PhD defence by Benedikt Merk

On Friday, 9 October 2026, at 10.00, Benedikt Merk from DTU Aqua will defend his PhD thesis, ’Lake Restoration and Fish Behavior’.

You can follow the defence online on Teams via this link   or in person at DTU Aqua, Vejlsøvej 39, 8600 Silkeborg.

In his PhD project, Benedikt Merk studied whether a better understanding of fish behaviour and physiology can improve the efficiency of lake restoration and how restoration can affect the behaviour of the remaining fish.

Many European lakes are turbid and algae-dominated due to excessive nutrient inputs from land. This eutrophication reduces biodiversity and the lakes’ value for drinking water, fisheries and recreation.

One lake restoration tool is biomanipulation, which involves removing fish such as roach that feed on zooplankton. With fewer roach present, zooplankton become more abundant and can graze the algae down, helping to keep the water clear. But removing thousands of fish is costly and must often be repeated regularly, so gains in efficiency matter. In addition, the effects of removal on the grouping behaviour and general condition of the remaining fish needs to be studied.

Benedikt Merk’s investigations involved tagging roach and pike in a Danish lake with acoustic transmitters and continuously tracking their movements over several years. Some pike also carried loggers recording their heart rate, providing a rare window into the physiology of a wild predator.

The results show that knowledge of fish behaviour can improve lake restoration efficency. For example, in the lake that was studied, roach aggregated on autumn mornings, enabling efficient removal. In contrast, fish removals should be avoided during warm, oxygen-poor periods to protect stressed pike to maintain their potential natural control of roach populations.

The PhD project was part of the research project ‘Sustainable Lake Stewardship’ funded by Poul Due Jensen Foundation.

Examiners

• Senior Researcher Jakob Hemmer Hansen, DTU Aqua (chair)
• Professor Thrond Oddvar Haugen, Norwegian University of Life Sciences, Norway
• Senior Scientist Mikko Johannes Olin, Natural Ressources Institute, Finland

Chair of defence

• Professor Dorte Bekkevold, DTU Aqua

Supervisors

• Principal Supervisor: Associate Professor Christian Skov, DTU Aqua
• Co-supervisor: Senior Researcher Henrik Baktoft, DTU Aqua
• Co-supervisor: Senior Researcher Martin Hage Larsen, DTU Aqua
• Co-supervisor: Professor Anders Koed, DTU Aqua

Learn more

Read the popular science summary of Benedikt Merk’s PhD thesis

A copy of the thesis is available for reading at DTU Aqua. Please contact PhD Coordinator Susan Zumbach Johannesen, szjo@aqua.dtu.dk