Predator-avoidance behaviours of sand-eel schools: why schools seldom split

In a large outdoor tank 250 sand-eels (Ammodytes sp.) cruised as a single school performing consistent behaviours when close encounters with faster swimming mackerel occurred. Sand-eels near fishing gear in the wild showed similar behaviours. Cruise and Avoid were the most frequent behaviours, followed by Herd, Mill, Split and Join. Flash Expansion, Hourglass and Vacuole were occasional, whereas Ball was rare. Cruise and Mill were apparently unconnected with predation. School manoeuvres can be generated by combinations of four behavioural mechanisms of individual fish: (1) schooling tendency, (2) startle response, (3) nearest neighbour distance, and (4) minimum approach distance allowed intruders. The minimum approach distance, bringing about behaviours like Herd and Vacuole, may represent an optimization of the relative swimming performance of sand-eels and their predators with the need to monitor predator behaviour. Paradoxically, minimum approach distance may come about simply through sand-eels schooling with intruders. Rapid reformation of the school after Flash Expansion, high polarity, and the integrity of the school maintained in the thin ‘neck’ of Hourglass all testify to the high degree of schooling tendency evolved by sand-eels. Schools were difficult to split as a consequence. When two schools join, the new swimming direction is the resultant vector of the two original directions. Leading fish join by swinging smoothly onto the new track, but fish downstream of the initial join are progressively more likely to continue in their original course, producing interdigitation. At the far side of the school, interdigitated peripheral fish take up a new course only after large ill-coordinated turns; this creates a ‘Confusion Zone’. The Confusion Zone, reported here for the first time, may result inevitably from ‘good schooling’. Such a potential danger zone created on joining may explain the evolution of reluctance to split.