Design of a Selective Waterjet Trawl Gear System for Flatfish Fishing

Abstract

Conventional beam trawling for sole is associated with high fuel consumption and significant seabed disturbance due to the use of tickler chains. With the ban of pulse trawling, waterjet stimulation has been proposed as a potential alternative startle mechanism. However, previous concepts were ineffective and continuous activation was considered economically infeasible because of its high energy demand. Combining waterjets with fish detection technology may offer a solution by activating the system only when marketable fish are present.
The objective of this thesis was to develop and evaluate a trawl concept that integrates waterjet stimulation and fish detection technology into an existing trawl configuration to minimize energy consumption and seabed disturbance while maintaining catch efficiency for sole. An extensive concept development and optimization was performed using knowledge on existing trawl systems, waterjet studies, fish behavior and fish detection technologies. This development and optimization began with the development of functional requirements and generating sub-concepts for different design aspects. These sub-concepts were combined into two main concepts and evaluated based on selected design criteria and stakeholder feedback. Finally, the selected concept was further developed and refined, and the hydraulic water supply system was optimized.
The resulting concept consists of multiple enclosed tunnel-shaped sections separated by side chains and netting. Each section contains its own fish detection gear and selectively activated waterjet housing, supplied by a water supply system positioned inside a wing configuration. The system is integrated into a conventional trawl configuration by placing its accumulator-based hydraulic system inside a conventional wing-with-shoes configuration, and uses a conventional netting and square-shaped net opening. Optimization of this system gave an estimated pump power range from approximately 32 kW at a towing speed of 4.5 knots to around 75 kW at 5.2 knots, which was estimated to be a realistic power limit.
The developed concept appears therefore technically feasible for towing speeds between 4.5 and 5.2 knots, and has the potential to reduce seabed disturbance and improve the practical feasibility of waterjet stimulation compared to continuous activation across the full trawl width. However, the physical response of sole to waterjets, the performance of the detection gear, and the operational behavior of the complete system remain uncertain. Although experimental validation is required before the practical effectiveness, seabed disturbance, catch efficiency, and overall energy performance of the concept can be determined, the developed design shows that selective waterjet stimulation can potentially be an alternative to conventional stimulation. The concept therefore serves as foundation for future experiments and further development in finding a
potentially less invasive and more energy-efficient trawl system.