(Read "Salmon Farm Escapees Threaten Wild Salmon Stocks" [June 16, 2003])
Some experts had cautioned that sterile triploids would present a different ecological hazard, because they would be able to devote more time and energy to feeding and would outcompete wild trout.
But the new study showed that the genetically modified fish were not more damaging than farmed diploid fish.
"Basically, triploid trout didn't perform any better in the wild than normal [farm] trout," Roberts said.
Both normally farmed and engineered fish lost weight in equal measure when introduced to rivers.
"The reason for this is that they're coming from a fish farm where they are fed a high-protein diet that makes for muscle bound, obese fish," Roberts said.
"Put them into a wild environment and they struggle, because there's a lot less food about."
And triploids proved no more voracious when it came to the number of prey fish they ate in the wild, the study found.
The key difference between farmed diploid fish and engineered triploids was that the all-female, infertile trout stayed away from spawning areas during the winter breeding season.
Farmed diploids joined their wild cousins on shallow, exposed river sections, where both were heavily predated by mink, otters, and herons, the study team reported.
"We found that over the winter the triploid fish were surviving a lot more than diploids and even the wild fish," Roberts said.
Still, all stocked trout—whether triploid or diploid—sustained heavy losses due to predation when they were first released.
Only 15 to 20 percent of introduced fish were still present at the study sites after three months, Roberts said, because "basically they don't know what an otter or a heron is."
Anglers also didn't seem to notice much difference between stocked diploids or triploids, Roberts said.
"They didn't find any difference in catchability or performance," he said.
The newly announced measure marks the first countrywide policy of stocking triploid-only farmed trout, said EA fisheries scientist Graham Lightfoot.
Other European countries don't have the same demand from anglers for readymade, meal-size trout, Lightfoot commented.
And in the U.S., where stocking policy is set by each state government, triploid trout are being introduced only in some parts of the country.
"In the United States there also tends to be more focus on stocking fish that are big enough [for anglers] to take," he noted. "So they seem to be going down the same route."
The Idaho Department of Fish and Game (IDFG), for example, has promoted the use of triploids as a means of conserving indigenous strains of rainbow and cutthroat trout.
Recent field studies of triploid rainbow trout introduced in Western streams suggest "they provide recreational fisheries of equal or superior quality to normal diploid fish," IDFG scientists said in the August 2006 issue of Fisheries magazine.
Despite their label as genetically modified animals, triploid trout aren't as controversial as they might sound, Lightfoot added.
"Elsewhere, triploidy has been used quite a lot in crop production," Lightfoot said. For example, "most of the bananas you buy are triploids.
"What people tend to get exercised about are transgenic situations where you're moving genetic material from one species to another," he said.
"In this case, all you're doing is retaining an extra set of trout chromosomes," he said. "And, of course, triploids can't breed."
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