Rob McCormack
This refers to taking fish or crayfish from the pond and holding then in a artificial tank to ensure a quality tasting product.
Silver Perch can have an off flavour (See Off Flavours) that requires purging to remove. Generally this is achieved by removing the fish from the ponds and transferring them to a recirculation or flow through system under controlled conditions for purging. Clean clear water is used with 3 – 4 grams of salt per litre of water added. Excellent water quality is required to ensure the health of the fish. Salt is added as a antiseptic for any injuries incurred in harvest, as a anti fungal and parasite control and as a stress reducer. Fish generally should not be purged in cold water. The off flavours they have are retained in the fatty tissue of the fish and the larger the fish or fattier the fish the longer it will take to purge. Temperature in purging is critical as the higher the temperature the faster the fishes metabolism will go and the faster the process. Generally most farmers purge for at least a week though in some cases 2-3 weeks may be required, taste testing fish prior to sale should be standard industry practice. Generally water is fully exchanged in the system within 2 hours of fish being deposited in them and then daily for the first few days to ensure good water quality as if the water quality is poor the fish will sicken and become valueless. Water exchange and volume all depends on the system you are using as some are flow through and some are recirculation, etc. the important thing is good water quality, designing a purging system to ensure your fish are healthy and happy is essential.
Yabbies are purged for approximately 3 – 4 days in clean clear freshwater with a dash of salt added. The aim of this exercise is to ensure the yabbies do not feed in the purging or holding tanks. Pool salt is usually used at a rate of 4 grams per litre of water and this helps to increase the taste of yabbies as salt is a flavour enhancer and increases the crustacean flavour in yabbies. Salt also helps clean the yabbies of ectocommensals and other parasites etc. This is only a bi product of purging as the aim of the exercise is to ensure the waste line through the meat in the tail from the stomach to the anus is clean and clear. If it is clean and clear the chefs at the restaurant do not need to devein the yabby and it is easier to prepare and the customers are happier. It also gives the advantage to the aquaculturalist over the wild catch which is not purged and gives you a marketing advantage.