A proposed m ethod fo r ... s ig n ific a n tly affected ...

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A proposed m ethod fo r assessing the exte nt o f the seabed
sig nifican tly affected by demersal fishing in the Greater North Sea
Diesing M arkus', David Stephens, and John A ldridge
1 Centre fo r Environm ent, Fisheries and A quaculture Science, Pakefield Road,
Low estoft NRBB OHT, UK
E-mail: m [email protected] cefas.co.uk
The widespread im pact o f b ottom tow ed fish in g gear on benthic species and com m unities has long
been recognised. The responses to a given inten sity o f fish in g disturbance can be influenced by the
e xten t to which these species and com m unities are preconditioned to disturbance by natural
processes, in particular waves and currents. The advent o f vessel m on itoring system and m odels o f
natural disturbance enable h igh-resolution and large-scale com parisons o f fish in g and natural
disturbance. Vessel m on itoring system data were em ployed to estim ate the traw led area per 12km
by 12km grid cell. We then qua ntifie d natural disturbance by estim ating the num ber o f days in a
year the seabed was disturbed by tides and waves. As natural disturbance acts on large spatial
scales, we assumed th a t each natural disturbance event affects whole grid cells. Frequencies could
thus be translated into an area o f impact, allow ing us to compare fish in g w ith natural disturbance.
We show how such com parisons can be used to estim ate the e xten t o f d iffe re n t seabed substrate
types sig n ifica n tly affected by dem ersal fishing. A measure o f the p ro b a b ility th a t fishing
disturbance exceeds natural disturbance provides one m etric fo r identifyin g areas o f sign ifican t
traw ling im pact on seabed habitats and m ig ht be used to measure progress tow ards achieving Good
Environm ental Status fo r sea-floor in te g rity w ith in the co nte xt o f the European U nion’s Marine
Strategy Framework Directive. For more than half the seabed in the English sector o f the Greater
North Sea, the results suggest th a t disturbance a ttrib utab le to dem ersal fish in g exceeds natural
disturbance based on data from the years 2006 to 2008. The imbalance between natural and fishing
disturbance is greatest in m uddy substrates and deep c irca litto ra l habitats.
Lay Summary
A ll human activities have an im pact on the environm ent, and th is is also true fo r fish in g gear th a t is
dragged over the seabed o f our continental shelves. However, it is often d iffic u lt to assess w hether
such an im pact ‘ m atters’ to the environm ent.
The significance o f b ottom -fish ing impacts depends on the nature o f the seabed habitats and the
levels o f natural seabed disturbance caused by waves and currents.
Ecological th e o ry predicts th a t animals living on and in the seabed are adapted to the naturally
occurring levels o f seabed disturbance. Shallow tide-sw ept and wave-im pacted sandy habitats
e x h ib it animal com m unities th a t are well adapted to high rates o f m o rta lity and natural disturbance.
As a consequence, these com m unities show greater resilience to fish in g disturbance as well.
Conversely, deep and stable seabed habitats are often characterised by slow -grow ing, habitatm odifying species fo r which b ottom fish in g can have m ajor and long-term im pacts on biomass and
diversity.
W hilst these relationships are relatively well understood, it remains a challenge to d ire c tly compare
seabed disturbance caused by bottom -tow ed fish in g gear w ith the natural disturbance o f the seabed
as d iffe re n t m etrics are used to measure these.
In a recent study, published in the ICES Journal o f Marine Science, Diesing et al. describe a
m ethodology th a t enables such a com parison o f fish in g and natural disturbance, based on data
from the English part o f the greater North Sea.
The presented results are particula rly relevant as th ey help id e n tify areas where fish in g disturbance
is at a level beyond the range o f natural background variability.
The proposed m ethodology m ig ht also be used to track progress tow ards Good Environm ental
Status o f sea-floor in te g rity (D escriptor 6 o f the European U nion’s Marine Strategy Framework
Directive).
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