In the present study the time course and spectral-amplitude distribution of hydrodynamic flow fields caused by moving fish, frogs, and crustaceans were investigated with the aid of laser-Doppler-anemometry. In the vicinity of a hovering fish sinusoidal water movements can be recorded whose velocity spectra peak below 10 Hz (Fig. 2). Single strokes during startle responses or during steady swimming of fish, frogs, and crustaceans cause short-lasting, low-frequency (<10 hz),="" transient="" water="" movements="" (fig.="" 3).="" low-frequency="" transients="" also="" occur="" if="" a="" frog="" approaches="" and="" passes="" a="" velocity-sensitive="" hydrodynamic="" sensor.="" in="" contrast,="" transient="" water="" movements="" caused="" by="" a="" rapidly="" struggling="" or="" startled="" fish="" or="" water="" motions="" measured="" in="" the="" wake="" of="" a="" slowly="" swimming="">47 cm/s) trout can be broadbanded, i.e., these water movements can contain frequency components up to at least 100 Hz (Figs. 4, 5A, 6). High-frequency hydrodynamic events can also be measured behind obstacles submerged in running water (Fig. 5C). The possible biological advantage of the ability to detect high-frequency hydroynamic events is discussed with respect to the natural occurrence of high frequencies and its potential role in orientation and predator-prey interactions of aquatic animals.