  Fluke oscillations of cetaceans (whales, dolphins and porpoises) imprint the
ocean surface with an oval patch that surface waves cannot enter. Scientists
have hypothesized that modification of surface tension by surfactants creates
flukeprints. Here, we show, on the contrary, that the formation of flukeprint
is primarily due to a wave-current interaction problem. We provide Particle
Image Velocimetry measurements of anisotropy and vorticity in the flow
generated by a mock whale fluke in a laboratory experiment. We explain for the
first time why long gravity waves enter the flukeprint, whereas short gravity
waves are blocked.
