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A bamboo barrier closes off a tidal lagoon to make a fish pond
A bamboo barrier closes off a tidal lagoon to make a fish pond
Courtesy of NACA Bangkok

Using natural habitats
Aquaculture began by man making small modifications to natural habitats so as to improve the survival and growth of his target species. Some of the oldest examples are in the rearing of freshwater fish in ponds, something that has been practised for thousands of years in Asia and at least for many centuries in Europe. The simple act of putting a mesh barrier across the outlet of a small pond or lake to prevent the fish escaping could make a big improvement in the food that could be supplied from the pond in the following months. Similarly along the coasts, tidal lagoons occur that can be turned into ponds with a minimum of effort. Closing off such naturally occurring water bodies was the start, centuries ago, of much fish and shrimp aquaculture in Asia and, in modern times, in South America.

Enhancing natural productivity

Removing predators and improving the conditions within the pond, (for instance by providing more area of preferred water depth), supplying additional food and later, adding seed animals collected outside, were further steps that moved aquaculture production close to where it is today. The farming of seaweeds and molluscs (oysters, clams, mussels, etc.) developed similarly, as people made improvements such as providing more settlement areas for the young, or the removal of predators from the growing area.

Farmers had a natural desire to improve the productivity of their systems and, as knowledge grew, they learned to stock more animals, increase feeding and manage the exchange of water to maintain the conditions, such as adequate oxygen levels, that the animals needed to survive.

Artificial structures

The range of facilities used for aquaculture subsequently broadened for a number of reasons. Farmers encountered difficulties with more intensive rearing because of the uncontrolled influence of pond soils, local water quality, weather. Some of these problems could be resolved by rearing in ponds built of concrete, or lined with plastic, by bringing the ponds indoors under cover, or treating the water before flowing it to the culture ponds. Secondly, as the naturally occurring ponds and lagoons all became utilised, prospective farmers had to take a broader approach and develop the technology to be able to use less naturally favoured sites. Net cages floating in protected coastal or inland waters were developed for fish culture, or fish were stocked in fenced areas of the sea or of large lakes.

Hatchery development

An indoor fish hatchery
An indoor fish hatchery
Courtesy of the South Pacific Commission

At the same time, the need for supplies of young animals (fry, seed) to stock the systems led to the development of hatchery techniques and dedicated hatchery facilities. For most species this aspect of production has proved more successful when conditions can be more closely controlled, for instance in indoor concrete and fibreglass tank systems, rather than in outdoor ponds. Often, the younger stages of aquatic animals are more sensitive than adults to physical and chemical conditions and these have to be managed within a smaller range, if production is to be successful. Thus hatchery facilities developed as a separate branch of the industry.

Closed systems

More recently, our knowledge has improved greatly regarding the complex interactions that occur in a rearing system, between nutrients, bacteria and the cultured organism. This and technological developments have allowed many aquatic organisms to be reared in completely closed recirculating facilities, including the farming of marine organisms at locations far from the sea. Closed systems have the added advantage of offering greater protection from the danger of disease entering from the natural environment and also of minimising adverse effects of the production system on that external environment.

Open ocean systems

Technology has also begun to open up the possibilities of growing fish in enclosures in the open ocean, something that could one-day transform the nature of human food production on the planet. With 70% of the earth's surface covered by water, the potential is clear. Earlier technology has restricted the cage farming of fish to sheltered coastal waters. Here the number of available sites is limited, environmental damage is more likely and conflicts exist with other users. As cages are developed that can withstand the demanding conditions of the open ocean, or which can be operated below the ocean surface, farming could move further offshore.

Ranching

The farming of the sea without enclosures has also made a small start, through so called 'ranching' programmes, notably in Japan, where fry are released in large numbers into the ocean with the aim of improving returns from the capture fishery. Sometimes artificial reef structures are created underwater as well, to increase the available habitat and natural food for the fish. Results with such initiatives have been mixed, but as we reach a better understanding of species interactions on the high seas, ranching could become more successful and make a difference to world fisheries production.

 
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