In research to be presented at the annual meeting of the Ecological Society of America on Aug. 7, U-M researchers Brian Kennedy, Andrea Klaue, and Joel Blum, along with Dartmouth College researcher Carol Folt, have found that the element strontium, relatively common in bedrock beneath streams, accumulates in the bony tissues of Atlantic salmon and leaves a specific chemical signature, depending on the geology of the watershed in which the fish is living. This discovery could reveal whether certain rivers or tributaries produce fish that are more likely to survive their time in the ocean and successfully make the return trip to spawn in the stream where they hatched.

Conventional methods of tracing fish movements involve tagging thousands of juvenile fish in hatcheries with fin clips, dyes, or PIT (passive integrated transponder) tags and then hoping that the tagged fish are among the fraction that get re-caught as returning adults years later. It is a labor-intensive procedure that does not yield as much information as scientists would like. Young fish do not necessarily stay in the streams into which they are released, so the tag on a recaptured adult fish may only indicate where the fish was released as a juvenile, not where it spent most of its life. By taking advantage of the natural variation in strontium isotopes (alternate forms of the element that are present in different watersheds), scientists now can differentiate fish from specific geologic areas without having to use a man-made marker previously attached to a fish.

"It's a natural tag," says Kennedy, a research fellow in the Department of Geological Sciences. "In addition to linking adult fish to their juvenile stream, now we can look at juvenile movements between streams, so it gives us a really good indication of where they are spending their juvenile phase."

At a given area in a watershed, strontium isotope ratios are very stable and show little seasonal or temporal variation. Kennedy and his colleagues identified 11 different geologic signatures for 18 regions of the Connecticut River and its tributaries in central and southern Vermont, an area that has been the focus of Atlantic salmon restoration efforts for more than 30 years. Then they looked at the strontium isotope ratios in backbone tissue of juvenile salmon and in otoliths—bits of bony material near the brain known as "ear stones"—of adult salmon. Additional tests with independently tagged fish provided a control to measure the natural variability of isotope ratios for neighbor fish.

The otoliths become a record of the fish's environment. "The neat thing about it," says Kennedy, "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."

Atlantic salmon generally spend two years inland in streams and rivers as juveniles, and then head out to the ocean for a few years before returning to their home stream to spawn. Knowing which streams produce salmon that successfully make this round trip will enable specific habitats to be targeted for protection and could provide valuable information about where to release hatchery fish or how regional habitat restoration efforts are influencing adult survival.

"We're letting nature apply the tag and then reading it, without incurring the potentially high financial costs or mortality rates of artificial tags," says Kennedy. "It could be very useful for distinguishing fish populations in both wild and managed settings."

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