Gillnet with sphere. Photo: Anne Schütz, TI-OF

PhD defence about gillnet modifications to reduce bycatch of harbor porpoises

Monday 18 Jan 21


Finn Larsen
Senior Researcher
DTU Aqua
+45 35 88 34 96

On 26 January 2021, Isabella Kratzer will defend her PhD thesis. The defence can be watched online.

Harbor porpoises are small whales inhabiting the coastal waters of the Northern hemisphere. They are regularly bycaught in gillnets, which are thin nylon nets standing vertically in the water column like curtains. The porpoises do not recognize that gillnets are impenetrable, get entangled in the netting and drown. 

This gives rise to a conflict between nature conservation goals and the widespread fishery with gillnets which otherwise have positive properties, e.g. low energy consumption and low sea bottom impact. 

To orientate themselves underwater, porpoises use their biosonar by emitting short acoustic signals and perceiving their surroundings through the returning echoes. To avoid bycatch of porpoises, one idea is to make the gillnets more “acoustically visible”, so that porpoises perceive the netting as an obstacle they cannot swim through.

In her PhD project, Isabella Kratzer, DTU Aqua has developed an acoustic reflector that can be used for this purpose, while being invisible for fish. This reflector is a small (8 mm diameter) sphere made from acrylic glass (Plexiglas ®) (see photo above). The spheres create the semblance of a barrier “visible” for porpoises if attached to the gillnets at approximately 30 cm distance from one another. 

Isabella Kratzer’s work shows that the echo returning from a gillnet with spheres is much stronger compared to a standard net, regardless from which angle the porpoise is approaching. These results are supported by findings in a first trial in the commercial fishery showing that a gillnet modified with acrylic glass spheres has the potential to catch fewer harbor porpoises than a standard net. 

About the PhD defence

Isabella Kratzer will defend her PhD thesis "Gillnet modifications to reduce bycatch of harbor porpoises” on Tuesday 26 January 2021 at 13:00 (CET) via Zoom. Find link to Zoom in the box below.


  • Principal supervisor: Senior Researcher Finn Larsen, DTU Aqua
  • Co-supervisor: Researcher Lotte Kindt-Larsen, DTU Aqua
  • Co-supervisor: Senior Researcher Daniel Stepputtis, Thünen Institute of Baltic Sea Fisheries, Germany


  • Senior Researcher Ole Ritzau Eigaard, DTU Aqua
  • Principal Scientist Arne Bjørge, Institute of Marine Research, Norway
  • Senior Scientist Jakob Tougaard, Dept. of Bioscience, Aarhus University

Chairperson at defence

  • Senior Researcher Stefan Neuenfeldt, DTU Aqua
Learn more

A popular science summary of the thesis can be downloaded here

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

Link to Zoom and guidelines for attending the defence

It is possible to attend Isabella Kratzer’s defence 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 both camera and microphone are turned off at all times. 

There will be instructions regarding the proceedings in the beginning of the defence.