Reactions of Herring Larvæ to Light: a Mechanism of Vertical Migration

Abstract

DURING the course of studies on the reactions of fish to light the opportunity arose for observing the behaviour of herring larvæ, Clupea harengus (Linn.). Initial observations were made on larvæ in shallow dishes illuminated from below by diffuse light. Each larva was dark-adapted and then observed for fifteen minutes at different light intensities, the total time spent moving in this period being recorded as a measure of its activity. The larvæ exhibited a positive photo-orthokinesis¹, their activity being proportional to the light intensity between two limits. The lower threshold for the kinesis was about 20 metre-candles, the larvæ being active for 20 per cent of the time below this intensity (down to at least 0.3 m.c.). At higher intensities, activity increased to a maximum at 4,000 m.c. when larvæ were active for 35 per cent of the total time; still higher intensities (up to 65,000 m.c.) did not increase activity further. When a light was shone at the side of the dish, larvæ swam directly towards it, showing a positive phototaxis.