One plus one is three: trawling differences from multi-beam imagery
Depestele Jochen12, Koen Degrendele3, Hans Polet1, Mare Roche 3, lom a Teal4, Bart Vanelslander1
and Adriaan R ijnsdorp4
1 Institute o f A g ricu ltu ra l and Fisheries Research (ILVO)
A nkerstraat 1, 8400 Oostende, Belgium
E-mail: iochen.depestele@ ilvo.vlaanderen.be
2 Marine Biology, Ghent University, Krijgslaan 2 8 1 -S8, B-9000 Ghent, Belgium
3 Federal Public Service Economy, Energy - C ontinental Shelf, Simon Bolivarlaan 30, WTCIII,
1000 Brussel, Belgium
4 Institute fo r Marine Resources and Ecosystem Studies, Wageningen IMARES, PO Box 68, 1 970 AB,
Ijm uiden, the Netherlands
Beam traw ling causes physical d isru p tio n o f the seafloor m ainly due to the effect o f the tic k le r
chains th a t penetrate into the seabed. A b o u t 40 Dutch beam traw lers have recently replaced tic k le r
chains by electrodes as alternative stim ulatio n fo r catching fla tfis h . This g reatly reduced fuel costs
and presum ably reduces benthic impacts. These newly developed fish in g gears (pulse traw ls) were
studied in the EU-FP7 project ‘ Benthis’ using a Before-After C ontrol-Im pact (BACI) setup. The
penetration depth o f the com m ercial ‘ D elm eco’ pulse traw l was investigated and com pared to the
penetration depth o f a conventional 4m com m ercial tic k le r chain beam traw l. Here we focused
specifically o nto the physical alteration to the seabed, illu stra tin g one o f the potentials o f modern
high resolution m ulti-beam echosounder im agery as registered onboard o f the RV Simon Stevin. The
experim ental area was m onitored p rior and a fte r beam traw l disturbance at d iffe re n t tim e intervals
up to 107 hours. A d ditio na lly, the penetration depth o f a single passage o f a tic k le r chain beam
traw l was characterized, as well as m ultiple passages o f a tic k le r chain and pulse traw l. While our
results confirm ed estim ates o f previous studies, subtle differences indicated th a t one beam traw l
disturbance is not the other. Trawl tracks o f m ultiple pulse traw l passages (>3 tim es) reached up to
6cm (median: 1.2 cm) at one day a fte r traw ling , while tic k le r chain traw ling penetrated up to
10.7cm w ith a median o f 2.0cm. In contrast, a single passage o f a tic k le r chain traw l penetrated less
deep (median: 0.9cm , max: 2.9cm). Moreover, u nide ntifie d traw l m arks o f com m ercial beam
traw ling were overall deeper than registered beam traw l disturbances, p ote ntia lly p ointin g at the
use o f yet another type o f com m ercial beam traw l. A lthough the experim ental study area was
relatively shallow (<25m), fading o f traw l marks seemed p rim a rily due to bed and suspended load o f
tid al currents, rather than wave induced currents as wave heights up to 2.5m did not result in faster
fading o f traw l tracks. We therefore conclude th a t the overall significance o f short-term beam
traw ling disturbance is o f considerable im portance in relation to natural disturbance fo r th is study
area, and th a t a d istin ct d iffe re n tia tio n between beam traw l types should be accounted fo r. One plus
one is not ju s t two.
References
w w w.benthis.eu
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