Lidar could offer a fast, efficient way to locate and capture lake trout, an invasive non-native fish that is upending the ecosystem in Yellowstone Lake. Aircraft-mounted lidar could allow lake managers 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. The lidar instrument aims a laser beam downward through a hole in the bottom of the plane. The beam is angled backward so that the light's reflection on the water is deflected away and does not saturate the receiver. Courtesy of Joseph A. Shaw, Montana State University. A series of test flights performed by researchers at Montana State University was successful in locating groups of two or more lake trout swimming as deep as 15 m below the surface, and in identifying previously unknown spawning areas. The lidar instrument, bolted to a small aircraft, is able to detect fish in a 5-m swath of water and cover 80 km 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, researchers 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 would be deflected away and not saturate the receiver. The team first tested the use of lidar technology at Yellowstone Lake 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 operations. 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. “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 professor Joseph A. Shaw. "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.” Michael Roddewig, then a doctoral student at Montana State University, setting up the lidar in the plane for a flight over Yellowstone Lake. Courtesy of Joseph A. Shaw, Montana State University. Shaw said the system could be further improved with a technique called pushbroom 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 plan 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.” 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 cloudier. Lake trout prey upon the lake's native cutthroat trout, which have historically been a key food source for many top predators. This substantially reduced the food supply for bears, birds, and other animals that cannot prey upon the invasive fish because, unlike cutthroat trout, the lake trout spend most of the year in deep water. 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 resource managers. The research was published in Applied Optics, a publication of The Optical Society (OSA) (doi:10.1364/AO.57.004111).