Genetically modifying plants may help them to grow better but the technology does not always work as well in fish, according to Canadian research.
Scientists at Canada's Department of Fisheries and Oceans found that inserting a growth hormone gene into wild trout made the fish grow bigger and fatter.
But it had little impact on commercial species that had been bred to grow quicker.
The researchers conclude that genetic engineering can increase the size and growth rate of wild fish but conventional breeding techniques might be just as effective in producing fish for farming.
In addition the GM trout had deformities not seen in the other fish, they report.
It comes as US regulators decide whether to allow the public to eat GM salmon.
Environmentalists oppose approval because they believe that the modified fish could escape into the wild and wipe out the population there - what they call the "Trojan gene" effect.
Risk assessment
The trout research, which was carried out by Canada's Department of Fisheries and Oceans in 1994, has just been published in the scientific journal Nature.
The work involved taking a growth hormone gene from salmon and injecting it into the eggs of rainbow trout.
Fisheries and Oceans' Don Noakes told BBC News Online that the purpose was to assess the risks of GM technology: "We were carrying out baseline research to provide us with the knowledge needed for effective regulation.
The environmental group Friends of the Earth (FoE) condemned the research.
"I think that the public would find these experiments with fish vulgar and unnecessary," FoE food campaigner Adrian Bebb told BBC News Online.
Breeding shortcut
Since the beginning of civilisation, humans have domesticated a variety of creatures and along the way they have selected the animals which best suited their purposes - the fattest, the hardiest or even the most tasty.
Such breeding means that domesticated animals are usually quite different from their wild relatives.
Genetic engineering is an attempt to speed up this process by directly inserting a desired gene from one species into another.
In this case, it appears that old-fashioned breeding and selection has already produced the best trout.