Developing Low Impact and Fuel Efficient Towed Demersal Fishing Gears

Demersal trawling is practiced worldwide and supports global food security, serves as a key economic driver, providing employments and contributing to the vital cultural identity of many coastal communities worldwide. However, demersal trawling is recognized as one of the largest anthropogenic sources of disturbance to the seabed and its biota of shallow shelf seas. This disturbance can have environmental and ecological consequences that affect primary production and threaten the biological sustainability and economic viability of fisheries. Further, demersal trawling can have global consequences associated with greenhouse gas emissions, either through the burning of fossil fuels to maintain fishing effort or through gear contact with carbon-rich seabed habitats. As a result, the use of demersal trawls have been banned or severely restricted in some countries, and there are many proposals to implement such restrictions elsewhere.

The present study deconstructs the bottom otter trawl gear into its individual components, focusing on particular gear components with the aim of reducing hydrodynamic and contact forces. The findings can facilitate the design and development of towed demersal fishing gears that are more fuel efficient and environmentally sustainable. Individual gear components generate varying magnitudes of hydrodynamic and contact forces; therefore, they can cause different level of disturbance to the seabed. This emphasizes the necessity for a systematic exploration of a bottom otter trawl system at the single gear component level. Notably, the otter boards, sweeps, lower bridles, and the ground gear are the components of a bottom otter trawl with the most contact with seabed, which require technical measures to reduce environmental impacts of demersal trawling.

In Chapter 2, I examined the engineering performance of a novel self-adjusting, semi-pelagic otter board (SAO) system, which have the ability to maintain a given height above the seabed, aiming at reducing the seabed contact. The SAO have two adjustable flaps, which are controlled by on-board altimeters and actuators, modifying their lift and drag and altering their position in the water column. The actuators are governed by a Proportional Integral Derivative feedback system, which uses the altimeter data to maintain the otter boards at a preset target height above the seabed. I measure, under operational conditions at sea, the hydrodynamic efficiency, the fuel consumption, and the extent to which the doors avoid contact with the seabed. I demonstrate that the SAO have high hydrodynamic efficiency and effectively reduces gear drag and fuel consumption. Utilizing the SAO successfully eliminates seabed contact, and consequently reduces the environmental impacts typically arise with conventional seabed-contacting otter boards.

In Chapter 3, I subsequently investigated whether replacing conventional seabed-contacting otter boards with SAO affects catch efficiency of a demersal fish trawl. The ability of SAO to control their position in the water column provides an opportunity to explore how varying heights above the seabed affects catch efficiency. In particular we focus on target heights of 1 and 5 m, because these two heights have been shown to lead to partial-contact and zero contact scenarios, which could have differential effects on the catch efficiency. Experimental fishing trials, using the alternate haul method, were conducted in the Kattegat and Skagerrak with three otter board configurations using: conventional seabed-contacting otter boards; the SAO set to maintain a target height over the seabed of 1 m; and the SAO set to maintain a target height 5 m. I have shown that replacing conventional seabed-contacting otter boards with SAO reduces the catch efficiency of a demersal trawl, with differences both within and between roundfish and flatfish species. Replacing conventional otter boards with SAO resulted in a loss of catch efficiency for haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and plaice (Hippoglossoides platessoides), while no significant difference was found for cod (Gadus morhua), common dab (Limanda limanda) and lemon sole (Microstomus kitt). Finally, I discussed pros and cons of adopting SAO in demersal trawl fisheries considering their catch efficiency, fuel saving, and seabed impact.

To obtain fundamental knowledge of hydrodynamics for individual gear components like sweeps, lower bridles, and ground gear, I investigate hydrodynamic coefficients of chain and cylinder models that resemble these components in Chapter 4. Two circular cylinder and two stud-less chain models were designed for a flume tank experiment. By examining the hydrodynamics of these models under various flow velocities, angles of attack, and positions in the water column, we gained insights into the hydrodynamic behavior of individual gear components in operational conditions.

Furthermore, a prediction model was established based on cross-flow principle method, which can be utilized to estimate the hydrodynamic drag of specific gear components like sweeps, lower bridles, and ground gear.

Drawing on the data obtained in chapters 2 and 4, I was able to evaluate the physical disturbance of individual gear components in a bottom otter trawl. Consequently, in Chapter 5, I explored modifications to the otter boards and sweeps in order to reduce the sediment resuspension caused by demersal trawls. I decomposed a bottom otter trawl into its main components that are in contact with the seabed, or towed just above the seabed such as otter boards, sweeps, lower bridles, ground gear, and bottom panels of the net. I estimated the hydrodynamic drag of each component from its frontal surface area, speed, and angle of attack. Specifically, the hydrodynamic drag coefficient values for sweeps, lower bridles, and ground gear were predicted based on model established in Chapter 4. Using the estimated hydrodynamic drag and silt fraction, I quantified the sediment put into the water column by each component. Finally, I discussed the contribution of each gear component in the overall sediment resuspension generated by the entire gear and potential modifications to the otter boards and sweeps to mitigate this impact.

The study shows that designing gear components with reduced hydrodynamic drag and reduced contact drag can significantly mitigate seabed contact, sediment resuspension, and fuel consumption. This thesis contributes to the ongoing efforts to develop demersal trawls with reduced environmental impacts, thereby contributing to the economic and sustainable development of demersal fisheries.