Cod liver with a high density of nematodes (19 nematodes per gram liver tissue). Photo: Marie Plambech Ryberg

PhD defence about liver worms in Eastern Baltic cod

Monday 18 Jan 21


Marie Plambech Ryberg
DTU Aqua
+45 93 51 14 14


Jane Behrens
Senior Researcher
DTU Aqua
+45 23 29 68 63

On 28 January 2021, Marie Plambech Ryberg will defend her PhD thesis. The defence can be watched online.

The Eastern Baltic cod stock is in severe distress. Fish are in a historically poor nutritional state, natural mortality is high, and the large fish have disappeared while the stock biomass has declined substantially since the late 1980s. 

Several ecosystem factors have been suggested to drive these changes, including infections with the parasitic nematode Contracaecum osculatum, which is found in cod livers. 

In her PhD project, Marie Plambech Ryberg, DTU Aqua has investigated how infections with the liver worm associate with the physiological and nutritional conditions in individual cod. The investigations include laboratory experiment, data collection in the field and integration of the results into bioenergetics and statistical models.

The results show that high infection density is associated with an impaired physiological condition of the infected cod, and individuals with high infection densities reveal signs of suffering from severe liver disease. Moreover, the growth is lower in infected fish and the probability of cod being in a critical condition increases with increasing infection density. Hence, Marie Plambech Ryberg concludes that parasite infections is a potential important driver of the poor status of Eastern Baltic cod.

Marie Plambech Ryberg has also analyzed whether the current method used in the monitoring of liver worm infection loads in the Baltic region is robust. Here cod livers are categorized into five stages based on the number of liver worms visible on the surface of the liver, and Marie Plambech Rygberg’s study confirms that this method is a good predictor of the total number of nematodes in the liver.

About the PhD defence

Marie Plambech Ryberg will defend her PhD thesis "Eastern Baltic cod infected with Contracaecum osculatum: physiological mechanisms and the importance of monitoring infection loads” on Thursday 28 January 2021 at 10:00 (CET) via Zoom. Find link to Zoom in the box below.


  • Principal Supervisor: Senior Researcher Jane Windfeldt Behrens, DTU Aqua
  • Co-supervisor: Associate Professor Peter Vilhelm Skov, DTU Aqua


  • Senior Researcher Manuel Gesto Rodriguez, DTU Aqua
  • Senior Lecturer Tara Marshall, University of Aberdeen, Scotland
  • Associate Professor Louise von Gersdorff Jørgensen, University of Copenhagen, Denmark

Chairperson at defence

  • Associate Professor Mikael van Deurs, 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 Marie Plambech Ryberg’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.