Traditional sandeel otter trawl. Illustration: Neil Bromhall.

PhD defence about the effect of lightweight fishing gears on the seabed

Friday 03 Mar 23


Katrina Bromhall
PhD Student
DTU Aqua


Ole Ritzau Eigaard
Senior Researcher
DTU Aqua
+45 21 15 45 65


This PhD project was funded by the European Maritime and Fisheries Fund and the Danish Fisheries Agency through the projects ‘Sandbanker og fiskeripåvirkning i relation til EU’s fiskeri- og miljøpolitik’ and ‘Development of benthic indicators for assessment of fisheries impacts on marine benthic fauna and habitats’.

EU-flag og det danske logo for Den Europæiske Hav- og Fiskerifond.

On 17 March 2023, Katrina Bromhall will defend her PhD thesis. It is possible to attend the defence at DTU in Lyngby or online.

Worldwide, the leading cause of physical seabed disturbance is fishing with bottom-towed fishing gears. Bottom-towed fishing gears are dragged along the seabed in order to harvest seafood that live on the seafloor.

A solution to mitigate damage caused by these fishing activities is to use fishing gears with lower seabed impact. Decreasing the weight of the gear reduces the amount of direct contact the fishing gear has with the seabed. As such, modified and lightweight fishing gears have already been introduced to commercial fisheries in Denmark. However, experimental assessments of their impact on seabed macrofauna have not yet been carried out. Now PhD Student Katrina Bromhall, DTU Aqua, has done that.

The PhD project “The effect of modified and lightweight fishing gears on benthic macrofauna” explored the effect of three lightweight- or modified fishing gears on benthic macrofauna. Benthic macrofauna are tightly associated with physical and chemical conditions of the seabed and therefore used as indicators of seabed health. The aim was to assess whether low-impact fishing gears caused less benthic faunal mortality relative to conventional fishing gears used widely in the fishing industry.

The fishing gears examined were a lightweight mussel dredge, and a Danish anchor seine, which are both commercially used in Denmark, as well as a modified sandeel otter trawl which had been designed and trialed as an alternative to the traditional sandeel fishing gear.

To evaluate the impact from fishing, an assessment was based on an experimental design called a BACI (Before-After-Control-Impact). This involved sampling seabed macrofauna before and after fishing, in a fished (impact) and unfished (control) location. A subsequent comparison of the benthic macrofauna determined the extent to which the fishing gear effected the seabed.

In her thesis, Katrina Bromhall overall concludes that the lightweight fishing gear, she has examined, reduce the impact of fishing to seabed macrofauna. However, some level of impact remains, and thus using lightweight gears does not completely eliminate physical damage. Instead, lightweight and modified gears may be seen as an improvement on existing fishing gears. In the thesis, the results are discussed in relation to fisheries management, and as to whether lightweight fishing gears offer a better alternative, particularly in areas of conservation interest such as Natura 2000 sites. 

About the PhD defence

Katrina Bromhall will defend her PhD thesis "The effect of modified and lightweight fishing gears on benthic macrofauna” on Friday 17 March 2023, 13:00 on Zoom and at DTU in Lyngby, building 303A, auditorium 41.


  • Principal supervisor: Senior Researcher Ole Ritzau Eigaard, DTU Aqua
  • Co-supervisor: Professor Barry O'Neill, DTU Aqua
  • Co-supervisor: Senior Consultant Grete E. Dinesen, DTU Aqua


  • Professor Ludvig Ahm Krag, DTU Aqua (chair)
  • Professor Clare Bradshaw, Stockholm University, Sweden
  • Senior Lecturer Bryce Stewart, University of York, UK

Chairperson at defence

  • Professor Karin Timmerman, DTU Aqua
Learn more

A copy of the thesis is available by e-mail on request. Please contact PhD secretary Karin Stubgaard, 

How på participate in the defence


Everybody is welcome to attend Katrina Bromhall's PhD defence at DTU, building 303 A, auditorium 41, 2800 Kgs. Lyngby. 


The defence can be followed online on Zoom using this link: 

Please, enter the meeting 10 minutes prior to the defence proceedings are scheduled to start. All participants are muted per default, but we ask you to double check that your microphone is turned off at all times. 


Friday 17 March 2023, 13:00-16:00.