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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
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Tickler chain vs Pulse trawling
3 OBJECTIVES Penetration depth
- Field trial (1) - Model (2)
Sediment resuspension: field (3)
intro
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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)
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Before – After assessment
4
(1) M&M
ICES Symposium
Fishing
Tickler chain (1.4x)
Pulse trawl (2.4x)
Tickler chain (1x)
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Before – After assessment in practice...
5
(1) M&M
Tickler chain (1x)
Tickler chain (1.4x)
Pulse trawl (2.4x)
Multi-Beam Fishing
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Before Fishing (1) M&M
ICES Symposium 6
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Fishing (1) M&M
ICES Symposium 7
Tickler chain (1.4x)
Pulse trawl (2.4x)
Tickler chain
(1x)
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After Fishing
Identification of trawl tracks
(1) M&M
8
Measurements of depths
Seabed alteration: differences of depth in and outside the track
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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 Cu
mu
lati
ve p
rob
abili
ty o
f o
ccu
rre
nce
(1) Results
~1cm, no differences between bottom gears (Lindeboom & de Groot, 1998)
<1 – 8cm, modelling, sediment dependent (Paschen et al, 1999)
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<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 Cu
mu
lati
ve p
rob
abili
ty o
f o
ccu
rre
nce
Can we confirm earlier results empirically?
(1) Results
~ 0.9cm Tickler chain (1x)
(after <0.5d, n=31)
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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 Cu
mu
lati
ve p
rob
abili
ty o
f o
ccu
rre
nce
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
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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 Cu
mu
lati
ve p
rob
abili
ty o
f o
ccu
rre
nce
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
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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 Cu
mu
lati
ve p
rob
abili
ty o
f o
ccu
rre
nce
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
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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 Cu
mu
lati
ve p
rob
abili
ty o
f o
ccu
rre
nce
Gear improvement?
(1) Results
Tickler chain (1.4x) (after <2d, n=82)
~ 1.2cm Pulse trawling (after <2.5d, n=246)
~ 2cm
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Fading of trawl tracks?
ICES Symposium 15
Penetration depth (cm)
0 2 4 6 8 10 12
Cu
mu
lati
ve p
rob
abili
ty o
f o
ccu
rre
nce
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
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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
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ICES Symposium 17
Trawl shoe (2)
~ 0.5cm ~ 3cm
Discrepancy with MBES?
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ICES Symposium 18
Tickler chains vs electrodes? (2)
Pe
net
rati
on
de
pth
(m
)
Weight on the seafloor (kg)
~ 1cm
Model only a single passage of a chain
Ongoing . . .
Links to the MB!
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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
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ICES Symposium 20
Equipment (3) M&M
a) Re-suspension Control & Laser Profiler
Sledge
5m Bridle
To vessel
<90m Rope/Warp
~40m
Float
Line
Sledge
5m Bridle
~40m
Float
Line
To vessel
20m tow line
20m extensions
added here
b) 25, 45, 65m Re-suspension
Safety Line
a) Re-suspension Control & Laser Profiler
SledgeSledge
5m Bridle
To vessel
<90m Rope/Warp
~40m
Float
Line
SledgeSledge
5m Bridle
~40m
Float
Line
To vessel
20m tow line
20m extensions
added here
b) 25, 45, 65m Re-suspension
Safety Line
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ICES Symposium 21
Particle size and concentration (3) Results
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ICES Symposium 22
Particle size and concentration (3) Results
Tickler chain beam trawl
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ICES Symposium 23
Particle size and concentration (3) Results
Pulse beam trawl Not significant
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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
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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
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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
[email protected]
Thanks!
Physical impact of beam trawling revisited Seabed disturbance and sediment re-suspension