In situ measurements of turbulence in fish shoals

Turbulence was measured in situ within shoals of juvenile perch Perca fluviatilis with a self-contained autonomous microstructure profiler near an artificial reef in Lake Constance, Germany. Depth-averaged dissipation rates of turbulent kinetic energy (TKE) correlated with the density of shoaling fish, providing evidence for fish-induced turbulence in a large and stratified lake. The observed range and the depth-averaged values of TKE dissipation rates associated with fish-generated turbulence were comparable with magnitudes of turbulence typically caused by internal waves or wind forcing. Enhanced turbulence within fish shoals could only be observed during periods of low background turbulence and high fish abundance. The observed rates of dissipation of TKE are about two orders of magnitude smaller than production rates of TKE estimated from empirical models on the basis of observed fish size and swimming speed.