General Information:
Callop, also known as Golden perch or Yellowbelly occur naturally mainly in the Murray- Darling system, Lake Eyre drainage system and some coastal streams of southern Queensland and northern New South Wales. Their prefered habitat is within the warm sluggish waters within these regions.
The largest Callop caught on record weighed in at 23kg, however the majority of fish caught by professional fishermen range between 1kg and 3kg. Callop are a hardy fish and can withstand extreme conditions. Females generally grow faster than males.
Aquaculture of callop has occurred extensively in farm dams on a limited basis. Although the potential culture of this species has been promising, at this stage it has been difficult to wean callop fingerlings onto an artificial pellet diet and has therefore limited the further development for the aquaculture of this species.
Reproduction:
In natural populations, spawning of callop is occurs at night near the water surface during periods of rising water. This normally occurs during spring and early summer when the water temperature rises above 230 C. Females attain sexual maturity at four years of age when they reach around 1 – 1.5 kg in weight. Males reach sexual maturity within 2 – 3 years.
Females can hold eggs at an advanced stage of development for 3 – 5 months until conditions are suitable for spawning. If suitable conditions do not occur, the eggs are reabsorbed. Up to 650,000 eggs may by produced by a female in season, depending on her size, and these are shed in a single spawning. Ripe eggs are 1.1mm in diameter, spherical and amber in colour.
Water-hardened eggs are about 45mm in diameter and are smooth, transparent and semi-bouyant. The eggs are demersal, noadhesive and float freely in the water column which aids in the dispersal of the species to various regions of the river. The eggs hatch after 24 – 33 hours, depending on temperature.
Small fish are migratory and will travel up and down the river system. Since 1972, considerable research of hormoneinduced spawning techniques for callop has been conducted at the NSW Inland Fisheries Research Station at Narrandera. Callop spawn within 39 hours of being injected with hormones. Fertilised eggs are then collected and hatch after a 30 hour period of incubation.
Newly hatched larvae are 3.2mm long, unpigmented, have no mouth, and are buoyant and float upside down. They will swim actively and commence to feed aver approximately 5 days and will develop adult features after 15 – 20 days of growth reaching around 12mm in length.
Diet:
Callop are active predators that will feed on anything that moves. Young callop will feed on zooplankton, consisting mainly of copepods and cladocerans. Older fish will move on to yabbies, shrimps, molluscs, small fish and insect larvae.
High survival of callop fingerings under culture conditions depends on the presence of high densities of zooplankton at the time of the first feed. This is necessary in aquaria, tanks or earthen ponds. Callop will consume around 5 – 6% of body weight per day.
As briefly discussed previously there has been little success on weaning juvenile callop onto an artificial pellet food. To ensure the success and development of the industry suitable weaning techniques will need to be established.
Recent research has demonstrated that callop can be weaned gradually using a plankton/crumble mix however further studies will need to be conducted to further establish these techniques.
Growth:
Growth of callop varies and will depend on environmental conditions. In the wild, callop will reach 500 – 800g within about 2 to 3 years. Within farm dams, callop fingerlings stocked at 50mm in length will reach 215mm (135g) in around 13 months.
At this stage, growth rates and stocking densities for callop within recirculating systems is unknown. This is mainly due to the lack of a suitable artificial feed for this species.
Marketing:
Callop flesh is white and firm and therefore the fish has a good market acceptance. Good quality fish receive around $17 - $20/kg for live product and between $10 - $15/kg for chilled product.
Callop often have a layer of fat under their skin which is responsible for the ”muddy” flavour that freshwater fish regularly has the reputation for. Fat tissues however can easily be identified and removed before cooking.
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