Method to observe large scale behavioural waves propagating through fish schools using 4D sonar

How animals interact collectively in groups is of interest for both ecologists and engineers, both from a mechanistic and functional point of view. A property of some large animal groups, regardless of their size, is their ability to perform highly coordinated anti-predatory reactions that can progress through entire groups. We developed a method using a true 3D sonar to quantify these behavioural waves in free-ranging fish schools, utilizing that rapid changes is backscattering strength is caused by changes in fish orientation rather than changes in density. Both simulated and real data were used for evaluation of the method. The method reliably estimated the speed of a simulated turning wave propagating through the school, and tests on real data gave similar wave speeds as observed in smaller scale experiments. In cases where the schools were highly irregular and dynamic, the results were less reliable. These cases were identified by high sensitivity to the classification parameters. Until now, the existing methodology has been restricted to small scale laboratory experiments or qualitative observations. Our approach provides a tool to validate the generalizations that are usually extrapolated from small scale tank studies, and we discuss its potential use.