Underwater Irradiance Distribution of Fishing Lights Used by Small-type Squid Jigging Boat

Abstract

The spectral distribution of downward irradiance in the sea around a squid jigging boat (<30Gt) was investigated with special reference to the type and total kW output of fishing lamps, in the fishing ground of Toyama Bay and off Tsushima Island in June and November 1994, respectively.
The downward irradiance distributions in the waters shallower than 20m depth and within the horizontal distance from the bow to the stern of the boat were coaxial semicircles in shape with a center at the midship position of the boat: Below 30m depth, the downward irradiances, however, were almost equally distributed in the horizontal direction for the same distance. The downward irradiances in the water just under the boat were considerably lower than those around the boat. Such differences decreased with increasing depth in irradiance intensity, and almost disappeared below 30m. When the vertical profiles (510nm wavelength) of underwater irradiances from 0-100m across-ship of the midship position were compared for different wattage fishing lamps, if the wattage of the fishing lights was in the ratio 80kW:150kW:240kW, the irradiated area of more than 10^-2μW•cm^-2•nm^-1 was in the ratio 1.0:1.4:1.8, respectively. This increase is not proportional to the total kW ratio output of the fishing lamps.