Acoustic telemetry consists of a tag, a transmitter, implanted in fish, that sends a signal which can be picked up by receiving instruments, fixed hydrophones under water, that listen for these sounds. When a tagged fish is close to a receiver, it gives a time stamp of when that particular fish was passing by, at a certain location. Jan Reubens at VLIZ, the Flanders Marine Institute, specialises in marine observations, while Pieterjan Verhelst, at the Aquatic Management team at INBO, the Rese...