Fig. 2. Experimental apparatus for stereoscopic digital particle-image velocimetry (SDPIV) and recording images of the fish motion. This sketch shows the setup for filming the `parasagittal' flow fields. The parrot cichlid swam freely in a transparent tank filled with still fresh water. The motion camera was placed below the tank. A mirror was placed on a side wall of the tank to enhance the intensity of backward scattered light received by the SDPIV camera. The viewing angle between the optical axis of each camera and the object plane norm was 30°.