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

Tickler chain vs Pulse trawling

3 OBJECTIVES Penetration depth

- Field trial (1) - Model (2)

Sediment resuspension: field (3)

intro

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)

Before – After assessment

4

(1) M&M

ICES Symposium

Fishing

Tickler chain (1.4x)

Pulse trawl (2.4x)

Tickler chain (1x)

Before – After assessment in practice...

5

(1) M&M

Tickler chain (1x)

Tickler chain (1.4x)

Pulse trawl (2.4x)

Multi-Beam Fishing

Before Fishing (1) M&M

ICES Symposium 6

Fishing (1) M&M

ICES Symposium 7

Tickler chain (1.4x)

Pulse trawl (2.4x)

Tickler chain

(1x)

After Fishing

Identification of trawl tracks

(1) M&M

8

Measurements of depths

Seabed alteration: differences of depth in and outside the track

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)

<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)

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

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

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

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

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

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

ICES Symposium 17

Trawl shoe (2)

~ 0.5cm ~ 3cm

Discrepancy with MBES?

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!

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

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

ICES Symposium 21

Particle size and concentration (3) Results

ICES Symposium 22

Particle size and concentration (3) Results

Tickler chain beam trawl

ICES Symposium 23

Particle size and concentration (3) Results

Pulse beam trawl Not significant

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

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

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