The Importance of Light in Shoaling, Avoidance of Nets and Vertical Migration by Herring

Abstract

Herring (Clupea harengus L.) are pelagic, shoaling fish. When established in tanks, they continue to shoal until the light intensity falls below a threshold value of about 0.1 metre candle. The fish then disperse. In daylight they avoid stationary nets placed across the tanks, but start to swim into them at a threshold light intensity of about 0·006 mc. Surveys, using an echo-sounder and underwater light meter, were made by a research vessel during the Scottish summer fishing season for herring (May–August) in the years 1957–1963. The usual pattern of vertical migration was found. The fish move to the surface at dusk when the light at the depth of the shoals reaches an intensity of 10−0·1 mc. The fish tend to disperse near the surface at the shoaling threshold (0·1 mc) forming “scattering layers”; they re-form and move down at dawn when the light intensity again reaches 0·1−10 mc. Regular records, taken at different depths throughout the night fishing period, show that the light intensity at the depth of the drift nets appears to be above the threshold for net avoidance (0·006 mc) during a considerable part of the period during which capture takes place, especially in June and July. This implies that fishing by drift net may be relatively inefficient at these times.