Scientists Looking at Ear Bones in Fish to Map out their Movements

New research coming out of the Flathead tracks the movement of a fish throughout its life. The unique chemistry of different streams across the area makes an imprint on the fish in the stream, and biologists can use this imprint to see all the places the fish has been.

The study takes a close look at the otoliths of a fish. The Otoliths are a fish’s ear bones. “As fish grow, just like rings on a tree, they experience new environments, and the chemical signatures get laid down in their ear bones,” said Research Aquatic Ecologist Clint Muhlfeld.

Muhlfeld said as a fish breathes it filters water through its gills leaving behind unique signatures. He works with the US Geological Survey and said this is the first fresh water application of this technology

It was conducted in the Flathead through a  partnership with Montana State University, Fish Wildlife and Parks, and the Massachusetts lab that’s been using the technology to map fish movements in salt water.

The first step was mapping out the water chemistries for more than 40-streams across the Flathead. Muhlfeld said the underlying geology in the Flathead is diverse which creates the distinct water chemistries, “we can basically map out, or fingerprint the water chemistry of the basin, and then those fingerprints are laid down in the ear bones of the fish as they grow. So we can match those together to figure out the entire life history of individual fish.”

He said the streams nearly all showed distinctly different chemistries, and those that were similar were geographically close. Currently, fisheries managers have a few different methods to tag and follow fish movements. One is radio telemetry in which a fish is caught, fitted with an electronic tracking device, and tracked.

“We’ve been somewhat restricted with the use of conventional tagging and tracking techniques like telemetry or batch marking individuals with tags where we only look at a sliver of time, throughout the course of a fish’s entire life cycle,” Muhlfeld said looking at the ear bones of a fish complements existing tagging techniques.

However, studying the ear bones can only happen after the fish dies. Muhlfeld said they’re also studying if and how they could use the fish scales in a non-lethal method to detect the same patterns, but, he says the scales are not as chemically stable, “fish die, and so, by capitalizing on any kind of mortality you have associated with netting, or you know, fish, for example cutthroat- 40 to 50 percent of them die after they spawn,” Muhlfeld said.

Muhlfeld said they can use this to see where natives like Westslope Cutthroat Trout came from, where they’re spawning, where they travel through to get there, and can therefore see if certain streams have adequate protection. It also has uses for non-native fish, like lake trout that scientists are finding in cutthroat and bull trout streams north of the border, “are they coming from Flathead Lake, are they coming from these new source populations in Glacier National Park. So, understanding these dynamics is going to really be important,” Muhlfeld said. “So, for situations like Quartz Lake or others that fish can move around, if you’re suppressing a population, your efforts might be compromised due to the continued invasion of lake trout.”

Muhlfeld has been conducting a lake trout suppression effort in Quartz Lake, one of Glacier Park’s high mountain lakes. The lake is home to and spawning grounds for native bull trout, and recently lake trout were discovered there too. Muhlfeld said a possible application for the technology is finding those sources of invasion, and targeting them for suppression efforts.

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