A trawl sensor device for monitoring trawl doors and the distance of the trawl to the bottom during trawling operations comprises at least one acoustic transmitter and receiver, the transmitter being adapted to be positioned such that an acoustic signal is transmitted towards the seabed at a forward-pointing angle, and means for communication between the sensor device and a vessel. The transmitted acoustic signal is reflected from the seabed and is received at this position and the distance between the acoustic transmitter and the seabed is calculated based on the difference in time between transmission and reception of the acoustic signal and the forward-pointing angle.