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What Could Happen If Genetically Modified Fish Get Loose

By Cheryl Simon Silver

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Two males simultaneously mate with female (center). The male on the female’s left is genetically modified; the male on her right is normal. In such pairings, wild males managed to sire about 25 percent of the young produced.
A few weeks ago, researchers advised us to cut back our consumption of farm-raised salmon due to high mercury levels and opt for wild salmon instead. Now, fish aficionados have a new worry: A recent study confirms that genetically modified fish, if released to the wild, may create havoc in natural stocks, hastening their extinction.

The scientists devised a computer model that predicts the outcome when genetically modified fish invade a wild population, and they simulated an invasion using a fish species called medaka. The model determined that a fish population invaded by transgenic fish could become extinct within 50 generations.

The issue stems from the increasingly common practice of inserting genes from one kind of fish into another to enhance a trait such as size. The resulting fish is called “transgenic” because it contains genes from another species. So far, about 20 species, including many commercially important species, have been genetically modified, mostly for size. But as yet, none in wholesale production.

These days it can be a short jump from the laboratory to the marketplace, however. While scientists acknowledge the benefits of bigger fish that provide more food, some worry about the environmental consequences of transgenic fish.

In an ideal world, transgenic fish won’t meet their wild counterparts, but the legions of farm-raised fish thriving today in wild settings show that the possibility is all too real. This concern prompted biologist Richard Howard and his colleagues at Purdue University in Lafayette, Indiana, to observe what happens when transgenic fish and wild strains interact.

The researchers inserted a salmon gene for a growth hormone into medaka. The transgenic fish were 83 percent larger than their wild counterparts. In a controlled setting, the larger, transgenic males mated three times more frequently than the smaller wild males.

The wild fish “were relentless in approaching females and attempting to mate,” the researchers report in the Proceedings of the National Academy of Sciences. But their attempts usually failed because the larger male fish chased the smaller ones away.

Another issue was that the offspring of the transgenic males lived less than 70 percent as long as offspring of normal males. This, combined with the fact that wild males do not mate as often as transgenic fish, could lead to a decline of the wild fish.

“The problem comes in the mating advantage of the transgenic males, which enables them to take over a wild population, combined with the disadvantage of producing inferior young,” says Howard.

The experience with farm-raised fish being released to the wild presents a sobering preview of what may lie ahead. The number of fish that escape from fish farms or impoundments is staggering, Howard says, “something on the order of 14 million fish a year.” There are places in the world, he adds, where the majority of fish are farmed fish living in natural settings.

An industry goal is to produce genetically modified fish that are sterile. But given the vast numbers of farm-raised fish, Howard says, the sterilization procedures would have to be 100 percent effective.

Muir, William. M. et al. Transgenic male mating advantage provides opportunity for Trojan gene effect in a fish. Proc. Natl. Acad. Sci USA. Published online.

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