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Deriving apparent optical properties from light measurements obtained using bottom-trawl-mounted archival tags
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2020
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[PDF-7.22 MB]
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Description:Since 2004, the National Marine Fisheries Service’s Alaska Fisheries Science Center has collected light data during bottom-trawl surveys using trawl-mounted archival tags equipped with a photoelectric cell. The archival tags continuously record light measurements during bottom-trawl survey hauls and provide vertical profiles of downwelling light during each deployment and retrieval (downcast and upcast) of the bottom-trawl gear. Given the temporal frequency and spatial resolution of bottom-trawl survey sampling, these measurements provide an unprecedented opportunity to monitor subsurface water clarity in Alaska’s marine ecosystems and to characterize the visual environment inhabited by deep-dwelling marine fauna. This technical memorandum describes an algorithm which applies quality control checks to archival tag light measurements obtained during bottom-trawl surveys, then derives apparent optical properties (optical depth and vertical attenuation coefficient of downwelling irradiance) from the bottom-trawl survey light data. To determine whether the data collection method and algorithm yields reproducible and precise apparent optical properties, we applied the algorithm to light data obtained during annual summer bottom-trawl surveys of the eastern Bering Sea continental shelf from 2004 to 2018. Out of 10,837 casts from the eastern Bering Sea continental shelf during 2004–2018, 7,887 (72.8%) passed quality control checks. Apparent optical properties derived from cast data were reproducible and precise. A description of an experimental data product containing apparent optical properties derived for the eastern Bering Sea is described herein.
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Rights Information:Public Domain
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