In many fishes, an upright swimming position is unstable: the fish's centre of mass is above its centre of buoyancy. How a fish moves its fins and where the fins are located determine how fins produce torques around the body. Most fish have two sets of paired fins (pelvic and pectoral) and three median fins (dorsal, anal and caudal). The integrated motion of multiple fins help fish control their instability. Some studies have looked at how individual fins are used during swimming. Little work has been done on multiple fin function, however. This dissertation uses high speed video analysis, quantitative flow visualization and electromyography to determine how fish use multiple fins during steady and unsteady swimming.

High speed video of fish showed that simultaneous dorsal and anal fin oscillation was driven by body undulation and independent fin control. Fin muscles control the curvature of each fin ray, as well as the ray's angle to the fin base independently, making fins incredibly flexible surfaces. Flow visualization indicated that dorsal and anal fins released lateral jets concurrently, balancing roll torque between fins. Fish appear to control heave and pitch oscillations of their dorsal and anal fins independently; differences in attack angle, fin shape, and oscillation amplitude help to explain how anal fins release lateral jets earlier in their oscillation cycle to match rolling torques produced by dorsal fins.

Flow conditions produced by upstream pelvic fins also affect the performance of anal fins. Electromyography during slow speed swimming indicated that trout actively oscillate their pelvic fins. Flow visualization showed pelvic fins change the flow along the trout's ventral aspect, slowing the fish and altering the incident flow experienced by the anal fin. At slow speeds, the anterior fins of trout act as brake and stabilization control while posterior fins produce thrust, suggesting complex and integrated fin hydrodynamic function. Results document a new hydrodynamic function for pelvic fins, and show that the hydrodynamic wakes of fins influence each other during swimming. Results also reveal fundamental control mechanisms found in fish fins that may be applied to further ideas of control in biorobotic applications.