Fig. 1. (A) Schematic drawing of the video tracking and recording setup. A white paper sheet (WP) covering the tank that is illuminated by a halogen lamp (HL) is used to provide an even background for the high-contrast images of the carp's silhouette. The emission spectrum of the halogen lamp peaks in the red to yellow region. The flow around the carp is exposed by an expanded light sheet from a continuous wave (CW) laser. A high-speed CCD camera (HSC) fitted with an optical band-pass filter was used to catch the particle images (image in C), and another CCD camera fitted with a red glass plate was used to catch the swimming kinematics (image in B). We set up a global coordinate system with its origin coinciding with the end of the Y rail and the beginning of the X rail (the side of a rail where a motor is fastened is defined as the beginning).