The beating tail of a swimming fish creates vortex rings (blue) in its wake. Predators detect their prey via such wake structures, which can persist for over one minute.

From the article:

Wake Tracking and the Detection of Vortex Rings by the Canal Lateral Line of Fish
Jan-Moritz P. Franosch, Hendrik J. A. Hagedorn, Julie Goulet, Jacob Engelmann, and J. Leo van Hemmen
Phys. Rev. Lett. 103, 078102 (2009)

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