Common eiders. Photo DTU Aqua.

PhD defence on bycatch of seabirds in Danish gillnet fisheries

Monday 08 Jun 20


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


Gildas Glemarec
Research Assistant
DTU Aqua

On 11 June 2020 Gildas Glemarec, DTU Aqua will defend his PhD thesis. The defence can be watched online.

Worldwide, incidental captures (bycatch) in gillnets are responsible for the drowning of hundreds of thousands of seabirds each year. Yet, little is known about the extent of this problem in Danish waters.

During his PhD project, Gildas Glemarec, DTU Aqua, has worked on increasing the comprehension of bycatch of seabirds in Danish commercial gillnet fisheries. Using electronic monitoring systems with closed-circuit television cameras, Gildas Glemarec analysed the entire fishing activity of 16 Danish commercial gillnetters, with data spanning from 2010 to 2018, and he identified more than 1400 bycaught seabirds. His work allows determining the spatio-temporal variability of seabird bycatch in gillnets and identifying the main environmental and operational drivers associated to it.

Results from part of the study conducted in the Western Baltic Sea demonstrate that electronic monitoring systems are useful for assessing seabird bycatch in small-scale gillnet fisheries. Of the 700 birds identified as bycatch in this part of the study, 90 percent belonged to three species only, common eider, great cormorant and common guillemot. Data analyses show an overwhelming influence of rare mass bycatch events on mean annual seabird bycatch rates, suggesting that the whole fishing activity of electronic monitoring vesselsand not only random samplesshould be processed to estimate bycatch rates accurately.

Additionally, statistical treatment of a larger portion of data from electronic monitoring demonstrate a strong effect of net length, duration of soaking, depth and distance to shore on seabird bycatch rates. Furthermore, scaling up seabird bycatch rate estimates to the entire commercial fleet shows concerning mortality levels, at least for the most impacted species, the common eider.

Gildas Glemarec has also tested the effect of light and sound as underwater bycatch deterrent in real commercial fishing conditions. Despite potentially encouraging results for the light-emitting devices, the effect of these technologies did not prove statistically significant and more work is underway to confirm these findings.

About the defence

Gildas Glemarec will defend his PhD thesis "Bycatch of seabirds in Danish gillnet fisheriesassessing scale and testing mitigation", Thursday 11 June 2020 at 13.00 CEST, online via Zoom. For the link to join the defence please contact Rikke Hansen,, no later than 10 June at 14:00 CEST.  


  • Principal supervisor: Senior Researcher Finn Larsen, DTU Aqua
  • Co-supervisor: Researcher Lotte Kindt-Larsen, DTU Aqua


  • Senior Researcher Ludvig Ahm Krag, DTU Aqua
  • Researcher Dominik Marchowski, Museum and Institute of Zoology, Polish Academy of Sciences, Warsaw, Poland
  • Senior Lecturer Simon Northridge, School of Biology, Sea Mammal Research Unit, Scottish Oceans Institute, UK

Chairperson at defence

  • Senior Researcher Stefan Neuenfeldt, DTU Aqua

The thesis

A copy of the PhD thesis “Bycatch of seabirds in Danish gillnet fisheries - assessing scale and testing mitigation” may be obtained via e-mail. Please send your request to Rikke Hansen,