Until relatively recently, man has acted only as a hunter-gatherer in exploiting the oceans. Now, with fish stocks falling and fisheries failing, there is an upsurge in interest in farming the oceans. Freshwater aquaculture already plays a very important part and some estimates suggest that one in five fish that end up on the dinner plate is a farmed fish. Increases in total aquaculture production (freshwater and marine) have nearly doubled in the past 10 years with most of this due to freshwater culture.
On a local scale, freshwater fish culture has been practised from early times, in many parts of the world, particularly in the warmer areas. The Chinese were rearing carp 4000 years ago and in developed countries, before the advent of modern refrigeration techniques, the fish pond provided a ready source of fresh protein. Many, usually fast-growing, vegetarian species are cultivated in shallow ponds. The growth of suitable pond weed for their food is encouraged by enriching the water with sewage or organic refuse. Where sunshine keeps the water temperature high, biological processes proceed very rapidly and remarkably high rates of food production can be obtained from efficiently managed fish ponds. Capital and labour costs may be low, and land unsuitable for ordinary agriculture can often be profitably farmed in this way although there are often problems with water supply.
Cultivating marine fish - mariculture - presents greater problems, but these problems are now being addressed. As well as finfish, a wide variety of other groups is cultivated worldwide, of which the most important are molluscs, crustaceans and seaweeds (see Table 9.2). The techniques and species used are described in detail in Barnabe (1990) and specific aspects relevant to the UK, in a number of laboratory leaflets produced by MAFF (see Appendix 4). Currently around 6 per cent of the total worldwide marine harvest comes from cultured sources. In the developed world, efforts are mainly concentrated on species that command a premium market price such as salmon. There has been a dramatic increase in commercial mariculture in Europe and America in the past 10 to 20 years. Cultivation of species such as salmon and sturgeon may take the pressure off wild stocks but does little at present to increase the supply of world fish protein. Most of these fish are carnivorous and are fed on fishmeal prepared from wild-caught fish. The high price they fetch also means they are destined mainly for those who can already afford other sources of protein. As better feeds are developed, possibly from vegetable sources, this situation may change.
At the other end of the scale, particularly in the East Asian seas region, traditional small-scale, low-technology techniques are used to rear a wide range of fish, crustaceans and molluscs. This provides cheap protein and employment for local people. However, systems such as the Indonesian tambak ponds take up a lot of space and still rely on the collection of wild-caught fry and, to a large extent, on natural tidal flushing to provide the feed. Intensive pond production
Table 9.2 The most commonly cultivated marine species.
Atlantic salmon (Salmo) Pacific salmon (Oncorhynchus) Flatfish (various)
Milkfish (Chanos) Mullet (Mugil) Yellowtail (Seriola) Groupers, bass, bream, snappers, etc. Oysters (Ostrea, Crassostrea)
Clams (Tapes, Mercenaria, Anadara, Mactra, Meretrix, Haliotis (abalone)) Shrimps and prawns (Penaeus,
Metapenaeus) Seaweeds (various)
Northern Europe North America, Japan Northern Europe, Japan, Korea
Asia-Pacific mainly SE Asia
Japan, Korea, SE Asia Mediterranean, Japan
Asia-Pacific, S. America Asia-Pacific
Anchored floating cages
Onshore tanks, cages, semi-open coastal waters Brackish coastal ponds
Anchored floating cages Anchored floating cages;
semi-open coastal waters Natural rocky seabed; hanging ropes
Natural rocky seabed; posts
(buchot); hanging ropes Seabed
Suspended culture systems using artificial feed and pumped water are being increasingly used. These can create pollution problems and may require clearance of valuable natural habitats such as mangroves.
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