Zhiyuan Gong, a biology professor at the National University of Singapore, has produced a fluorescent green-and-red zebra fish, and his laboratory has the technology to create a kaleido-scopic palette of five colours in genetically modified fish, he told the Pacific Rim Biotechnology Conference in Vancouver.

Prof Gong is developing GM zebra fish in which the fluorescent colour genes are switched on by specific chemicals in the water - so far, oestrogen hormone and heavy metals. The goal would be an indicator fish that turned a certain colour if a particular pollutant was present in the water.

The Singapore scientists have inserted genes for green and red fluorescent proteins - derived from jellyfish - into zebra fish, which are normally black and white. The colours show up well in ordinary light and glow vividly under a blue or ultra-violet lamp. The colours remain distinct, rather than fusing into a muddy brown, because the researchers have developed technology to activate different colours in different tissues.

Biotechnology researchers elsewhere are concentrating on adapting fish for aquaculture and particularly on making them grow more rapidly. The fast growth champion so far is a genetically modified strain of mud loach, a small freshwater fish that is a national delicacy in Korea.

Its creator, Dong Soo Kim of Pukyong University, told the Vancouver conference that his GM mud loaches, which have extra growth hormone genes, grow 30 times faster and become 30 times heavier than unmodified fish - up to 400g.

But GM fish grown for human consumption face consumer resistance. None has yet received regulatory approval, though the US Food and Drug Administration is reviewing an application to commercialise GM salmon.

In Korea, Prof Kim is feeding his GM loaches to laboratory mice to test their safety. "My colleagues and I eat them ourselves," he added. "They taste just the same as normal fish."