Angling authorities are concerned over the ultimate size of artificially produced catches and environmentalists fear the effect on natural ecosystems of the ones that get away.
The idea of tinkering with fish genes is one step beyond fattening them up on special diets. Such 'cultivated' fish are now judged apart from wild game fish in competition - a distinction recently hammered out by the British Rod-caught Fish Committee.
Scientists say the most serious risk is that genetically engineered fish might be more competitive than natural fish, upsetting balances between predator and prey populations.
One of Britain's leading genetic engineers voiced his anxieties over 'transgenic' fish at an international genetics meeting in Strasbourg last month.
Professor Mick Crawley, from the Silwood Park laboratories of Imperial College, London, said: 'My concern is . . . introducing predators into a new area. I want to raise the notion that this might not be a particularly sensible thing to do.'
So far, results of only a couple of field trials have been published. Research has concentrated on adding genes to enhance growth by lifting controls on growth hormones. Similar work on pigs and cows produced unexpected side effects, including rheumatism.
Genetic manipulation in fish is even less predictable, since little is known about their molecular make-up, according to Dr David Penman, a research fellow at Stirling University's Institute of Aquaculture.
'All this work is very much at the basic research, or speculation stage,' he said, 'but the technology is there, or almost there.'
Exploration by the fishing industry includes the possibility of giving Atlantic salmon 'anti-freeze' genes from cold-tolerant fish to extend their range into colder waters.
Other scientists are working on fish with delayed breeding seasons that allow them to get fatter earlier.
Dr Penman has been commissioned by the Department of the Environment to produce a report on transgenic fish, due for publication next spring. He said the main brake on commercial interest was consumer acceptability and ethical concerns. 'People are not so worried about eating engineered plants, but when it comes to animals that is a bit different.'