbane. Prices to the grower are $10-15/kg, and the fish retail for $20-45/kg. This relatively small but lucrative market is expected to increase as Asian communities in Australia grow, and the demand for high-quality, fresh product increases across our society.

Some farmed silver perch are sent whole and chilled to the Sydney Fish Markets where they can bring $5-13/kg at auction, depending on presentation, quality and daily market fluctuations. Several farmers are developing local markets in clubs, hotels and restaurants. There appears to be much scope for local marketing in inland regions where silver perch is well known and regarded, and where there is a shortage of fresh fish.

Large-scale marketing and promotion of silver perch have not yet been undertaken because of the present limited supply of the fish.

By far the largest market available is the domestic market for processed product e.g. fresh fillets; frozen, and packaged fillets. Much of the fish eaten in Australia is in this category, and most of it is imported. The great potential for development of the silver perch industry is in replacing some of the fish sold through retail outlets, supermarkets, clubs, hotels, plus that provided in institutions such as hospitals, nursing homes, military bases and so on, with farmed silver perch. Processing increases product types, greatly expands market niches, and will enable the clear delineation of farmed fish from wild-caught fish. The latter is important because of the growing consumer concerns about the increasing pollution of estuaries and the marine environment, both in Australia and overseas, and the subsequent effects on the quality of seafood.

The number of farms and the production of silver perch have now reached levels that guarantee increasing availability and continuity of supply. Markets outside the niche live-fish trade can now be developed and the species promoted to a wider range of consumers.

Production requirements

Silver perch is a temperate, warm-water species that can tolerate water temperatures of 2º to 38ºC. Although it can live over a wide area of Australia, optimum water temperatures for growth are between 20º and 30ºC. The length of the growing season, and therefore the time taken to reach market size, will be determined by the temperature regime. Regions with a growing season (> 20ºC) of at least 5 months, preferably 7 or 8 months, are most suitable for the commercial aquaculture of silver perch. This excludes much of Victoria and the southern tableland areas of NSW. Successful farms are located from the Murray River to northern Queensland.

Site selection is the first and most critical step in establishing an aquaculture facility. An abundant supply of good quality water is essential. Large, permanent rivers and creeks are most commonly used, and underground water is ideal, assuming the quality is acceptable. If rain run-off is to be used as the main supply, water budget and storage requirements must be closely estimated. Domestic water supplies should be avoided because of the limited supply, high cost and chlorine and ammonia content.

An experimental freshwater aquaculture facility has been established at the Grafton Research Centre. Water is pumped from the Clarence River to two earthen reservoirs (capacity 17.5 ML) from where it is gravity fed to the 19 ponds (15 @ 0.1 ha; 4 @ 0.3 ha) and buildings; effluent water from all facilities drains into an effluent/settlement dam (43 ML) from where it is re-used by pumping to the reservoirs or stored for irrigation of pastures and crops. The NSW Fisheries' Silver Perch Aquaculture Policy requires that no effluent water is released to natural waterways.

Ponds should be constructed from impervious soils to eliminate or reduce the loss of water by seepage. Clay or clay-loams are ideal. Ponds should be 0.1 to 0.5 ha surface area, and those larger than 0.1-ha should be rectangular. The bottom should slope evenly to the deep section (2.5 m) which may contain a harvest sump. Each pond should have an inlet and a screened outlet structure, and should be completely drainable by gravity. Banks should be wide enough to ensure strength, stability and vehicular access. Electricity should be available for the aeration of each pond. Fingerling ponds should be netted or covered with wire to prevent predation by cormorants. A building(s) containing an office, laboratory, quarantine/purging tanks, equipment and work area is also an essential part of a fish farm.

Varieties

All research, hatchery production and grow-out of silver perch to date has used the progeny of `wild' broodfish, originally collected from the Murray-Darling river system. There has been no artificial selection. Recent research has identified several distinct genetic stocks or `strains' of silver perch, and a selective breeding program is now under way at the Grafton Research Centre using fish from these stocks. These silver perch have been made available to industry for use as future broodfish.

Key statistics
NSW production (t)

Fish husbandry

A three-phase production strategy is recommended for the pond culture of silver perch: I - Hatchery; II - Fingerling; III - Grow-out. This strategy, combined with a single-batch system where each pond has only fish of the same age or batch, which are harvested completely before the next batch is stocked, contributes to good aquaculture practice. The three-phase production strategy, however, is not rigid and there is scope for flexibility to suit each farm according to its facilities and production potential, and to enable continuity of supply to markets.

Some farms are involved in only one or two of the phases. A hatchery is a specialised operation requiring broodfish, spawning and incubation facilities, larval rearing ponds, and a relatively high level of technical expertise. Farmers can either purchase fry from a hatchery and then use Phases II and III to rear fingerlings, then market-size fish, or purchase fingerlings only and grow these out to 500 g. In cooler regions, where there are relatively short growing seasons, it would be an advantage to stock, in early spring, large (50-100 g) fingerlings that could be reared to 500 g by the end of the growing season in March.

