The physical impact of towed fishing gears on the seabed

Towed demersal fishing gears interact with the water column and the seabed through which and on which they are towed. On soft sediments their physical effects have been classified as being either geotechnical or hydrodynamic. The geotechnical effects refer to the contact drag, the penetration and piercing of the substrate, lateral displacement of sediment and the influence of the pressure field transmitted through the sediment; whereas the hydrodynamic effects refer to the hydrodynamic drag and the mobilisation of sediment into the water column (O’Neill and lvanovic, 2016).
These physical effects can have broader ecological, environmental and biological implications and many studies have shown that towed demersal fishing gears damage habitats, cause benthic mortality, release nutrients and resuspend phytoplankton cysts and copepod eggs. There can also be economic consequences and studies of the fuel efficiency of fishing trawlers have demonstrated that the drag of a demersal towed gear can account for up to 80% of the fuel consumed. In order to be able to assess and quantify the implications of fishinq to the wider ecosystem; provide advice to policymakers in relation to the establishment of marine protected areas and the prioritisation of seabed usage; and develop more fuel efficient and environmentally friendly fishing techniques; we must understand the
underpinning physical and mechanical processes taking place.
Experimental and theoretical studies undertaken at the UoA have successfully identified the relationship between different gear elements and the resistance force/failure mechanism within the soil as the gear moves. Experiments took place, under drained ( dry) and
saturated conditions, where the element was (i) fixed and (ii) free to move vertically. The former was to identify associated forces and the latter to give an indication of penetration. (Casanovas-Revilla and lvanovic, 2017) The aim of this proposal is to further develop an
understanding of these key mechanisms and resolve some of the challenges that have been identified. This would greatly improve the assessment of the disturbance fishing gears make and identify ways in which these impacts can be reduced. The approach will be both theoretical and experimental. Theoretical approach will take into account the mode of sediment deformation and failure beneath the ground gear during its movement which is deemed to be analogous to common geotechnical processes. Analytical models based on passive earth pressure and retaining wall theories will be further developed in this project to allow for the saturation conditions to be properly defined and consequently included in analytical expressions to be used for the predictive capacity of the impact of the gear examined. Complementary to this extensive laboratory tests will be carried out in a specially designed sand towing channel in the UoA Geotechnical Laboratory. A number of relevant scaled gear components (groundgears, sweeps, trawl doors) will be tested on different types of sand of different densities, at both dried and saturated conditions. In addition, the candidate may be involved in sea trials on a scientific research vessel where full scale gear components will be tested. The student would mainly undertake experimental study under saturated conditions, improving the method currently used so that more reliable and consistent results can be obtained and compared with analytical models developed. The student will therefore design a new method to measure the penetration with a view to include laser elevation methods for underwater conditions.

Application Process
Please apply for admission to the ’Degree of Doctor of Philosophy in Biological Science’ to ensure that your application is passed to the correct college for processing.
Please provide a copy of the degree certificate and transcript for each previous degree undertaken, a copy of your English language proficiency certificate (if relevant), and contact details of two referees who can comment on your previous academic performance (at least one should be from your current degree programme). References will be requested if you are selected for interview. Incomplete applications will not be considered.