Invasion of the salmon

Chinook are rapidly spreading into the rivers of Chile and Argentina.

Hard to believe it’s the same species. But the chinook salmon, conservation heartbreak of the U.S. West Coast, is invading and thriving in South America.

SALMON SPREAD Chinook salmon, introduced in South America for aquaculture, have now started self-sustaining populations in the wild. The species has expanded its range rapidly at the southern end of the continent (dark watersheds starting from left), and could easily spread to more river systems (right panel, shaded area). Correa, Biological Invasions

BIG CATCH The large Chinook salmon colonizing the rivers of Chile and Argentina outweigh the native fish species and join other big invaders like brown trout in shaking up the region’s water life. Correa

NUTRIENT SUPPLEMENT Chinook salmon bring a big pulse of nutrients into rivers when they return to freshwater to breed and die. Biologists now wonder if that unaccustomed blast is changing the ecosystems of the South American rivers that salmon are invading. Correa

Chinook, or king salmon, largest of the five North American salmon species, reached South America some 25 years ago as people tried to farm them there, says Cristián Correa of McGill University in Montreal. Now a broad survey of records and stream visits finds chinook reproducing on their own in at least 10 Andean watersheds that empty into the Pacific plus more along the coast, and three Atlantic watersheds, Correa and Mart Gross of the University of Toronto report in the June Biological Invasions. Correa says he is worried that the invaders could disrupt both freshwater and marine ecosystems.

The dearth of the same species, Oncorhynchus tschawytscha, so alarmed U.S. government fisheries managers this year that they closed both commercial and recreational chinook fisheries off California and much of Oregon for 2008. Of 17 chinook populations in the U.S. Northwest, two rank as endangered and seven as threatened on the U.S. endangered species list.

The news of chinook colonizing South America “absolutely, unequivocally proves how stupid we’ve been in managing our fish,” says Jack A. Stanford, who directs the University of Montana’s Flathead Lake Biological Station and also works with the WildSalmonCenter in Portland, Oregon.

Unlike their North American relatives, the South American chinook don’t have to cope with dams, extensive fishing and genetic mixing with hatchery fish that dilute the wild stock’s local adaptations, Stanford says.

The South American fish also find cool mountain rivers and rich offshore feeding grounds in the southern part of the continent, Correa says.

To trace the spread of chinook, Correa interviewed passionate anglers along the Chilean coast, scrutinizing their photos of trophy fish and scouring written records. Then he surveyed rivers himself finding dozens and in rare places a hundred chinook swimming in a stretch of river the length of a city block.

Commercial operations in South America had raised chinook but now rely mostly on other salmon species, one piece of evidence Correa sites for saying the fish he saw weren’t farm escapees, but members of a population that sustains itself in the wild. Also the ones he examined didn’t have the eroded fins typical of farm fish, and he found them in some remote watersheds. He reports witnessing chinook creating nests and spawning in rivers he visited.

Correa and Gross “are right in identifying chinook as a particularly invasive species,” says Miguel Pascual of Argentina’s National Scientific and Technical Research Council, who has also published on the invasion.

Chinook probably won’t colonize much farther north than Chile’s Toltén watershed, where they are currently found, Correa predicts.

The exotic chinook adds punch to other fish invasions already slamming rivers in South America, say both Stanford and Correa. Among other big fish, brown trout are also establishing themselves in South American rivers, and woe to less competitive natives such as Galaxias species that grow only several inches long. Stanford, who studies fish in the Rio Grande in Patagonia, says one of his students surveying there has seen two individual Galaxias fish in four years.

Correa frets that most of the southern region’s river systems used to run on a sparse budget of nutrients. He says he’s not sure what will happen to their ecosystems as the salmon return to breed and die, bringing massive seasonal pulses of nutrients.

“Salmon are voracious,” Pascual says. He and his colleagues have found that salmon at sea overlap in diet with established species such as the penguins along the southern Patagonian shelf. He hesitates to predict that that salmon would hog the food, but he does say, “we should scrutinize them closely.”

Chinook salmon have gone wild in New Zealand too, where they joined invader cousins such as brown trout, says Martin Unwin of the National Institute of the Water and Atmospheric Research in Christchurch. Fishing for trout and salmon has become popular, and about half of the rivers given special protection have won their honor based on the value of these alien species. “Thus we have here the slightly paradoxical situation of an exotic species acquiring a substantial conservation value,” Unwin says.

The invasions could have silver lining for science. In North America, salmon populations have adapted to the particular watershed where the fish hatch and eventually return to breed. With the ongoing invasion in South America, “we can study evolution in action,” Correa says. “I think this is going to be a good model system.”

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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