Relationship between Underwater Irradiance and Distribution of Japanese Common Squid under Fishing Lights of a Squid Jigging Boat

Abstract

The relationship between the distribution of Japanese common squid beneath a boat and underwater irradiance of fishing lights was investigated during nighttime jigging operations off the Oki Islands, Shimane Prefecture in November 1995. The downward spectral irradiance under fishing lights of a power of 300 kW (metal-halide type; white) was continuously measured with an underwater radiometer. The spatial distributions of squid attracted by the fishing lights were simultaneously detected with color sector scanning sonar. Sonar images of squid from a vertical-section across the boat were approximately triangular-shaped. The squid were distributed in the water layer between 30 and 70 meters deep, the densest part of the image occurring in the layer between 55 and 65 meters deep. Values of the downward spectral irradiance, when schools were distributed from 30 to 70 meters, ranged 1.8 × 10-2 to 5.4 × 10-4 μW·cm-2·nm-1 at 510 nm, and 5.8 × 10-2 to 6.4 × 10-4 μmol·m-2·s-1 at PAR. Further, vertical profiles of the squid corresponded closely to the distribution of underwater irradiance intensity. We also determined that the upper limit of the squid distribution approximated the 2 × 10-2 μW·cm-2·nm-1 contour at 510 nm.