for National Geographic News
A recent study in Norway suggests that wild salmon lose out to sexually precocious fish-farm invaders when breeding in rivers.
It's the first time scientists have shown that escapees from salmon farms can out-compete native populations, heightening fears conservationists have for the future of wild stocks.
Although it's been known for some time that farm salmon interbreed with genetically distinct wild populations, escapees are handicapped by less-competitive breeding behavior. The same can't be said for their offspring.
While adult farm salmon have been shown to be 84 percent less successful than native fish at reproducing in rivers, the male youngor parrthey produce are four times more successful than their wild counterparts, the study found.
A proportion of male parr become sexually mature despite being at a juvenile stage of development. They are a significant factor in reproduction among Atlantic salmon. Being a fraction of the size of sea-feeding adults, the parr are able to sneak up and fertilize a female's eggs without being noticed. Up to 40 percent of hatchlings are fathered by these parr.
The Norway experiment suggests that both farm and hybrid parr are able to fertilize many more eggs than wild fish. The research team, which included scientists from Canada, Britain, Norway, and the United States, believes this is due to genetic differences resulting from artificial selection by salmon farmers who prefer big, quick-growing fish.
"They have a higher growth rate and are more aggressive than wild parr," said Dany Garant, an ecologist at the zoology department of Oxford University in England. "Also, they don't respond as much to the risk of predation."
The breeding behavior of equal numbers of farmed, hybrid, and native parr was monitored using overhead surveillance cameras, underwater cameras, and tiny microchips inserted into the fish. They were introduced to artificial spawning beds containing wild adults collected from a local river. Relative breeding success was confirmed by DNA analysis of fertilized eggs.
Results of the experiment revealed that wild parr had a breeding success of just 25 percent compared with farm parr, and were less than half as successful as hybrid parr.
Garant and his colleagues conclude in this month's scientific journal Ecology Letters, that the presence of farmed and hybrid male parr in salmon rivers "ultimately threatens the long-term genetic integrity of native populations".
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