Two independent surveys showed that aquatic insects can generate echoes comparable with the echoes of small fish. In the open water of two fishless mountain acidic waters of the Czech Republic, a very distinct population of well-defined single targets resembling fish (target strength -66 to -50 dB at 120 kHz) was discovered. These targets were found in the depth range of 5-7 m during the day (June-July 1997). At night, most targets were recorded by horizontal beaming (depth