Some Aspects of the Schooling Behaviour of Fish

The purpose of this paper is to present a hypothesis on the nature of the schooling behaviour of fish based on an ethological investigation of schooling. Recognizing the disadvantages of a limited amount of data and of the use of different species for different parts of the study, the following tentative picture is suggested. Schooling may be considered an instinct as defined by TINBERGEN and is at a relatively low level in the hierarchical organization of behaviour. It has typical appetitive behaviour and a consummatory situation. A single fish separated from its school searches until perceiving a group of fish. It then approaches the group. In most cases vision is the only sense involved in this approach. If more detailed specific stimuli are then perceived (possibly through any of the sense organs) the fish ceases searching and remains with the school; if not, it soon leaves, and appetitive behaviour continues until the appropriate consummatory situation (being in a school of the same species) is attained. This hypothesis is based on the following points: 1. A school of fish is an aggregation formed when one fish reacts to others by remaining near them. 2. Typical features of Gasterosleits ticuleatus and Scardinius erythrophthalmus schools are: performance of the same activity at the same time by all fish, lack of aggressivcness between members and equality of rank of all members. 3. Blinded Scardinius fail to show typical schooling but remain in an area where odours from other Scardinius can be detected. This response may keep schools of this species from scattering widely at night. 4. Visual perception of a school of fish releases approach in single Scardinius and Pristella riddlei. 5. When presented with two different-sized schools of their own species single Gasterosteus, Scardinius and Leuciscus ntlilus prefer the larger to the smaller group. 6. A small Gasterosteus prefers six large to six small Gasterosteus. 7. A single Gasterosteus prefers a school of its own species to a school of Rhodeus amarus but shows no consistent preference when either Pygosteus pungitius or Leuciscus are presented together with Gasterosteus. 8. A single Pristella prefers a school of unoperated Pristella to a school with amputated dorsal fins. The dorsal fin with its conspicuous black patch is jerked more rapidly after alarm. This structure and its special movement may be considered a social releaser. 9. Increased feeding motivation leads to limited dispersal of a school of Gasterosteus. The head-down feeding posture is a signal attracting others in a school to a source of food. 10. Alarm causes an increase in density of a school of Gasterosteus. 11. With increasing reproductive motivation male Gasterosteus cease schooling and try to hold territories. Females disperse to a limited extent.