Nature has a built-in tagging system for Atlantic salmon that yields more information than conventional identification systems do, researchers say. According to new findings, presented yesterday at a meeting of the Ecological Society of America, chemical signals contained in a fish's bony tissues can document its travels. In this case, scientists at the University of Michigan and Dartmouth College used the element strontium to trace the movements of salmon in the Connecticut River system.

By exploiting natural variations in isotopes of strontium¿a constituent of the underlying bedrock¿among different watersheds, the scientists were able to deduce where adult salmon spent their juvenile years. Strontium leached from the bedrock becomes incorporated into the backbones of young fish, and in older fish bony otoliths, or "ear stones," absorb the strontium. "The neat thing about it," team member Brian Kennedy says, "is the chemical information is laid down in the otoliths on a daily basis, and they can be ¿read¿ much like tree rings, but on an even finer scale."

Salmon are anadromous; born in freshwater, they migrate to the ocean and then return to freshwater to spawn. Because newborn fish may not stay in the same stream, the strontium markers, which record information throughout a fish's life, are especially informative.

The scientists identified 11 distinct strontium signatures from 18 regions of the Connecticut River and its tributaries in Vermont. Atlantic salmon are indigenous to the area but became threatened with extinction in the 1800s as a result of dam construction, overfishing and pollution. Efforts to conserve and restore Atlantic salmon in the region have been ongoing since 1967. The new information from strontium will help in the identification of both wild and managed populations of fish, the team notes.

"In addition to linking adult fish to their juvenile stream, now we can look at juvenile movements between streams," Kennedy says. "So it gives us a really good indication of where they are spending their juvenile phase." And if fish from a particular stream regularly make successful spawning trips, the location could be targeted for conservation or hatchery release programs.