Lidar TS measurements on Northeast Atlantic mackerel (Scomber scombrus)
Abstract
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.
Publisher
ICESSeries
ICES CM documents2003/V:05