Collecting bags used to quantify trawl disturbance of aquatic organisms

The environmental impact of demersal trawling activities has been documented by collecting samples before and after the fishing process or by comparing areas of different fishing intensity. These investigations are expensive and labor demanding and are inappropriate for documenting effects of smaller technological changes aimed at reducing trawling impact on commercial fishing grounds.

DTU Aqua has developed and tested small-meshed collecting bags designed to retain juvenile fish and benthic megafauna. See video at the top of this page.

The collecting bags are mounted in the wings and body of the trawl. The catches in the collection bags provide information of the species and size of organisms that encounter the trawl, but are not retained by it. Such catch represents an unaccounted trawl disturbance of the organisms associated with the seabed.

In combination with knowledge on behavior and habitat utilization of these organisms, their presence can be used to indicate differences in the mechanical effect, such as penetration depth of the ground gear of different trawl designs.

Collecting bags are also mounted to the different sections of the trawl in pairs, one to collect the organisms that pass through the trawl meshes and one to collect the population. Standard selectivity analyses thereby allow us to estimate the size-selection of the caught organisms in different parts of the trawl.

Application of the collecting bags

The collecting bags are easy to handle and in June 2014 they were tested in full scale on board a commercial trawler (see band of pictures below).

  • The paired collecting bag system allows quantification of the effect of small changes in the gear thus facilitating the development of low impact gears.
  • The paired design of the collecting bags allows use of traditional selectivity analysis of the collected species in different parts of the trawl.
  • The collecting bag system can be used for large scale cheap benthic data collecting e.g. in existing observer programs.
  • The collecting bag system catches several species of juvenile commercial fish and crustaceans. Such catch information can be used for modeling e.g. full gear selectivity in trawls or length based behavior in relation to different parts of a trawl.

Trawl

Catch

Catch

Contact

Ludvig Ahm Krag
Senior Researcher
DTU Aqua
+45 35 88 32 06
Rikke Frandsen
Member of Technical and Aministrative Staff
DTU Aqua
+45 35 88 32 41
Grete E. Dinesen
Senior Consultant
DTU Aqua
+45 35 88 33 59

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