A common mesopelagic fish species, Müller’s pearlside (Maurolicus muelleri). Photo: Kjetil Gjeitsund Thorvaldsen.

PhD defence about population ecology of mesopelagic fishes

Wednesday 09 Nov 22


Stefan Neuenfeldt
Senior Researcher
DTU Aqua
+45 35 88 34 02

On 18 November 2022, Kjetil Gjeitsund Thorvaldsen will defend his PhD thesis. It is possible to attend the defence at DTU in Lyngby or online.

About 20 percent of the volume of the ocean is covered by the mesopelagic zone at depths between 200 and 1000, also known as the oceanic twilight zone. Small bony fishes, crustaceans, squids, sharks and jellyfish inhabit this zone. 

The fish component of these mesopelagic layers is believed to be the largest vertebrate biomass on earth. This biomass is of increasing interest as a source of proteins and lipids to cover the needs of a growing global population.

To ensure that a potential exploitation of the mesopelagic resources is conducted in a sustainable way, more knowledge about the population ecology is needed. This knowledge is a prerequisite for valid abundance estimations and for assessing the carbon binding potential of these organisms. Their assumed large role in active carbon sequestration is unresolved, and harvest without proper knowledge can have disastrous consequences.

Identifying mesopelagic fishes is challenging. As a PhD student at DTU Aqua, Kjetil Gjeitsund Thorvaldsen has focused on improving the methods for collecting data on the mesopelagic fish Müller’s pearlside (Maurolicus muelleri). This is a globally common species and ecologically and potentially commercially important.  

Kjetil Gjeitsund Thorvaldsen has conducted field studies in the Norwegian Sea and in fjords. He has used a lowered hydroacoustic observation platform to identify and separate mesopelagic fishes from other taxa and a split beam echosounder to provide three dimensional locations of the targets to track and quantify the behaviour of the pearlside and its prey, the copepod Calanus finmarchicus

This PhD project is part of the EU H2020 projects MEESO and PANDORA. Learn more on meeso.org and pandora-h2020.eu.

About the PhD defence

Kjetil Gjeitsund Thorvaldsen will defend his PhD thesis "Population ecology of mesopelagic fishes" on Friday 18 November 2022, 13:00 on Zoom (see below) and at DTU, Asmussens Allé, 2800 Kgs. Lyngby, building 303A, auditorium 41.


  • Principal supervisor: Senior Researcher Stefan Neuenfeldt, DTU Aqua
  • Co-supervisor: Professor Patrizio Mariani, DTU Aqua
  • Co-supervisor: Professor J. Rasmus Nielsen, DTU Aqua


  • Professor Torkel Gissel Nielsen, DTU Aqua (chair)
  • Professor Iain Couzin, Department of Collective Behaviour, Max Plank Institute of Animal Behavior, Germany
  • Principal Scientist Nils Olav Handegaard, Institute of Marine Research, Norway

Chairperson at defence

  • Senior Researcher Cornelia Jaspers, DTU Aqua
Learn more

A copy of the thesis is available for reading at DTU Aqua. Please contact Karin Stubgaard, stub@aqua.dtu.dk 

How to participate in the defence


Everybody is welcome to attend Kjetil Gjeitsund Thorvaldsen's PhD defence at DTU, Asmussens Allé, 2800 Kgs. Lyngby, building 303A, auditorium 41.


The defence can be followed online on Zoom using this link: https://dtudk.zoom.us/j/66848444884

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


Friday 18 November 2022, 13:00-16:00.