Degrendele, K.

Depestele, J.

Esmaeili, M.

Ivanovic, A.

Kröger, S.

O’Neill, F.G.

Parker, R.

Polet, H.

Rijnsdorp, A.D.

Roche, M.

Summerbell, K.

Teal, L.R.

Vanelslander, B.

Open Marine Archive

102 
Poster presentation Competition poster 
Seabed impacts after mechanical disturbance by beam and pulse trawls 
in two North Sea experiments  
Depestele Jochen1, Degrendele Koen2, Esmaeili Moosa3, Ivanović Ana3, Kröger Silke4, O’Neill 
Finbarr5, Parker Ruth4, Polet Hans1, Roche Marc2, Summerbell Keith6, Teal Lorna7, Vanelslander 
Bart1 and Rijnsdorp Adriaan7 
1  Animal Sciences Unit, Fisheries and Aquatic Production, ILVO, Ankerstraat 1 8400 Oostende, 
Belgium  
 E-mail: jochen.depestele@ilvo.vlaanderen.be 
2 FPS Economy, Belgian Continental Shelf,  
3  School of Engineering, Fraser Noble Building, University of Aberdeen, Aberdeen, AB24 3UE, 
United Kingdom  
4  Centre for Environment, Fisheries and Aquaculture Science (Cefas), Lowestoft Laboratory, 
Pakefield Road, Lowestoft, Suffolk NR33 0HT, United Kingdom  
5  Technical University of Denmark, National Institute of Aquatic Resources (DTU – AQUA), 
Willemoesvej 2, 9850 Hirtshals, Denmark 
6  Marine Scotland-Science, PO Box 101, 375 Victoria Road, Aberdeen AB11 9DB, United Kingdom  
7  Wageningen Marine Research, Wageningen UR, PO Box 68, 1970 AB, IJmuiden, The Netherlands 
 
Flatfish live in, at or near the seabed. Commercial bottom trawl fisheries that target these fish 
species require close contact with the sediment to increase their encounter and to avoid their 
escape from the trawl. Close seabed contact is insufficient in itself to result in high flatfish 
catches. The flatfish-directed trawlers further stimulate flatfish by tickling the flatfish using 
tickler chains (beam trawls) or by using electricity (pulse trawls). The catch stimuli are distinctly 
different and so we hypothesized that the expected mechanical impacts and abrasion of beam 
trawls is distinctly different from pulse trawls. Two Before-After Control-Impact (BACI) 
experiments were conducted in sandy sediments to compare the mechanical disturbance to the 
seabed for both gears. The results showed, indeed, that beam trawls penetrated deeper into the 
sediment than pulse trawls (4.1 cm vs 1.8 cm). The penetration was estimated as a combination 
of the resuspended sediment and the depth of the disturbed sediment layer assessed from 
Sediment Profile Imagery (SPI). Sediment resuspension was similar for both gears, as it was 
primarily caused by hydrodynamic drag of towing the trawl net through the water rather than 
the different catch stimuli. We assigned the difference in penetration entirely to the gear 
elements that are in close contact with the seabed, i.e. the tickler chains and the electrodes. The 
increased penetration depth of beam trawls further caused other differences in seabed impacts. 
The sediment grain sizes were reworked into deeper layers (up to 4 cm depth) by beam trawls 
than by pulse trawls (~1 cm). The bathymetrical profile was consequently flattened to a greater 
extent, trawl tracks were deepened more (beam trawls: 1.5 cm vs pulse trawls: 0.7 cm) and the 
oxygenated layer, reflecting benthic life, was disturbed deeper and did not recover with the 
duration of the experiments (~48 h). We conclude that these experiments have quantified the 
more profound mechanical effects of commercially deployed beam and pulse trawls in a coastal 
and in an offshore fishing ground. The implication of these findings suggest a decreased seabed 
effect of pulse trawling as opposed to beam trawling, but its interpretation requires caution 
when extrapolated to fleet level. Extrapolation requires that the variation in sediment and gear 
characteristics are compared to the sediment and gear types used in our experiment, and that 
this variation is accounted for when assessing seabed impacts at ecosystem (or fleet) level. 
 
Keywords: Beam trawl; Biogeochemistry; Habitat impacts; Particle size distribution; Penetration 
depth; Pulse trawl; Seafloor integrity; Sediment resuspension 
  


Seabed impacts after mechanical disturbance by beam and pulse trawls in two North Sea experiments

http://www.vliz.be/imisdocs/publications/324875.pdf

Seabed impacts after mechanical disturbance by beam and pulse trawls in two North Sea experiments

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