Measurement of sardine-generated turbulence in a large tank

While swimming organisms may cause turbulence, it is not clear how strong the turbulence is and if eddies are large enough to mix stratified water columns. We conducted an observational experiment in a large aquarium tank containing several thousand Japanese sardines Sardinops melanostictus. Turbulence data were collected from inside the sardine school using a turbulence microstructure profiler. The averaged turbulent kinetic energy dissipation rate was 2.3 x 10(-4) W kg(-1) inside the school, while the averaged background value was 6.7 x 10(-6) W kg(-1). The school displayed fast and non-continuous `avoidance behavior', or fast and long-lasting `feeding behavior' during the measurements. A noticeable difference between the 2 behaviors was found in turbulent shear spectra: the avoidance behavior spectra showed a power decline in comparison with the Nasmyth empirical spectrum in the inertial sub-range, but the feeding behavior spectra exhibited no power decline, even in the inertial sub-range. In the latter case, the sardine school imparted kinetic energy into scales larger than the average individual body size of 0.173 m. This result is a counter-example to a general hypothesis that swimming organisms cannot impart kinetic energy at scales larger than their individual body size.