Since antiquity, man has pondered the question: How many fish are lurking in the waters below? Getting an accurate picture of a fish population is still a challenge to the fishery manager who must set catch limits so as not to deplete the stock but must avoid inflicting unnecessary economic hardship on communities that depend on fishing. Airborne lidar promises to monitor fish populations more effectively than current sonar or direct sampling techniques. A graph of the data taken off the coast of Southern California in 1999 reveals schools of squid. Because the aircraft carrying the lidar system was flying at 75 m/s, the X-axis represents a linear survey of 4.5 km of ocean. The traditional methods for sizing up fish populations include sonar and direct sampling, but both are limited by the costs of launching research vessels and by low sampling rates. Now researchers from the US National Oceanic and Atmospheric Administration may have found a solution in a compact, airborne lidar system. For the setup, the team needed a light source that could penetrate coastal waters. It also had to be commercially available and rugged enough to operate aboard a small aircraft. The group settled on a flashlamp-pumped, frequency-doubled, Q-switched Nd:YAG laser. "This laser is very nearly perfect for our application," said James H. Churnside, a researcher on the project. "The wavelength is very close to the wavelength with the greatest depth penetration in coastal waters, where most of the fish live. The normal Q-switched pulse length provides about a meter of depth resolution, which is sufficient for profiling most fish schools." Faster, cheaper, better The researchers tested the lidar system off the coasts of Southern California and the Iberian Peninsula and over Puget Sound. They also deployed a research vessel to map the same areas using acoustics and direct sampling to verify the data collected from the air. They discovered that the lidar is effective in gathering information not only about the fish populations, but also about plankton layers. Moreover, it could discriminate between schools of fish and whales. "The main advantage over ship surveys is the cost," Churnside said. The hourly operating costs of a small plane and a research vessel are roughly equivalent, but the aircraft's cost per mile is much lower because it covers a greater distance in the same amount of time. The slow speed of a ship also leads to less accurate results because the distribution of fish may change between the beginning and end of a survey. Another advantage to the airborne system is that fish do not avoid aircraft as they sometimes do ships. The group is still working on tweaking the system, with the goal of improving the signal processing to better distinguish between fish and other objects that scatter light. Details of the research appear in the March issue of Optical Engineering.