Ocean-swapping Pacific salmon are moving into Atlantic waters, scientists say.

©Miguel Pascual, Centro Nacional Patagónico
A researcher displays a chinook salmon found in the headwaters of the Santa Cruz River in Argentina. A new study has found that the invasive salmon, native to the North Pacific, are invading the South Atlantic, posing a threat to penguins and sea mammals that compete with the fish for food.

The fish, native to the North Pacific, have started colonizing and breeding in rivers in southern Argentina, a new study shows (see map).

Although the sight of salmon leaping in Argentina's world-renowned trout rivers may be enticing to anglers, the silvery predators could become a nightmare for the region's marine life.

The invaders threaten to deprive penguins and sea mammals of food - an ever-increasing risk given the number of invasive salmon currently escaping from fish farms in neighboring Chile, researchers say.

The warning stems from the first study to show salmon swimming from the Pacific to the South Atlantic, where salmon don't naturally occur.

The study focused on chinook salmon, a Pacific species that has recently become established in the Santa Cruz River system in the Patagonia region of Argentina.

DNA analysis of the Santa Cruz salmon traced the fish back to failed salmon-ranching experiments on Chile's Pacific Coast during the 1980s.

Don Staniford, who was not involved in the new study, is the European representative for the Washington, D.C.-based environmental group Pure Salmon Campaign. He said the new findings could mean dire consequences for the region's marine habitat.

"Salmon have a very healthy appetite, so they're going to consume native fish and prey that other species are dependent on," he said.

"You've got a recipe for potential ecological disaster."

Threat to Penguins

Chinook salmon were first discovered accidentally in the headwaters of the Santa Cruz in 1998 as researchers surveyed trout spawning sites.

Another breeding population has recently been identified in a river in Tierra del Fuego, at the southern tip of Argentina.

Eastern-flowing ocean currents and fish-rich seas off southern Patagonia have likely allowed the salmon to spread to Atlantic waters, said study team member Miguel Pascual of the Centro Nacional Patagónico in Chubut, Argentina.

"Salmon can migrate long distances in the ocean, and they can be caught almost anywhere in the Southern Ocean," Pascual said, referring to the waters that surround Antarctica.

One salmon was recorded as far north as Uruguay, he added.

The cooler waters of southern Argentina make the region most vulnerable to invasion, Pascual said, and the area's trout rivers are likely targets for the invasive salmon.

While anglers have yet to hook any of the salmon, "[the fish] are creating a stir among sport fishermen who regard them as an addition to other valuable fisheries for steelhead and sea trout," Pascual said.

But the impact of these sea-feeding fish on the marine environment may prove severe, according to the latest research carried out by Pascual and his colleagues.

A new study, yet to be published, found that 96 percent of the chinook salmon's diet in Patagonian seas is made up of sprats, small herring-like fish that are key prey for Magellanic penguins, a species classified as "near threatened" by the World Conservation Union.

While the number of chinook salmon in the region isn't yet known, models indicate that a "medium-size population" could match the food consumption of the entire penguin population of southern Patagonia, Pascual said.

Chile's Escaping Fish

The team also warns that the number of salmon finding their way to Argentina is likely to grow "as Chile moves forward to become the largest [farmed] salmon producer in the world."

Staniford, of the Pure Salmon Campaign, said salmon escapes from Chilean farms are spiraling out of control.

Millions of fish reportedly escaped in a single incident last year, when an earthquake triggered a mini-tsunami that hit salmon farms in Chile's Aysen region, Staniford noted.

In addition to competing with penguins and sea mammals for prey, escapees can spread disease and parasitic sea lice that affect wild fish, Staniford added.

"Escaped farm salmon, unlike domesticated cows or sheep, can swim vast distances and are mobile pollutants," he said.