Homing In On Migrating Salmon

Using a high precision isotope ratio measurement first developed by lunar scientists to analyze the geology of the moon, Dartmouth biologists Brian Kennedy and Carol Folt and geochemists Joel Blum and Page Chamberlain have shown that a unique isotope signature present in the water of a stream is incorporated into the bony structures of young salmon, or fry, soon after they begin to feed. This natural signal, derived from the geologic formations that underlie the rearing stream, can allow an adult salmon's origins to be traced with unprecedented accuracy.

Under natural conditions, anadromous or "sea-going" salmon are born in freshwater streams. One to five years later, as adolescents called smolts, they swim downstream from their freshwater habitats to the ocean where they spend up to three years growing and maturing. Then they return to their home stream to breed. Extraordinary navigators, salmon sometimes cross thousands of miles to return to the site where they were born.

Over the past century, dams on many rivers have stopped these natural migrations and have eliminated salmon from much of their historic range. Loss or degradation of habitat and changes in ocean climate also appear to be causing a reduction in salmon populations worldwide, and efforts to restore salmon to their native ranges are under way in many places.

As part of the restoration efforts along the Connecticut River, returning adult salmon are captured at the Holyoke dam and taken away to be bred in hatcheries. Millions of young are produced this way annually, hand-stocked into fresh water when they are fingerlings - about the size of a adult's thumb. "Even though it would be useful to know the home streams of migrating fish" and to know which streams produce the most successful fish "there is no practical method of marking and tracking fish on this large a scale," says Kennedy.

To address this problem, Dartmouth investigators set out to find a signal that could identify a fish with a particular stream. They chose the trace element strontium, which is dissolved in stream water in several different forms, or atomic weights, called isotopes. The proportion of different strontium isotopes in a stream is related to the composition of rocks and minerals found in the stream's watershed, giving the water in any stream a distinctive isotopic signature. The researchers proposed that this signature would be detected in the bones of salmon reared in that stream.

Applying a technique that measures the relative proportions of strontium isotopes (86Sr and 87Sr) the Dartmouth group discovered that within three months of being put into a tributary, salmon incorporated the isotopic signal of that tributary into their backbones and ear-stones (called otoliths). All but two of the 20 fish measured could be precisely matched to a home stream; both of the non-matching fish are believed to have traveled from a distant waterway and retained the isotopic signature of their home stream. Of the 10 sites that were studied, the scientists could distinguish eight unique isotope signatures, suggesting that this technique may be one solution to determining the stream origins of adult salmon.

"Our initial results are very promising," says Carol Folt, associate professor of biological sciences. "We are now looking at the use of additional isotopes, like nitrogen, to pinpoint salmon rearing origins with greater accuracy."

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