Ponds are prepared by drying, removal of excess organic matter, and in some cases tilling to provide the basis for optimum water quality and fish health. Fish are quarantined in tanks before stocking, to reduce stress and to ensure they are free of disease. Recommended stocking rates are 20,000-100,000 fish/ha in the fingerling phase and 5,000-21,000/ha in the grow-out phase. Grading is necessary after the fingerling phase because a large size-range develops irrespective of stocking density. Silver perch reach a mean weight of ca 500 g in 15 months from spawning, and fingerlings (15 g) stocked in May reach 500 g in 10 months. Annual production rates up to 10 tonnes/ha have been achieved in static ponds at the Grafton Research Centre. Research to date suggests that although silver perch survive and feed well in cages and tanks, growth rates are slower than fish cultured in ponds.

Silver perch are fed a formulated diet containing 35% protein and made up of meatmeal, field peas, canola, peanut meal and lupins, plus a small amount of imported fish meal. Fingerlings are fed up to 7.5% and larger fish to 3% body weight per day. Food conversion ratios (FCR -weight of food fed: gain in wet weight of fish) of 1.2:1 for fingerlings and 1.5:1 during grow-out are now readily achieved using good quality feeds, appropriate feeding regimes, and by maintaining good water quality and fish health. Nutritional research by NSW Fisheries scientists has resulted in a lowering of the cost of feed from about $1,000 to $750/t; the cost of feeding silver perch is now around $1.00/kg of fish. Future improvements in the formulation and quality of diets, and feeding strategies should see a further decrease in the cost of feeding silver perch, hence a lowering of overall production costs.

Fish are sampled monthly from each pond using a seine net, to determine the mean weight, estimate the biomass of fish in the pond and to adjust the daily ration accordingly. Several fish are also examined for disease.

Although silver perch is a hardy species, successful aquaculture depends on the maintenance of good water quality. Temperature, dissolved oxygen concentration, pH, total ammonia and unionised ammonia are monitored two or thee times a week because they can change from acceptable to stressful or lethal levels within several days, particularly during warmer months. The main techniques used to manage water quality are aeration, water exchange, regulation of feeding and stocking density, and the drying of ponds between crops. Dissolved oxygen is the most critical variable and aeration of ponds is essential in semi-intensive and intensive aquaculture. Silver perch ponds are aerated for at least 6 hours nightly (0200-0800h) usually with electrically-powered paddlewheel aerators at a rate of 6-10 hp/ha.

Pest and disease control

The diseases of silver perch are well known, and techniques for the diagnosis and treatment have been developed and can be readily implemented on farms. The major ectoparasitic diseases and the pathogens are: white spot, Ichthyophthirius multifiliis; chilodonelliasis, Chilodonella hexasticha; trichodiniasis, Trichodina sp.; ichthyobodiasis, Ichthyobodo nectator. Fungal diseases are: fungus or cotton-wool disease, Saprolegnia sp.; Epizootic Ulcerative Syndrome (EUS) or red spot, Aphonomyces sp. The major bacterial diseases and pathogens are: tail rot, Flexibacter sp. and/or Aeromonas sp.; columnaris, Flexibacter columnaris and Goldfish Ulcer Disease (GUD), Aeromonas salmonicida.

Rapid diagnosis of most diseases can be made on site using a high-powered microscope. The incidence of disease is much lower on well designed and managed farms, and the following factors contribute to a health management program; a suitable site with a good water supply and facilities; purchase of disease-free fish; use of quarantine procedures; appropriate stocking densities; close observation of fish during feeding; maintenance of good water quality; regular disease checks; careful harvesting and handling; and the drying of ponds between crops. Silver perch are transported in enclosed, fibreglass tanks on trucks. The water is filtered and oxygenated. Although silver perch are now displayed live in many Asian restaurants, they can be killed humanely and rapidly by placing them in an ice slurry or by Ike Jime, in which case they are then packed on ice in polystyrene boxes, labelled and transported to market. As with all seafood, freshness, quality and presentation are vital for successful marketing.

Economics of production

The economics of farming silver perch have not been determined because of the infancy of the industry. Most farms are either under construction or have been built only in the last three years. Aquaculture is an intensive animal industry that requires substantial capital investment, labour input and operating costs. The profitability of silver perch farming will be sensitive to the stocking densities used, the selling price and the cost of feed.

The following figures are based on a business plan which includes all costs associated with the purchase of land and equipment, and construction of 10 ha of ponds. Estimated capital costs are $1 million. Annual costs (excluding depreciation) are $272,000 and $344,000, and the minimum selling prices required to achieve 6% return on investment are $6.80 and $4.30/kg at annual production levels of 50 and 80 tonnes respectively. The costs and minimum selling prices will be lower on farms with available land, infrastructure and equipment; and in the future as the costs of feed and fingerlings decrease, the advantages of genetic selection, higher stocking densities and improved feeding regimes are implemented, and farms in general become more efficient.

Key contacts

Key references

Rowland, S.J. and Bryant, C. (Eds) 1995 `Silver Perch Culture: Proceedings of Silver Perch Aquaculture Workshops, Grafton and Narrandera, April 1994.' Austasia Aquaculture, Hobart for NSW Fisheries, Sydney.

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