The ICES Symposium “Effects of fishing on benthic fauna, habitat and ecosystem function” took place in Tromsø, Norway from 16-19th June 2014. Abstract: Beam trawling causes physical disruption to the seafloor through physical contact of the gear components on the sediment and the resuspension of sediment into the water column in the turbulent wake of the gear. Recently Dutch beam trawlers have replaced tickler chains by electrodes as alternative stimulation for catching flatfish. It is claimed that benthic impacts are reduced. Here we report on trials in a medium sand fishing ground to compare the physical impact of a conventional 4m commercial tickler chain beam trawl with that of the new commercial “Delmeco” pulse trawl. We use a Kongsberg EM2040 multibeam echo sounder (MBES) to measure the extent to which the beam trawls modify the topography of the substrate and a particle size analyser (LISST 100X) to measure the concentration and particle size distribution of the sediment mobilized into the water column. MBES measurements reveal that the disturbed sediment in the trawl track was on average located at a centimetre deeper after trawling of the conventional beam trawl than after pulse trawling. Particle size distributions of the sediment plumes were measured at 25m, 45m and 65m behind the gear and did not reveal any differences in concentrations between the two trawls. Whereas the empirical data serve comparative purposes, their lack of predictive capacity limits extrapolation to fleet level. Finite element (FE) models have shown to overcome this for otter trawls by predicting the penetration depth and sediment displacement associated with each gear component in different sediment types. In this study, FE models were developed for the conventional tickler chain beam trawl and the pulse trawl. Predictions were validated by results obtained during sea trials. As such, this study attempts to provide the basis for future predictions of physical impacts of beam trawling and its technical advances on a larger spatial scale.

1. Physical impact of
beam trawling revisited
Seabed disturbance and sediment re-suspension
“Effects of fishing on benthic fauna, habitat and ecosystem function”
ICES Symposium, Tromsø, Norway
16-19 June 2014
J. Depestele, A. Ivanović, K. Degrendele, M. Esmaeili, H. Polet, M. Roche,
K. Summerbell, L. Teal, B. Vanelslander and F. G. O’Neill

2. Tickler chain vs Pulse trawling
3 OBJECTIVES
 Penetration depth
- Field trial (1)
- Model (2)
 Sediment resuspension: field (3)
intro

3. Field trial
(1) M&M
 southern North Sea
 No beam trawlers >300HP
- Shrimp beam trawling
- Eurocutter (<300HP beam trawlers)
 15 – 22m depth
 fine / muddy sand
(EUNIS 2007-11)

4. Before – After assessment
4
(1) M&M
ICES Symposium
Fishing
Tickler chain (1.4x)
Pulse trawl (2.4x)
Tickler chain (1x)

5. Before – After assessment in practice...
5
(1) M&M
Tickler chain (1x)
Tickler chain (1.4x)
Pulse trawl (2.4x)
Multi-Beam
Fishing

6. Before Fishing
(1) M&M
ICES Symposium 6

7. Fishing
(1) M&M
ICES Symposium 7
Tickler chain (1.4x)
Pulse trawl (2.4x)
Ticklerchain(1x)

8. After Fishing
Identification
of trawl
tracks
(1) M&M
8
Measurements
of depths
Seabed alteration: differences of
depth in and outside the track

9. Penetration depth
ICES Symposium 9
Penetration depth (cm)
0 2 4 6 8 10 12
1.0
0.8
0.6
0.4
0.2
0.0
Cumulativeprobabilityofoccurrence(1) Results
~1cm, no differences between
bottom gears (Lindeboom & de Groot, 1998)
<1 – 8cm, modelling, sediment
dependent (Paschen et al, 1999)

10. <1 – 8cm
Penetration depth
ICES Symposium 10
Penetration depth (cm)
0 2 4 6 8 10 12
1.0
0.8
0.6
0.4
0.2
0.0
Cumulativeprobabilityofoccurrence
Can we confirm earlier
results empirically?
(1) Results
~ 0.9cm
Tickler chain (1x)
(after <0.5d, n=31)

11. One vs multiple passages?
ICES Symposium 11
Penetration depth (cm)
0 2 4 6 8 10 12
1.0
0.8
0.6
0.4
0.2
0.0
Cumulativeprobabilityofoccurrence
Tickler chain (1.4x)
(after <0.5d, n=112)
Fishing intensity?
(1) Results
Tickler chain (1x)
(after <0.5d, n=31)
~ 2cm
~ 0.9cm
<1 – 8cm

