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A salmon engineered to contain a certain growth hormone gene (rear) grows twice as fast as its non-transgenic counterpart the same age. (AquaBounty Technologies/Associated Press)

Genes from genetically engineered Atlantic salmon could potentially enter wild populations through natural interbreeding, a new study suggests.

Male fish carrying a growth hormone gene that causes them to grow twice as quickly as regular salmon can engage in normal breeding behavior and breed with wild females under natural conditions, scientists from Memorial University in St. John's, have found.

AquaBounty connection

The large adult salmon used in the experiment are descended from the same Newfoundland Atlantic salmon populations and contain the same growth hormone gene as AquaBounty Technologies’ AquaAdvantage salmon. The GM parr in the experiment are directly related, as their fathers were all AquaBounty fish. Their mothers were wild fish.

The U.S. Food and Drug Administration is currently deciding whether to approve very similar genetically engineered salmon produced by AquaBounty Technologies from eggs at their P.E.I. facility.

Males of the GM or transgenic fish had reduced breeding performance compared to wild fish, said Darek Moreau, lead author of the paper published this week in the Journal Evolutionary Applications. But they did show the interest and ability to participate in natural spawning, he said.

"That alone shows that it is possible for the genetic modification to enter wild populations through sexual reproduction," added Moreau, who is completing his Ph.D. in ecology.

So far no GM salmon have ever escaped into the wild and it isn't clear how well they would survive. But the researchers believe it is important to be cautious.

They suggested that rearing sterile GM salmon in contained, land-based facilities would minimize the risk.

2 kinds of breeding

Moreau and his colleagues studied two kinds of breeding in an artificial stream in their laboratory — a tank with gravel at the bottom and water flowing over it to mimic a river current.

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Darek Moreau holds a full-grown anadromous male Atlantic salmon in his lab at Memorial University. (Courtesy of Darek Moreau)

Female Atlantic salmon use their tails to dig nests in the gravel, and full-grown adult males compete for the right to be close to her. The male who is close to the female when she releases her eggs can release his sperm and fertilize them.

Some small, smelt-sized young males called parr are also capable of breeding. They don't compete with the large males, but hide among the rocks.

"When the eggs and sperm are released, the little males dart in there and try to release as much sperm into the cloud as they possibly can. It's quite romantic, actually," Moreau said with a laugh.

Both the large adult males and the parr of the transgenic salmon were capable of natural breeding with wild females.

In the case of the full-grown males, the wild males were more aggressive, were stronger competitors against other males, spent more time with the female around the time of spawning, and were more likely to participate during actual spawning, even though they were on average smaller than the transgenic males. While the growth hormone gene causes the fish to grow faster, they still reach the same adult size, so the sizes of transgenic and wild adult males would probably overlap in the wild, Moreau said.

Atlantic salmon life cycle

Atlantic salmon lay their eggs in freshwater streams. That is where the young hatch and spend the first years of their life. During that time, some males may reach sexual maturity as parr.  After two to four years, the salmon migrate to the ocean, where they spend one to three years before returning to fresh water to breed.

In the case of the parr, the wild fish had higher fertilization success and fathered more offspring than the transgenic fish.

Moreau said the researchers didn't look at transgenic females because they didn't have access to suitable females of breeding age.

The paper did note that the conditions in the study did not perfectly mimic wild conditions. For one thing, each trial included just one female and two males, whereas there would be many males and many females in a wild situation. Also, in general, captive-bred males tend to be less aggressive than wild males, regardless of whether they are genetically engineered.

"There are a lot of unanswered questions," Moreau said. "Nature is a complicated place and it's hard to predict what's actually going to happen in nature if you're not in nature."

However, he thinks a decision on whether to approve GM fish will likely have to be made before all the questions are answered.

In June, the U.S. Congress voted in favour of amending a draft bill in order to stop the FDA's review of the salmon. However, the company said the FDA decision could come before the bill is passed or the amendment might not survive the final vote.