Lidar detection of fish schools

Abstract

Direct measurement of fish populations in the open ocean is difficult at best. Ship-based technologies, such as trolling and acoustic sounding, are limited in coverage by the speed of the surface ship. Current airborne techniques, such as photography and direct visual observation, are not very quantitative and are limited in depth coverage to the top few meters, depending on conditions. The author is developing lidar to provide an airborne technology that will provide quantitative information about fish schools to depths of a few tens of meters. A Q-switched, frequency-doubled Nd:YAG laser provides 15-nsec pulses of 532-nm light. The backscattered return is collected by a telescope, detected, and digitized. The digitized signal is processed to separate fish-school returns from returns from other scatters in the water. Lidar observations of fish schools have been made from a ship and from aircraft. Fish reflectivity has been measured in a sea-water tank in order to convert lidar return into quantitative biomass statistics. Work is currently being done on algorithms for automatic detection and classification of schools.