WHITEFISH – Like a tree’s concentric growth rings, a small bone within a fish’s ear reveals a history of its growth. And according to a new study of westslope cutthroat trout in the Flathead River system, the bone also contains a record of its migration pathways – a kind of geochemical diary of its life.

The bone, called an otolith, acquires a new ring every day of the fish’s life. All fish have them, and for decades scientists have counted the bands of the bone to determine the age of a fish, as well as estimate population growth.

But a study published last week by a slate of Montana researchers in the Canadian Journal of Fisheries and Aquatic Sciences brings to light new information that the ear bones also contain a unique fingerprint of the water chemistry where the fish swims on a given day, which can be used to map the entire life history of a fish within a river network.

When a fish leaves its natal stream and begins its life cycle, it drifts between geographically separate spawning and rearing beds, and embarks on the occasional foray to distant waters within a river system. During these sojourns, the fish absorbs the unique combination of isotopes and chemical elements contained within the water, and the otolith records them like a passport stamp.

“It worked so well. The values in the water matched those in the otoliths, which grow like rings in a tree,” said Clint Muhlfeld, an aquatic ecologist at the U.S. Geological Survey's Glacier Park field office and lead author of the study. “As fish grow and move into new environments, the otoliths record that information and we matched that with stream statistics to reconstruct the entire life cycle of a fish.”

Using study streams and sampling locations in the upper Flathead River system, researchers relied on stable isotopes and elements like strontium. By pairing the otolith’s chemical data with the isotopic signature of a particular stream, the researchers were able to pinpoint a fish’s whereabouts with extremely high resolution, Muhlfeld said.

Although similar studies have been used to determine a fish’s movements in a marine environment, the research is the first to show that the use of isotopes in water and ear bones accurately assesses movement of trout wholly within a freshwater system. The research was conducted by scientists from the U.S. Geological Survey’s Northern Rocky Mountain Science Center, the Woods Hole Oceanographic Institution, Montana State University, and Montana Fish, Wildlife and Parks.

According to Muhlfeld, one of the greatest challenges of fisheries science is accurately tracking fishes’ migration routes in order to inform stream management and preserve keystone species like cutthroat. Tag-and-recapture methods are often too labor intensive for large-scale surveys, and provide only a snapshot of the fish’s lifespan.

The otolith, however, is a natural tag – a protein skeleton that accumulates calcium carbonate and trace metals from the surrounding water and reflects the fish’s environment.

“That then gives you this awesome map of all these specific stream signatures of strontium which are derived from the geographic basin,” Muhlfeld said.


The researchers evaluated the “isoscapes” to identify natal origins, reconstruct environmental history, and estimate the spawning site fidelity of westslope cutthroat trout. The research provides a reliable method that compliments traditional fish tracking techniques, like telemetry, and may allow biologists to investigate non-native species invasions – lake trout, for example – as well as identify important populations and quantify life histories of freshwater fish in entire river networks.

“All life is literally a product of its chemical environment, and there is no more dramatic demonstration of that fact than the ability to retrace the life history of fish from the variations in the chemicals deposited in their ear bones as they grow and migrate through space and time,” explained USGS director Marcia McNutt. “As this technique has already proven its scientific value in understanding the movement of fish in the marine environment and those which migrate from freshwater streams to the ocean, it is truly an achievement that could inspire ‘CSI.’ ”

Westslope cutthroat trout are an important native fish species in western North America, where their populations have declined because of habitat destruction, fragmentation and competition from non-native species. Cutthroat make extensive migrations among spawning, growth and refuge habitats, yet conventional tracking techniques have not been able to unravel the extent of their movements.

Knowing exactly what habitat the fish use during each life stage is an important component to understanding their ecology and more effectively managing the species, Muhlfeld said. As Montana’s state fish, it is appropriate that the groundbreaking research occurred in the Flathead.

“For biologists, this is like the holy grail,” Muhlfeld said. “Understanding the life histories of fish is critical to maintain, conserve and preserve the species and populations we are trying to manage and these movements are really crucial to preservation and conservation biology and understanding the fish’s environment.”

“We are always trying to tag fish and figure out where they are moving,” Muhlfeld continued. “Traditional tracking techniques like telemetry are great tools, but the batteries run out. You are only looking at a short time in a fish’s life span. You are limited to these snapshots in time to try to piece everything together to tell the story of the fish. So this technique is extremely powerful.”

Telemetry and tagging techniques are nonfatal, however, whereas the new ear bone research requires destruction of the fish. That presents a stumbling block for conservation-minded researchers studying a sensitive species like cutthroat.

But future applications of the technique may allow biologists to assess habitat restoration activities and investigate non-native species invasions, like that of lake trout, in which fatalities are inconsequential.

“This approach could be very useful in understanding life history strategies and conservation needs of freshwater fishes worldwide,” said Muhlfeld. “Biologists are typically limited to examining movements of fish at checkpoints throughout their lives or over small periods of time. This approach allows examination of a fish’s entire life with significant accuracy.”

Flathead Valley bureau reporter Tristan Scott can be reached at (406) 730-1067 or at tscott@missoulian.com.

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