Fig. 2. Experimental apparatus. Fish swam in a multi-speed flow tank. High-speed cameras filmed ventral and lateral views simultaneously, enabling three-dimensional analysis of fin motion. A mirror slightly angled along the right side of the fish allowed full visualization of the right pelvic fin. Fin motion was measured relative to the water flow (flow axes) and relative to the fish body position (fish axes, which change as the fish turns during manoeuvres).