Researchers use LiDAR to locate
invasive fish and preserve a national treasure
For decades the National Park
Service has been locked in a battle against lake trout, an invasive fish
with a voracious appetite that has overtaken Yellowstone Lake and
upended its formerly thriving ecosystem. According to new research, an
aircraft-mounted instrument could offer a faster way to locate and
capture the non-native fish during the brief weeks each year when they
come into shallow water to spawn.
The view from
the aircraft used to scan Yellowstone Lake for invasive lake trout.
The instrument, which uses the
light-based imaging technology Light Detection and Ranging (LiDAR),
could allow those who manage the lake to hunt for invasive fish across a
wider area at lower cost, making more efficient use of the approximately
$2 million spent on lake trout control each year.
In The Optical Society (OSA) journal Applied Optics, researchers report
on a series of test flights that were successful in locating groups of
lake trout and identifying previously unknown spawning areas. They
demonstrate the technique's ability to find groups of two or more lake
trout swimming as deep as 15 meters below the surface.
"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," said Joseph A.
Shaw, Montana State University. "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."
A new way to look at lakes
LiDAR has been used to track fish in marine ecosystems, but this is the
first time it has been used to study fish in lakes, where the water is
In addition, while other LiDAR applications require expensive,
sophisticated equipment, Montana State University engineers developed
the new instrument for less than $100,000 and optimized it for operation
on a single-engine airplane that can be flown for $500 per day, making
it a practical solution for ecologists and local fishery and water
"The relatively low-cost and small size of our LiDAR system will make it
useful for other types of ecological studies, such as mapping plankton
layers and plumes from underwater vents," said Shaw. "Remote lakes that
would be difficult to access from the ground can be easily mapped and
studied within the span of a single day."
Managing a threatened ecosystem
A centerpiece of Yellowstone National Park, Yellowstone Lake has long
been beloved by anglers and revered for the populations of grizzlies,
bald eagles, otters and other wildlife it supports. The introduction of
non-native lake trout in the 1990s dramatically disrupted the area's
intricate ecological balance.
Lake trout prey upon the lake's native cutthroat trout, which have
historically been a key food source for many top predators. As lake
trout populations spiked, the cutthroat trout population collapsed,
dropping by 90 percent between 2000 and 2005. This substantially reduced
the food supply for bears, birds and other animals. These animals cannot
prey upon the invasive lake trout because lake trout spend most of the
year in deep water, unlike cutthroat trout.
To cull lake trout populations and give cutthroat trout a chance to
recover, lake managers use gill nets, among other methods, to capture
lake trout during spawning season. While highly effective, this method
requires knowing when - and more importantly, where - the fish are
To locate the fish, biologists currently implant individual fish with
acoustic transmitters and then track them with in-lake and on-boat
receivers, a time-consuming and labor-intensive process that provides
incomplete coverage of the invasive fish's whereabouts.
Taking to the air to cover more ground
The research team sought to develop a more effective way to quickly
locate large groups of lake trout without tagging. Their solution, a
LiDAR instrument bolted to a small aircraft, allows them to detect fish
in a 5-meter swath of water and cover 80 kilometers per hour.
The device works by transmitting a short pulse of laser light from the
airplane through the air and into the water. The LiDAR receiver measures
backscattered light, allowing researchers to pick out fish from the
surrounding water. To optimize the setup for use on the lake, they used
a green-beam laser, which penetrates water better than other types of
lasers used for LiDAR applications on the ground. The beam was angled
backward so that the light's reflection on the water is deflected away
and does not saturate the receiver.
Putting the system to the test
The team first tested the use of LiDAR technology at Yellowstone Lake
using the NOAA fish LiDAR in 2004. Data collected during those test
flights helped National Park Service personnel find previously unknown
spawning areas that were then validated with on-the-ground gill netting
The team then designed and built their own system to provide sufficient
optical power at the lowest possible cost. Tests of the new setup, in
2015 and 2016, successfully identified numerous trout groupings.
said the system could be further improved with a technique called
push-broom scanning, in which the laser beam is scanned in a line to
cover a wider swath. This would allow scanning the full lake area more
quickly than the single fixed-angle laser used in the current setup.
The researchers also aim to develop additional tools to help users
rapidly translate LiDAR-generated data into actionable information, and
to adapt the system for other types of freshwater ecosystems.
"We are interested in developing automated fish-detection algorithms and
in using this method as a routine tool to help the fisheries biologists
in their battle against invasive lake trout," said Shaw. "We also are
exploring options for using this LiDAR, along with multispectral and
hyperspectral imaging systems, to monitor river health."