Studies on the Shoaling Behaviour of Fish

Abstract

1. Hepsitia stipes and Bathystoma rimator swim in shoals during the day but disperse at night. 2. The fish become increasingly active as illumination decreases and shoals begin to disperse when it falls below about 0.1 f.c. The change from day to night patterns of behaviour is progressive; there is no sudden switch from one to the other at any single level of illumination. 3. Exploratory feeding behaviour similar to normal nocturnal activity can be induced during the daytime by olfactory stimuli. When so stimulated the average swimming speed is about twice as fast as in unactivated fish. 4. Aqueous extracts of mammalian tissues, of marine animals and water in which live plankton has been kept evoke exploratory feeding behaviour. Fish so stimulated attack live plankton which they can see but not reach and will pick up and sometimes swallow pieces of filter-paper which they had previously ignored. 5. The substance in the adductor muscle of Area zebra that excites exploratory feeding behaviour is dialysable and unaffected by boiling. It is effective in concentrations of the order of 1 x10-9. 6. Fish can detect trace amounts of many organic substances, to a few of which they respond in a way similar to exploratory feeding, though of less intensity and shorter duration than is the case with extracts of natural food substances. 7. Blinded Bathystoma respond to extracts of Arca muscle with a modified type of exploratory feeding behaviour and can locate the position of pieces of food. 8. These experiments are discussed in relation to the behaviour of fish shoals in the sea.