Technique can trace birthplaces of salmon

Scientists have discovered a way to match salmon to their native streams by comparing strontium patterns in their bones with levels of the element found in the stream water.

Dartmouth College researchers hope their finding will help worldwide salmon conservation efforts. If scientists can figure out where healthy, adult salmon grew up, they can figure out which streams are benefiting from habitat restoration.

Eventually, as they try to restore salmon populations in rivers where the fish had become extinct, wildlife biologists may be able to put salmon fry back into the streams where their parents grew up, strengthening their genetic adaption to those streams and their migratory patterns.

Janice Rowan, a fish biologist with the U.S. Fish and Wildlife Service, called the study “a really nifty piece of science” that could help other dwindling species of migratory fish.

Under natural conditions, salmon are born in freshwater streams. Between one and five years later, adolescents called smolts swim to the ocean, where they spend the next two to three years maturing. They then return upriver, seeking the streams they were born in to spawn.

But over the past two centuries, dams, pollution and shoreline development have stopped most natural migration and decimated the salmon population.

On the Pacific coast, sockeye salmon have been declared an endangered species, as have runs of Chinook salmon on some rivers. On the Atlantic coast, salmon became extinct in most of the major rivers, except for some Maine waterways.

Since the 1970s, the Fish and Wildlife Service has been trying to reintroduce the Atlantic salmon in the Connecticut River, which traverses four states, and the Penobscot River in Maine.

Fish lifts and ladders have been installed in dams, so salmon can migrate upstream.

Wildlife biologists catch most of the adults as they swim upriver, then take them to hatcheries to breed. The salmon fry are raised until they are finger-sized, then released into streams with the hope they will mature and repeat the cycle.

But after about 10 years of releasing millions of young salmon into the Connecticut River’s tributaries, only about 250 adults make it back upriver each year, Rowan said. The numbers should improve as the program continues.

The study published in Thursday’s edition of Nature looked at maturing salmon in 10 Vermont streams that feed into the Connecticut River.

Strontium is an element that stream water picks up from rocks and fish pick up from the water.