These researchers at MIT and CCNY have built a detailed replica of a bluefin tuna to study how fish propel themselves so efficiently. They found that the efficiency is closely tied up with the way the vortices are arrange in the wake. The shedding of vortices is described by a non-dimensional frequence, called the Strouhal number, defined as given in the caption.

Picture taken from the article "An Efficient Swimming Machine," by Michael S. Triantafyllou and George S. Triantafyllou, Scientific American, March 1995.