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Food and Agriculture Organization of the United Nationsfor a world without hunger
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Fish can feed themselves by pushing the rod on this autofeeder
Fish can feed themselves by pushing the rod on this autofeeder
FAO/12987/M.Pickstock

Background

Aquaculturists have developed or adapted many specific techniques to improve their operations - some drawn readily from other fields and some which have had to be devised by farmers and technicians themselves. They range from simple field tricks like moistening soil and rolling it into an elongated shape to test whether there is enough clay in the soil to make watertight pond dykes -- through to advanced biotechnology such as gene transfer. As the sector has expanded to new regions, new species and to achieve control over more of the life cycle of farmed animals and plants, fish farmers have proven very innovative in devising solutions to the new problems they faced.

The culture of carp was originally restricted to the home range of each species, where fry (seed) could be caught from the rivers and stocked in ponds. A study of how water temperatures, changes in day length and other factors affected the reproductive cycle of fish subsequently led to the ability to breed carp far from their native waters and where the natural conditions would not normally allow breeding. Manipulation of water temperatures and day length remains important in the successful hatchery production of many farmed species to this day. As hormones became identified and their action understood in higher vertebrates, fish farmers began to experiment too, with extracts of hormone producing organs in fish, and found that egg development and spawning could be promoted in many species by the injection of hormone extracts from pituitary gland. These techniques are used today in the production of fish like carp, salmon and bream. Oysters and other molluscs are brought to produce eggs by manipulating water temperatures and shrimp are made to develop their ovaries by removing a gland that produces an inhibitory hormone, sited in the eyestalk. Hormones are also used in processes like the sex-reversal of tilapia to produce all-male populations that give better production. There is a short period in the early life of the fish when their sex is not fully determined and feeding them with a hormone treated feed can push them to develop male characteristics.

Many fish are stripped of their eggs (females) and milt (males) in hatchery production and the fertilisation is carried out externally. Typically this is done in a bowl with the eggs and sperm being mixed with a feather. Shrimp that do not readily mate in captivity are artificially inseminated by extracting the sperm capsule from the male and attaching it to a female that has ripe ovaries to replicate the natural mating process.

Reliable supplies of feed

The young stages of many farmed animals need quite specific feeds to survive and many techniques have had to be developed to produce reliable supplies of feeds, both live and inert, to support hatchery operations. Particular species of microscopic algae are selected from the thousands that occur in the sea or freshwater and grown in tanks with just the right mixture of nutrients to make them flourish. These algae are then fed to the young fish or shrimp. Many cultured fish and shrimp species require live microscopic animals at certain stages of their development and the type of feed they need can be quite specific. Aquaculturists have found that in many cases, animals such as Artemia and rotifers can fill this need and have developed the techniques to rear these two to an advanced level. Artemia (brine shrimp) are tiny shrimp like creatures that grow in salty lakes in places like Utah, USA and Iran. When the salt content increases to a certain level the Artemia turn into an egg-like cyst and this can be stored dry for many years. When it is put back in water, a tiny Artemia hatches out and this has proven to be an ideal feed for the early stages of many fish and shrimp.

Eggs stripped from an Atlantic salmon
Eggs stripped from an Atlantic salmon
Courtesy of Luther Goldman/USWFS

Because of the problems and costs faced in rearing live feeds however, techniques have also been developed to produce artificial feeds with the right density to float in the water, the right taste and feel to be attractive to the fry and the right nutritional content to give good growth and survival. Vitamins dissolve out of such feeds quite quickly and so methods have been developed to coat the vitamins or hide them inside minute 'microcapsules'.

In ponds, it is often hard to see how much the animals are eating - a fish farmer can not see how much his animals leave, like a cattle or chicken farmer can - and so techniques have had to be developed to make sure the fish or shrimp are well fed, but feed is not wasted. Some farmers use demand feeders where the fish learn to push a lever to get feed. Others place feed on trays and then pull up the tray to check what feed is left. In fish cages, farmers nowadays use close circuit TV to see what is happening underwater.

Future techniques

Many techniques important to the industry are in the health sector. Plating samples of water and tissue on agar plates to test for bacteria and fungi, the use of electron microscopy and DNA based 'probes' to check for viruses, the use of 'probiotics' or 'friendly' bacteria to keep water in good condition, to mention just a few. Then there are fields like cryogenics -- the freezing of eggs, sperm and embryos at ultra low temperatures for storage and use at a later date; the use of anaesthetics to calm fish down for live transport (also possible by lowering the temperature); treatments like dipping the shells of freshly caught shrimp in an anti-oxidation mixture to keep them fresh longer. The techniques useful to aquaculture are many and will continue to expand and improve, to help bring more cultured fish to the table.

 
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