Silver Perch

Silver perch Bidyanus bidyanus, is a freshwater finfish species exotic to Western Australia, and has been imported into this State since 1950 for the purpose of stocking inland farm dams. Imports have only been allowed under license, subject to disease-free certification and on the basis that the fish were being placed into impounded or confined waters. Lawrence (1995) discussed the prospects for aquaculture of silver perch in Western Australia and since then the industry has only expanded slowly. A similar pattern existed in the early days of the silver perch industry in NSW, where rapid progress is now being made, and this species should still be regarded as having significant potential in WA. However, its limited salinity tolerance may restrict its usage for highly saline inland waters. This publication updates the information provided by Lawrence (1995).

Silver Perch research

Silver Perch distribution

Silver Perch distribution within Western Australia

The areas in which silver perch are permitted are determined by the conservation value of drainage basins:

Category 1: Drainage basins, or areas within drainage basins in which silver perch farms and stocking are not permitted;

Category 2: Drainage basins in which silver perch farms and stocking may be permitted, subject to conditions; and

Category 3: Drainage basins in which silver perch farms and stocking will be permitted, unless otherwise indicated by the Executive Director of the
Department of Fisheries.
The appropriate temperature for growing this species in aquaculture is 22oC to 28oC (Rowland, 1995c), indicating most areas below the Tropic of Capricorn would be suited to silver perch farming. This coincides with the current distribution of yabbies in Western Australia.

Silver Perch Habitat

Silver Perch General Biology

The female Silver Perch

Spawning behaviour is very aggressive and injuries such as scale loss and frayed fins are sustained by both males and females. Female silver perch usually release most of their eggs in a single spawning. Water-hardened silver perch eggs are between 2.5 to 3.0 mm in diameter, are non-adhesive and slightly negatively buoyant (Thurstan & Rowland, 1995).

Silver perch can tolerate relatively poor water quality, such as dissolved oxygen down to 2.2 mg/L; pH up to 10.2; and un-ionised ammonia up to 0.7 mg/L, although long-term exposure to un-ionised ammonia concentrations above 0.1 mg/L may adversely affect growth (Rowland, 1995c). Silver perch tolerate long-term exposure to salinities up to 5 parts per thousand (ppt) (Rowland & Bryant, 1995). However, Guo et al. (1995) showed that if acclimated correctly, silver perch are able to survive in salinities up to 12 ppt but die within 18 hours in salinities of 15 ppt or higher.

Culturing Silver Perch

Hatchery Phase: Most growers will not be involved with this phase. It lasts roughly eight weeks, involves collection and spawning of brood fish, incubation of eggs, and rearing of larvae through to fry (30 mm total length or 0.5 g).
Fingerling Phase: Fry are grown in farmers' ponds until they are about 15 to 20 g (three to four months at summer temperatures), when they should be harvested and graded. Grading is particularly important, as research has shown that fingerling size is hugely variable from 2 to 50 grams total length after just three to four months.
Growout Phase: Graded fingerlings are stocked in ponds for growout to market size (10 to 15 months).

Silver Perch Hatchery Phase

Silver perch broodstock are removed from ponds and are then induced to spawn in tanks. Broodstock will mature naturally but generally females must be induced to spawn by injection of a hormone, human chorionic gonadotrophin (hCG) at a dose rate of 200 International Units per kilogram of body weight (IU/kg). After hormonal injection, ripe fish placed in tanks spawn in around 36 to 40 hours and the eggs are fertilised naturally (MacKinnon, 1989).

In a spawning season, a large broodstock female weighing 1.8 kg can produce approximately 500,000 eggs. Fertilised eggs are collected from tanks using a 2 mm mesh and placed in cone-shaped fibreglass tanks with gentle aeration at a density of up to 500 eggs per litre. Eggs hatch in approximately 24 to 30 hours at 24oC. Larval development takes 18 days. WA producers have had to adapt eastern states culture techniques to WA conditions and this has resulted in slight changes to methodology (G. Whisson, pers. comm.).

Although both silver perch eggs and juveniles are susceptible to saline conditions, at low salinities (<5 ppt) juvenile silver perch demonstrate a better survival rate over a 20-day period than those hatched in fresh water. The increased survival in low salinities may be due to the salt's inhibition of microbial pathogens (Rowland, 1984; Merrick & Schmida, 1984; Allen, 1989). This provides an opportunity for the farming of this species in farm dams in the wheat belt area of WA which are moderately salt-affected.
Larval-rearing ponds need to be filled 10 to 14 days before stocking, with water pre-filtered through a 0.5 mm screen to remove potential predators such as aquatic insects and other fish species. When full, ponds are fertilised with inorganic and organic fertilisers to produce phytoplankton followed by zooplankton blooms. Larvae completely resorb their yolk sac five to six days after hatching and are now stocked directly into ponds after hatching (Thurstan & Rowland, 1995).

Fertilisation of ponds promotes populations of micro-organisms (bacteria, protozoans) and blooms of microscopic algae, which are then grazed on by zooplankton (copepods, rotifers, cladocerans). Silver perch larvae feed first on freshwater rotifers (Brachionus spp.), followed by copepods, then cladocerans as the larvae increase in size. Rotifer numbers need to be around five per millilitre (ml) to support a stocking ratio of 50 to 100 larvae m2.
During this period frequent monitoring of water quality (temperature, pH, dissolved oxygen and total ammonia nitrogen), zooplankton populations and examination of larvae for parasites are essential (Thurstan & Rowland, 1995c; Lambert, 1998).

Metamorphosis begins with the development of fin rays after about seven days and is completed at around day 10 to 14 or when larvae are 10 mm total length (TL). After five to six weeks the fish should have full scale development and be between 25 and 35 mm TL (0.5 grams). Once larvae have metamorphosed they need to be weaned onto artificial feed - this should begin in the third week after stocking (S. Rowland, pers. comm.).