12. Fading of trawl tracks?
ICES Symposium 12
Penetration depth (cm)
0 2 4 6 8 10 12
1.0
0.8
0.6
0.4
0.2
0.0
Cumulativeprobabilityofoccurrence
Tickler chain (1.4x)
(after <0.5d, n=112)
Fading of trawl tracks?
(1) Results
Tickler chain (1.4x)
(after <2d, n=82)
~ 2cm
~ 2cm

13. Fading of trawl tracks?
ICES Symposium 13
Penetration depth (cm)
0 2 4 6 8 10 12
1.0
0.8
0.6
0.4
0.2
0.0
Cumulativeprobabilityofoccurrence
Tickler chain (1.4x)
(after <0.5d, n=112)
Fading of trawl tracks?
(1) Results
Tickler chain (1.4x)
(after <2d, n=82)
~ 2cm
~ 2cm
Marks faded <0.5d (Fonteyne, 2000)
Fine-medium sand, Scheveningen area
RoxAnn Survey

14. Gear improvement?
ICES Symposium 14
Penetration depth (cm)
0 2 4 6 8 10 12
1.0
0.8
0.6
0.4
0.2
0.0
Cumulativeprobabilityofoccurrence
Gear improvement?
(1) Results
Tickler chain (1.4x)
(after <2d, n=82)
~ 1.2cmPulse trawling (after <2.5d, n=246)
~ 2cm

15. Fading of trawl tracks?
ICES Symposium 15
Penetration depth (cm)
0 2 4 6 8 10 12
Cumulativeprobabilityofoccurrence
Pulse trawling (after <4.5d, n=236)
Fading of trawl tracks?
(1) Results
Pulse trawling
(after <2.5d, n=246)
~ 1.2cm
~ 1.0cm
1.0
0.8
0.6
0.4
0.2
0.0

16. ICES Symposium 16
‘Generic’ trawl tracks (2)
Traits-based gear approach
Which gear element causes
differences in trawl tracks from MB?
- Preliminary work . . .
- Based on FE model

17. ICES Symposium 17
Trawl shoe
(2)
~ 0.5cm ~ 3cm
 Discrepancy with MBES?

18. ICES Symposium 18
Tickler chains vs electrodes?
(2)
Penetrationdepth(m)
Weight on the seafloor (kg)
~ 1cm
 Model only a single
passage of a chain
 Ongoing . . .
Links to the MB!

19. ICES Symposium 19
Sediment re-suspension (3)
 What happens to the sediment that is disturbed?
 Re-suspension
 Also depending on turbulence...
 Proxy for nutrient displacement, turbidity changes...
(c) Digital Globe

20. ICES Symposium 20
Equipment
(3) M&M
a) Re-suspension Control & Laser Profiler
5m Bridle<90m Rope/Warp
Sledge
5m Bridle
~40m
FloatLine
To vessel
20m tow line
20m extensions
added here
b) 25, 45, 65m Re-suspension
Safety Line
a) Re-suspension Control & Laser Profiler
5m Bridle<90m Rope/Warp
SledgeSledge
5m Bridle
~40m
FloatLine
To vessel
20m tow line
20m extensions
added here
b) 25, 45, 65m Re-suspension
Safety Line

21. ICES Symposium 21
Particle size and concentration
(3) Results

22. ICES Symposium 22
Particle size and concentration
(3) Results
Tickler chain beam trawl

23. ICES Symposium 23
Particle size and concentration
(3) Results
Pulse beam trawl Not significant

24. ICES Symposium 24
Conclusion field trial
No significant difference in
sediment resuspension
between tickler chain and
pulse trawl
Tickler chains penetrate
deeper than electrodes
Trawl tracks remain
at least 4days
Penetration depth:
1-2cm
Modelling is
ongoing

25. ICES Symposium 25
Future perspectives . . .
 Trial 1: high natural disturbance
- Bottom shear stress: up to 1.5N/m²
- Physical impact!
- Biological impact?
 Trial 2: low natural disturbance
- Different sediment
- 12m beam trawls
 Question: How to move from empirical BACI-study to predictions?
- Based on literature + trial 1 + trial 2
- Traits-based approach of beam trawls modelling

26. J. Depestele1,2, A. Ivanović3, K. Degrendele4, M. Esmaeili3, H. Polet1, M. Roche4, K. Summerbell5, L. Teal6, B.
Vanelslander1 and F. G. O’Neill5
1 2 3 4 5 6
More @
www.benthis.eu
Jochen.Depestele@ilvo.vlaanderen.be
Thanks!
Physical impact of beam trawling revisited
Seabed disturbance and sediment re-suspension