Fish can be harvested using a number of different techniques depending on the method and type of aquaculture. Angling is a good method of harvest from aquaculture ponds when only small numbers of fish are required.
However the most practiced method of harvesting native fish from an aquaculture pond is by netting with a seine net.
This method will catch the majority of fish in the pond after several pulls. It is important that this method of harvesting commences during the morning when temperatures are cooler, and that water quality such as oxygen and pH, is suitable.
Harvesting from recirculation systems is a lot easier as the native fish are housed in tanks. Fish can be pulled out using a dip net and placed within purging facilities.
As freshwater native finfish have the reputation for possessing a “muddy” flavour it is important that purging occurs before they are shipped off to market or the processors.
Purging will expel any compounds which cause the off taste and ensure that a clean safe product reaches the market.
Purging is achieved by housing the fish in holding tanks filled with clean fresh water at a stocking rate of around 20kg/1000 litres. After 2–3 hours, a dose of 3 – 5 g/litre of salt is added under static conditions. This is repeated over 3 to 4 days by flushing the water once its quality deteriorates and repeating the salt bathing process.
Once purging has been completed the fish can then be sold live or processed as a freshly chilled whole product, gilled and gutted, or filleted.
Diseases and Parasites:
Like all fish, Australian natives are susceptible to a number of diseases and parasites. Most pathogens and parasites occur naturally in ponds and water supply (exception of bore water) and healthy fish are usually able to ward of any infections that may potentially occur.
However under aquaculture conditions fish can be placed under stress by transportation, harvesting and stocking at high densities. This may decrease their immunity to various diseases and therefore cause the fish to become more susceptible to infection.
Good husbandry and management techniques will decrease the chance for diseases to occur. It is suggested that all new stock that is introduced to the site should be bathed with regular salt baths of 5 – 10ppt for at least 1 hour before they are placed into culture ponds or tanks to help prevent the transfer of pathogens and parasites. Optimum water quality should be maintained and regular checks of stock should be conducted to monitor health.
Should a substantial number of mortalities occur or there is a presence of infection, the PIRSA Aquaculture SA should be notified immediately to arrange relevant testing and treatment of the fish.
To prevent the translocation of diseases into South Australia from other states it is policy that all fish that are brought into our state need to be certified of all notifiable diseases.
The certification is undertaken in the state that the fish are sourced from and a copy of the certification must be sent to PIRSA Aquaculture SA.
Native Fish in Farm Dams:
Many farm dam owners enjoy stocking native fish in their farm dams for angling and personal consumption. Fingerlings are available from a number of hatcheries that are listed in Appendix 2.
Cost of fingerlings will depend on the species and the hatchery that you purchase them from however generally silver perch and callop fingerlings are generally cheaper than Murray cod and catfish fingerlings.
To import fish to South Australia from interstate, approval must be obtained in accordance with the Fisheries (Exotic fish, Fish Farmers and Fish Diseases) Regulations, 1984 under the Fisheries Act 1982, from the PIRSA Aquaculture SA, including the provision of a disease-free certification from an accredited fish disease laboratory.
To ensure optimum survival and growth the following stocking rates are recommended.
Silver perch, callop and catfish can be stocked within the same pond however due to the territorial nature of Murray cod it is advised that they are stocked separately. Silver perch require dams that have plenty of weed, shrimp and aquatic insects while the other species prefer yabbies, aquatic worms, snails tadpoles, shrimps and other aquatic molluscs and crustaceans to feed upon.
Silver perch and callop will not breed in the farm dam however if logs are supplied for Murray cod or gravel for catfish, breeding may occur after five or so years. All species are considered to be excellent fish for angling and can be taken on both bait such as yabbies, worms and grubs or artificial lures.
Care should be taken however when handling catfish as their dorsal and pectoral spines can inflict nasty wounds.
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