Field studies on behavioural ecology are rare – the technical challenges involved in facilitating the continuous and real-time observation of fish and how they behave in natural freshwaters are simply too immense. Consequently, only very little is known about the interrelations between the behavioural type, individual fitness and population dynamics of fish in their natural environment. We have taken on this challenge: IGB has installed a modern fish-detection device at the lake Kleiner Döllnsee (Schorfheide, north Brandenburg). The telemetry system consists of 20 wireless hydrophones. Several thousand fish can be located simultaneously with a high degree of precision (average position error 2 ± 6 metres) in three-dimensional resolution. The principle is based on acoustic transmitters implanted in the abdominal cavity of free-swimming fish. The transmitters emit an encoded acoustic signal at regular intervals meaning that, ideally, several positions per fish per minute can be recorded over a long period.