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LiDAR came to fame finding lost cities hidden under centuries of jungle overgrowth. Turns out, the repurposed laser technology makes a pretty good fish-finder, too.

That makes it easier to spot schools of spawning lake trout, something fisheries biologists must do to protect the dwindling populations of cutthroat and bull trout that the invasive lakers eat. Instead of spending days scouting with gillnets from boats, a LiDAR-equipped airplane can survey all of Yellowstone Lake's 87,000 acres in a few hours.

LiDAR scan

A scanning image from a LiDAR flight shows the location of a school of lake trout holding along an underwater shelf in the lake bed. 

“You can pinpoint where the spawning fish congregate, and then go net them and get them out of the lake,” said Ric Hauer, director of the University of Montana’s Center for Integrated Research on the Environment. “It’s perfect for targeting an invasive species like lake trout.”

Hauer co-authored a study on the technique with Montana State University electrical engineering professor Joseph Shaw and several other colleagues. LiDAR stands for Light Detection and Ranging. It uses laser lights of various colors that can pass through some substances but not others, producing 3-D images of places that visible light can’t reveal. Blue laser light penetrates jungle foliage, exposing archaeological sites. Green lasers shine through up to 15 meters of water, revealing groups of two or more fish.

Lake trout have upended the native ecosystems of many mountain lakes including Yellowstone, Flathead and westside lakes in Glacier National Park. The lakers prey on many other species of fish, having eliminated most of Flathead’s native cutthroats and bull trout as well as introduced kokanee salmon. In Yellowstone National Park, the lake trout have virtually eliminated a native cutthroat population that had thrived for centuries. The park currently spends about $2 million a year trying to suppress the lake trout population and give the cutthroat a chance to recover.

Loading plane

The LiDAR scanning system was modified to fit in the back of University of Montana researcher Ric Hauer's single-engine airplane. It can inspect Yellowstone Lake's entire 136 square miles in a few hours for spawning lake trout.

“The key problem we address with this research is the need for a method to find where the invasive lake trout spawn so fisheries biologists can deploy various methods of reducing their population,” Shaw said. “There are several other methods being explored for tracking these fish, including acoustic sensing, but an airplane can cover the large lake in a much shorter time than is possible for boats.”

Hauer outfitted his personal single-engine airplane to house the LiDAR system. A new design simplified the much costlier and bulkier version used for archaeological studies. Other refinements produced what the researchers called “push-broom scanning,” which allowed laser sweeps of even larger areas in less time. The result was an inspection of Yellowstone Lake’s 136 square miles of surface for about $500 in aircraft expense.

Test flight

MSU electrical engineering professor Joe Shaw, left, and study co-author Ric Hauer, director of the University of Montana's Center for Integrated Research on the Environment, are shown during an Oct. 2014 flight that tested the ability of an MSU prototype to detect fish in Flathead Lake.

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“Two big plusses of using the LiDAR tool are that we could fly the whole lake in a couple hours, and that we could detect any fish in shallow water, not just the ones that have transmitters,” said Yellowstone fisheries biologist Patricia Bigelow. That could save time and money.

Glacier National Park natural resources program manager Mark Biel said he was looking forward to trying the method in places like Quartz and Logging lakes, where park biologists struggle to suppress lake trout infestations. Crews are traveling into those roadless lakes in early June to start four-week netting operations with a boat they have to airlift over the mountains.

The LiDAR system can’t distinguish one fish species from another. Shaw said it depends on biologists’ understanding of fish behavior to target a problem species. In the Rocky Mountains, lake trout spawn in large schools in shallow water along particular rock formations. That differentiates them from cutthroats and bull trout, which spawn in creeks at different times of the year.

Shaw said eventually the system might be automated to further simplify detecting invasive species like lake trout. Other improvements could allow the system to work in moving water like river channels.

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