Fish put to work on weeds

Press, 18 December 1986, Page 17

Fish put to work on weeds

Fish have cut the cost of clearing water weed by 30 per cent. Wessex Water Authority in south-west England used to spend $550,000 a year on clearing water weed with herbicides and machinery, but now a tonne of grass carp are chewing their way through their own weight of weed each day. After a nine-year experiment, the authority can now look at a weed-choked area of water and say how many fish and of what size are needed to clear it, and how long it will take. Its biology unit has also solved the problems of breeding artificially a fish which, left to itself, breeds only in rivers of Siberia. While grass carp is obtainable in most parts of the world and mature carp cost more than fresh salmon, Wessex Water not only breeds enough for its own use but sells occasional surplus to research organisations. The authority is also experimenting with other types of fish to solve water probelms. In one pond, silver carp has been introduced in an experiment "to create an underwater polycuyIture. This fish cruises through the middle layers of water, straining out phyto-plankton with its specially-adapted wide mouth. Above it swims ornamental koi carp, feeding on insects, while above them the grass carp eats weeds. These digest only half the grass they consume, and the excreted remainder falls to the pool bed where it joins other debris as food for crayfish.