Using hydrodynamics to improve the selectivity of towed fishing gears

Demersal trawling is one of the most commonly used fishing methods worldwide. It is a socioeconomically important industry and accounts for a large portion of the global landings of wild fish species. The demersal trawl fishing gear is dragged over the seabed and can have a damaging impact on benthic habitats, resulting in habitat destruction, benthic mortality, and the release of nutrients from the seabed. Further, they are multi-species fishing gears where many of the fish in the path of the net get caught, and there may be low selectivity, leading to large numbers of discards, fish that are not kept or are thrown back to sea dead or dying.
This study investigates how hydrodynamics can be used to improve the selectivity of towed fishing gear with focus on the groundgear in a standard demersal trawl gear and the beam in a beam trawl. These forward gear components are expected to have a large impact on the selectivity since they are positioned at or close to the mouth of the fishing net hence are the last places the species can escape before they enter the fishing net. The flow around a simplified model of a forward gear component, a disc-attached cylinder near a wall, was investigated experimentally in a current flume. The instantaneous flow velocities were measured using Laser Doppler Velocimetry from which the mean flow velocities were determined. From the mean flow, several flow regions were identified. At a sufficient distance from the disc, the effect of the disc on the flow is insignificant and statistically, the flow has two-dimensional properties. The wake of the structure was also investigated where the effect of the disc is present in the near wake and fades further downstream. The turbulent kinetic energy was also determined from the velocity component and the experimental data was used to validate a computational fluid dynamics (CFD) model. The numerical model and the experimental data were in good agreement and it was seen from both that vortex shedding from the cylinder is disturbed by the disc. After validation, the numerical model can be utilized to investigate how the hydrodynamics are affected by the size and number of discs.
The Danish sea star fishery is used as a case study to investigate the effect of the hydrodynamics on the catch efficiency. The beam is a special case of the modeled forward gear component where no discs are attached to the cylinder. The beam trawl is used in the sea star fishery. Sea stars are fished for two main reasons. Firstly, they predate on shellfish and can have a large impact on mussel production and shellfish populations. Secondly, the sea star fishery has become a fishery in its own right since the sea stars can be exploited as a protein supplement for animal feed. In both instances, a clean catch with little mussel bycatch is required. This study investigates how sea stars can be lifted from the seabed by the hydrodynamics around the beam instead of being mechanically scraped from the bottom. To optimize the fishing gear, the hydrodynamics should raise a large proportion of sea stars from the seabed while having a low mussel bycatch and a minimal impact on the seabed.
A particle path model is used to model how the hydrodynamics around the beam of a beam trawl raise the sea stars from the seabed and their further trajectory in the wake. The particle path model takes the velocity field obtained with the CFD model as input as well as the settling velocity of the sea stars. The morphology of the sea stars and the settling velocity were measured for this purpose. A geometric model of the sea stars was also created and their drag coefficient was determined. The sea stars were modeled in the particle path model as passive particles lying on the seabed with a given thickness. The model shows that an upward flow, which occurs when a vortex is formed at the lower side of the beam, causes the sea stars to be ejected. The model also demonstrates that the trajectories of the sea stars depend on the clearance of the beam. Increasing the clearance causes fewer sea stars to be ejected from the seabed and hence decreases the number of sea stars that will get caught. The effect of the diameter of the beam was also investigated where it was seen that a large beam with the same relative clearance, hence a different relative thickness of the sea stars, decreases the number of ejected sea stars.
Sea trials were conducted on a commercial fishing vessel where modifications were made to the fishing gear to investigate how systematic adjustments to the hydrodynamics cause systematic changes in the catch rate. The clearance of the beam was varied in the field trials where the catch rate decreased with increasing clearance. Further, a larger diameter beam was utilized where the catch rate was decreased for the same relative clearance, which is in good agreement with the particle path model. The catch rate was shown to be independent of the distance between the beam and the fishing line in the investigated range in the sea trials, which again is consistent with the results of the numerical model, which predicts that the sea stars are in the water column in this range.
The study shows that hydrodynamics can be used to improve the efficiency and selectivity of towed fishing gears. It is expected that the approach can be expanded to other fisheries and that a better understanding of the hydrodynamics around towed gears will lead to the design and development of low-impact, selective fishing gears.