FISH FOOT PRINTS - MORPHOLOGY AND ENERGETICS OF THE WAKE BEHIND A CONTINUOUSLY SWIMMING MULLET (CHELON LABROSUS RISSO)

The structure of the wake behind a continuously swimming mullet was an alysed qualitatively and quantitatively by applying two-dimensional pa rticle image velocimetry. A detailed analysis of the flow pattern and of the swimming movements of the fish allowed us to derive a kinematic explanation of the flow pattern as well as an estimate of the relativ e contributions of the body and the tail to thrust production. During active propulsion, the undulatory swimming fish shed a wake consisting in the medio-frontal plane of a rearward, zigzagging jet flow between alternating vortices. The fish shed one vortex per half tailbeat when the tail reached its most lateral position, Part of the circulation s hed in the vortices had been generated previously on the body by the t ransverse body wave travelling down the body, This undulatory pump mec hanism accounted for less than half of the energy shed in the wake, Th e remainder was generated by the tail, The vortex spacing matched the tailbeat amplitude and the stride length.