Silver perch aquaculture

Market and industry

The silver perch has biological and behavioural traits that make the species suitable for aquaculture:

• natural schooling behaviour means silver perch can be stocked in high densities
• a high survival rate (over 90%)
• hardy and grows rapidly (10–12 months for marketable fish)
• fingerlings are widely available
• good taste and texture, with a high meat recovery (around 40%).

Most silver perch are sold live to restaurants. Some live and chilled products are also sold at fish markets in major Australian cities.

There has been considerable investment in establishing further silver perch grow out systems using the large water storage facilities used by agricultural sectors such as the cotton industry. These production systems should increase production of silver perch significantly.

There are a number of challenges facing the industry. Serious consideration should be given to profitability, economies of scale and successful marketing strategies when considering silver perch farming as an investment opportunity.

Culture and production systems

Silver perch are suited to aquaculture because of their high survival and rapid growth rates. Marketable fish of 400–600g can be grown in 10–12 months.

Pond systems

Commercial production normally occurs in purpose-built earthen ponds. Recommended pond size is 0.7–2.5m deep with a surface area of 0.1–0.5ha.

Prior to stocking, ponds are drained, dried and cultivated. They are then filled with water that has been passed through a filter screen.

Ponds should be easily drainable and slope to a concrete sump to help farmers harvest stock. Properly installed netting or placing nylon scarewire across the pond may prevent predation by birds.

Water temperature

Temperature tolerance ranges from 20°C to 35°C. Optimum growth rates occur between 20°C to 30°C, with negligible growth below 12°C.

Water quality

Although silver perch are able to tolerate poor water quality conditions, maintenance of water quality is necessary to achieve optimum growth rates.

Dissolved oxygen levels of 4mg per litre or greater encourage maximum growth and survival. Supplementary aeration (e.g. paddlewheel) is required at densities greater than 5,000 fish per hectare and is essential for commercial production.

The excreted metabolic waste ammonia will limit production at levels greater than 0.1mg per litre and becomes lethal at 0.6mg per litre. Careful management of stocking densities and feed rates will prevent excessive ammonia concentrations.

Silver perch are able to tolerate pH levels between 6 and 10, with the desired range between 6.5 and 9.

Salinity levels of 5g per litre of sodium chloride are acceptable for long–term exposure and can be used to treat ectoparasite and fungal diseases.

Breeding

In aquaculture farms, sexually mature silver perch broodstock are induced to spawn, often with a hormone injection of human chorionic gonadotropin (HCG). Chemicals used in aquaculture, including hormones like HCG require veterinary advice and prescriptions before use.

After hormone treatment, male and female pairs are placed in 2000L covered, strongly aerated tanks maintained at 25°C. The ripe fish will then spawn and natural fertilisation will occur.

Each pair can produce up to 125,000 eggs per kilogram of female bodyweight. The eggs hatch in 36 hours at 25°C and development continues for 5 days until the yolk sack is absorbed. The first feeding larvae are 5mm long and can then be reared in a pond environment.

In the pond, high levels of small zooplankton (rotifers and copepods) are encouraged with chemical and organic fertilisation. Survival rates of 30% or higher are possible, with larvae turning into fingerlings in 3–4 weeks (15mm long). The fry reach 30mm in 5–7 weeks. Fingerlings 30–50mm are usually produced by a commercial fish hatchery, which in turn sells the fingerlings to growers.

Feeding and growing

Successful silver perch farming is a function of stocking rate, water quality, diet and stock management. In well–managed dams, production of 5–10t per hectare is attainable.

Growout stocking rates

Growers usually purchase fingerlings and produce fish to market size (500g) in 10–12 months or even less.

To prevent large variations in size at harvest and inefficient feeding rates, farmers usually divide grow out into 2 stages:

• nursery stage – where fingerlings are stocked at a range of 20,000 to 100,000 fish per hectare
• harvest – at about 9 months fingerlings are harvested from nursery ponds, counted, graded and then restocked into grow out ponds at 5,000 to 21,000 fish per hectare.

Higher stocking rates require careful monitoring of the pond environment to avoid mass deaths.

Feeding

Silver perch are omnivores that feed on:

• zooplankton and algae
• small crustaceans
• aquatic insects
• molluscs
• plant material.

Juveniles prefer to feed on crustaceans and zooplankton with the proportion of algae and plant material increasing with age.

Silver perch will also accept commercially-available artificial diets at both fingerling and adult stages. Pellet size varies with the size of the animal. The food conversion ratio (FCR) ranges from 1.2:1 for fingerlings, to 1.5:1 during grow out. Generally it takes 1.2–1.5kg of food to produce 1kg wet weight of fish.

The optimum dietary protein level is between 32% and 36%. Along with protein, silver perch need an artificial diet with sufficient lipids, carbohydrates, vitamins and minerals for energy and growth.

Harvesting

Harvesting methods depend on the amount of product required and pond management needs. Harvesting techniques include total drain harvesting and the use of seine nets for partial harvests. Off flavours are normally removed by purging the fish in clean water for 5–10 days before they are marketed. We recommend adding salt during the purging process.