This thesis is motivated by the possible benefits of a more precise trawl control system with respect to both environmental impact and fishing efficiency. A mathematical model of the trawl system is developed, including an accurate model of the hydrodynamic forces on the trawl doors. This model estimates both the steady state and the transient forces on trawl doors moving in six degrees of freedom. Various concepts for trawl door control are evaluated, and a new trawl door control concept is proposed. The trawl door control concept is developed to fulfill the demands on both energy...