Lidar TS measurements on Northeast Atlantic mackerel (Scomber scombrus)
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A green linearly polarized laser and a digital video camera were used to find the average reflectivity and lidar target strength of live mackerel. The light reflected from the fish was compared to that reflected from a target having a known total reflectivity of 20 %. The target was 50 % depolarizing, resulting in 10 % reflectivity in the copolarized plane and 10 % in the cross-polarized plane. Using a lidar having two receivers with different polarization, one might be able to distinguish between the reflected laser light from mackerel and other fish (e.g. herring) as they depolarize the light differently. Mackerel was found to reflect 8.6 % of the light in the plane copolarized with the laser and 6.1 % in the cross-polarized plane, giving a total average reflectivity of 14.7 %. A similar experiment on sardines gave a co-polarized return of 9.7 % and a cross-polarized return of 3.1 %. The large difference in depolarization between the two (41 % for mackerel and 24 % for sardines, respectively) can be used for species identification. The average reflectivity was combined with the size of the fish to find the lidar target strength of mackerel at 532 [nm]. When the video camera and laser were co-polarized, the target strength was found to be between 1.06x10-4 and 3.61x10-4 [m2sr-1]. The cross-polarized setup resulted in a target strength between 8.92x10-5 and 2.21x10-4 [m2sr-1]. Multiplying this by fish density, we get the lidar volume backscatter. Adding a second receiver gives the lidar great new species identification capabilities.
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