Experimental study of fish synergistic movement of pectoral and caudal fins in narrow space

In general, the turning motion to one side of fish is often driven by the pectoral fin on the opposite side of turning (OST), which’s a common behavior observed in sunfish and box fish etc. Nevertheless, our study presents an unconventional turning behavior exhibited by koi in narrow spaces. We have observed that koi typically turns with the use of the pectoral fin on the same side of turning (SST) while bending its tail into a “C” shape. Through high-speed camera analysis and tomographic particle image velocimetry (TOMO-PIV), we have identified three main flows generated during koi’s turning maneuver in confined areas, two of which are induced by the bending tail, and one is created by the rowing fin. The function of fin jet (jet produced by pectoral fin) is further discussed in this paper. Furthermore, to discuss the advantages of this kinematic pattern, we develop a multi-fin robot fish with a pair of pectoral fins and wire-driven tail to compare the turning ability with other turning motions.