Abstract: Method of controlling two separately operable hydraulic circuits involving monitoring the pressure conditions in one of the circuits, comparing this pressure with a reference pressure which is established in the control system to produce a first error signal which is used to correct if necessary the pressure within the one circuit. Performing the same operation in the case of the second circuit using the same reference pressure. In particular the system and apparatus of the invention is used for controlling the tensions in the warps of a trawl automatically throughout the shooting, trawling and hauling-in of a trawl.