The spectral distribution patterns of downward irradiance were studied in the squid jigging grounds of the Sea of Japan to examine fishing light transmittance according to the optical charac teristics in areas and seasons. The optical water types in the summer season in the Sea of Japan can be classified into oceanic type IB-II for Shakotan peninsula offshore to western Akita prefecture, oceanic type II for northern Hyogo prefecture to San-in offshore, oceanic type II-III for northern Noto

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... sula, northern San-in offshore and Tsushima offshore waters. In autumn, oceanic type II-III for San-in and III for Tsushima offshore waters were determined. The fishing light transmittance at 510nm wavelength was calculated as 6.27% at the depth of 50m for the optical condition of ocean ic type IB-II, 2.77% for oceanic type II and 1.54% for oceanic type II-III. According to optical obser vations in the San-in and Tsushima fishing ground in autumn, the relative irradiance of fishing light at the depth of 50m can be estimated to be 34-51% of that in summer.