A Measurement Method for Speed Distribution of Collective Motion with Optical Flow and Its Applications to School of Fish

Abstract

We propose a measurement method for the mean speed distribution of collective motions of highly dense groups with optical flow based on our previous work. This measurement is fundamental for ecological investigations and mathematical modeling of collective animal behaviors. Our method is applicable to highly dense homogeneous groups wherein individual movements are approximately uniform locally. To measure speed distributions, we partition a group into regions and estimate mean speeds in each region by extracting only flows that are relevant to collective motions and averaging them over a period of time. We apply our method to schools of sardines to measure a kind of speed distribution called rotation curve (RC). As experiments, we first performed simulations and found that our method can estimate flows and RCs accurately. We then performed experiments with videos of real fish. The RCs estimated by our method and ones by manual tracking were approximately equal. We applied our method to a longer video and confirmed that time series of RCs can be estimated automatically. In addition, we detected noticeable events by two features derived from flows. We also applied our method to a school of anchovies. The results indicate that our method is practically useful.