The invention relates to a module (100) which is attached to a trawl, and which at least comprises measuring sensors (330) and a power supply unit. (320) The sensors are basically inactive, but can be activated as the need arises to measure different parameters. In an embodiment the module also includes a power-generating movement device connected to a charging device that can supply energy to the power supply unit. The module may also include communication means. In an alternative embodiment the module ma include a trawl door that can drag the drawl in a desired direction. Several modules may be (combined to form a system that both measures different parameters and positions the trawl in order to achieve an optimal trawling process.