Sea bed

PhD defence about the importance of habitat structure for demersal fish

Monday 02 May 16

Time & place

18 May 2016, 1:00 p.m. 

DTU Aqua
Charlottenlund Castle
Jægersborg Alle 1

On 18 May 2016 Louise Dahl Kristensen will defend her PhD thesis about the importance of habitat structure for the distribution and behaviour of demersal fish

PhD student Louise Dahl Kristensen will defend her thesis "The importance of habitat structure for the distribution and behaviour of demersal fish" Wednesday, 18 May 2016, at 1 p.m. at DTU Aqua in Charlottenlund, Denmark. 


  • Principal supervisor: Dr. Josianne Gatt Støttrup, DTU Aqua
  • Co supervisors: Senior statistical programmer Claus Stenberg, Novo Nordisk and Associate Professor Peter Grønkjær, Aarhus University


  • Senior Research Scientist Niels Gerner Andersen, DTU Aqua (chairman)
  • Associate Professor Kjell Magnus Norderhaug, University of Oslo 
  • Associate Professor Lena Bergstrøm, Swedish University of Agricultural Sciences


  • Professor Henrik Gislason, DTU Aqua

The thesis

A copy of the thesis is available for reading at the institute. Please, contact the secretary at DTU Aqua's PhD school Marian Solrun Probst,


This PhD project investigated the effects of coastal habitat structural characteristics on the biodiversity, abundance, size range and behaviour of fish, whilst maintaining a particular focus on the effect of habitat restoration. Fish distribution and behaviour was measured using a combination of gill net sampling, video recordings and acoustic telemetry.

Some biogenic temperate reefs have declined to commercial extinction, in several countries, due to a host of impacts, including overexploitation, parasites and the loss of hard biogenic substrate. The recovery of these reefs may be slow or, without sufficient hard bottom, even impossible. Bivalves are ecosystem engineers as they modify the benthic environment and influence the health of other organisms. Additionally, biogenic reefs provide ecosystem services such as reducing turbidity and improving water quality which make bivalves ideal organisms for consideration in habitat restoration project. In the present thesis, blue mussel (Mytilus edulis) beds were established costeffectively using crowdsourcing and the help of local volunteer fishermen. A total of 44 tons of blue mussels were produced and established in beds over an area of 121,000 m2. The effect of the artificial mussel beds was most evident on a small scale resulting in an increased biodiversity and a three times higher abundance of small fish on the introduced mussel structures. To our knowledge, this is the first attempt to use established mussel beds for improving fish habitats and the new method is a potentially useful management tool in areas where mussel spat are abundant.

Globally bottom trawling and dredging reduces the complexity of benthic structures by spreading and flattening marine boulder reefs which results in a reduced abundance and biodiversity of marine species. In addition, boulder reefs have been destroyed through targeted extraction of boulders for the construction of piers and jetties with a presumed high loss of biomass and numbers of hard bottom species. As boulder reefs are unable to restore themselves, they depend entirely on habitat restoration. Therefore, a boulder reef was successfully restored with the addition of 100,000 tons Norwegian quarry boulders deployed on approximately 27,400 m2 of seabed. The boulders stabilized the existing reef and reintroduced the cave forming reef structures. The restored boulder reef increased the biodiversity and the abundance of reef associated fish such as Atlantic cod (Gadus morhua), saithe (Pollachius virens) and goldsinny wrasses (Ctenolabrus rupestris). Restoration also increased the proportion of larger individuals present, both within species and across the whole fish assemblage. In addition, the restoration increased the abundance of invertebrates fivefold and the biomass 14-fold. This increase in food availability was also evident in cod stomach contents, where the biomass increased threefold. Cod stomach contents indicated a shift from a diet based on smaller crustaceans towards larger crustaceans and fish. Furthermore, using telemetry and acoustically tagged cod, results showed that a larger fraction of the tagged cod remained in the study area with the restored boulder reef compared to before restoration, and with this an increase in residence time was also observed. This thesis show unique results from the first ever restored boulder reef and the results are thus highly relevant for future management of degraded hard bottom habitats. Our study indicates that boulder reef restoration could be a valuable management tool to improve habitats for temperate fish species.