Net. Foto: Esther Savina

PhD defence about Danish bottom set nets fisheries

Wednesday 31 May 17


Esther Savina
DTU Aqua
+45 35 88 32 02

Time & Place

Thursday, 8 June 2017, 13:00

Nordsøen Forskerpark
Willemoesvej 2
9850 Hirtshals

The thesis

A copy of the PhD thesis is available for reading at the Institute. Contact PhD Secretary Rikke Hansen,

PhD student Esther Savina defends her thesis about ecosystem effects of bottom set nets on 8 June 2017

Gill- and trammel nets represent about 80% of the Danish fleet in number of vessels. These nets have the advantage of low energy consumption and good size selectivity, but little is known about the ecosystem effects of bottom set nets.

In her PhD project, Esther Savina, DTU Aqua has focused on methodological development, catch pattern and habitat effects of bottom set nets and she has identified, adapted, tested and used new methods to properly assess the ecosystem effects.

Esther Savina defends her PhD thesis "Gear technical contributions to an Ecosystem Approach in the Danish bottom set nets Fisheries" on 8 June 2017 at 13.00 at Nordsøen Forskerpark in Hirtshals.


  • Principal supervisor: Senior Researcher Ludvig Ahm Krag,DTU Aqua
  • Co supervisors: Senior Researcher Finn Larsen, DTU Aqua


  • Senior Researcher, PhD Ole Ritzau Eigaard, DTU Aqua
  • Doctor F.G. O’Neill, Marine Scotland Science
  • Doctor Marie-Joëlle Rochet, Ifremer

Chairperson at defence

  • Senior Researcher Ivar Lund,DTU Aqua


Summary of the thesis

The European Union is implementing a sustainable fisheries management framework called the Ecosystem Approach to Fisheries, with the main basis provided in the objectives of the Marine Strategy Framework Directive and the Common Fishery Policy (landing obligation). As fishing can affect other components and not just targeted species, with for example physical damage to habitats or discarding of non-target species, the ecosystem as a whole must be considered.

Although the fleet has reduced since the mid-1990s, gill- and trammel nets still represent about 80% of the Danish fleet in number of vessels. Gill- and trammel nets have the advantage of low energy consumption and good size selectivity. However, there is limited knowledge about the ecosystem effects of bottom set nets. Focus was given to methodological development, catch pattern and habitat effects. Regarding catch pattern, one can intend to minimize the catch that is unwanted, or to maximize the part of the catch that is wanted, e.g., by adjusting the fishing tactic or by improving catch quality of the target species.

The limited information on passive gears is partly due to historical focus on active gears, but also because data collection and analysis calls for the development of appropriate innovative assessment methodologies to properly assess the new type of information which has to be gathered as part of an Ecosystem Approach to Fisheries. A stereo imaging method to assess in-situ the dynamic behavior of passive gears was identified, adapted, tested and used.

Comparing bottom set nets fishing operations can be challenging as the measure of fishing effort depends on various factors such as the combination of netting characteristics, net length, or soak time. Statistical methods that have recently been developed were identified and used for estimating the relative catch efficiency between two different designs of a passive fishing gear or to standardize data to a wide range of effort variables by including the landed portion of the fishing operation with the use of discard ratios.

Gear technologists can play a key role in searching for win-win solutions so that fishing can continue in an ecologically sustainable manner, i.e., avoiding unwanted catch and habitat damage.  The selection properties of gillnets may be improved by changing the gear characteristics, e.g., mesh size or netting material, but in many cases the fisher’s operational tactic plays a preponderant role, as new selective technologies involving more complex gear are usually limited in passive fisheries. Gear technological considerations, i.e., gear design and operational tactics, can help to implement an Ecosystem Approach to the Danish bottom set nets fisheries. The effects of gear design, i.e., light and heavy nets, on habitat effects and fisher’s tactic, i.e., soak duration or choice of target species, on catch pattern and quality were explored.