Chinook Salmon Rapidly Colonize Rivers

It's taken less than thirty years for chinook salmon that were let loose in South America to establish spawning runs along 1500 kilometres of the South Pacific coast.

Salmon from these introductions have also swum through the Strait of Magellan and into the Atlantic Ocean.

Although North American salmon were released many times since 1924 into Chilean waters, investigators trace the origins of the naturalized fish mainly to a period of ocean ranching lasting from 1978 to 1989. Fish hatcheries at two locations in southern Chile raised and released hundreds of thousands of chinook smolts into streams flowing into the Pacific Ocean. Their stocks came from the Cowlitz and Kalama Rivers, both tributaries of the Columbia River in southwest Washington state.

Farmed salmon that escaped from net pens during the 1990s are another possible source of the naturalized runs. This study's authors nevertheless assert that fish farm fugitives haven't contributed much to the wild Chilean populations.

Adult chinook at first returned only to the release sites of ocean ranches. Yet before long the salmon were spotted in two unstocked rivers. This was the beginning of a rapid range extension that's still continuing. During the 1990s, chinook runs were documented in five major South American watersheds. By 2004, the exotic salmon had been reported from over ten basins that flow out of the Andes into the South Pacific.

Their known spawning grounds in 2004 stretched from the Toltén River, north of Valdivia, at latitude 39° S to the Grande River near Punta Arena at 53° S. A landlocked, nonmigrating group also inhabits Puyehue Lake. These chinook, which probably descended from fish farm escapees, are so successful they now comprise 28% of the fish caught in that lake.

Trawlers in the Atlantic Ocean first hooked chinook in 2002. A population of spawning chinooks were subsequently discovered in the Caterina River, a tributary of Argentina's Santa Cruz River that drains into the Atlantic. No other anadramous fish has invaded such a vast range in South America.

The salmon's rate of river colonization in South America mirrors that of chinook released on New Zealand's South Island between 1901 to 1907. Basins within 200 kilometres of the two introduction points in Chile became populated by chinook within 15 years. The new runs were initiated by a small proportion of spawning salmon which strayed from their natal river.

Similarities between their native and the Patagonian landscapes have set the salmon up for survival in the southern hemisphere. Scientists expect chinook to continue colonizing rivers farther south of the fish's current range, in both Pacific and Atlantic drainages. Their eventual southern range will likely correspond in latitude to the salmon's presence in the northern hemisphere, and comprise short rivers like those where the ocean ranching stocks originated.

The newcomers stand out as the largest freshwater fish in South America. Thus their arrival already is a boon for sports anglers, and the species might one day support a commercial fisheries.

The implications this successful invader poses for the native ecology, though, are still unclear. The salmon's presence is no doubt being felt within the freshwater and marine food webs. Chinook in the South Pacific might even prey heavily on Patagonian grenadier and southern hake, both commercial Chilean species.