Comparison of airborne lidar with echosounders: a case study in the coastal Atlantic waters of southern Europe

The feasibility of using airborne lidar (Light Detection and Ranging) was studied to assess the early juvenile fractions of the main pelagic fish species of the coastal Atlantic waters of southern Europe (anchovy, sardine, mackerel, and horse mackerel). Field comparisons with more established echosounder methods were undertaken in the summers of 1998 and 1999 during the recruitment period of sardine and anchovy in the selected areas, in the presence of a variety of oceanographic and environmental conditions. Backscattered energies as well as the types of target recorded by both devices were compared. The distributions of energies and the shape of the targets were generally similar for both techniques, with moderate numerical correlation between sensors, demonstrating the potential of lidar for assessment of anchovy, sardine, and juvenile mackerel. However, differences in received backscattering energy were found, especially in the presence of certain plankton assemblages (to which lidar is more sensitive) and isolated schools with large vertical dimensions (for which shadowing is more significant for light than sound). Experimental ad hoc optical reflectivity measurements of fish and plankton are proposed to discriminate these two types of targets. In addition, an improvement on lidar implementation and data processing is suggested to achieve fish abundance estimates.

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