Detailed visual seabed survey at drilling site 7218/11-1
Detailed visual seabed survey at drilling site 7218/11-1
Detailed visual seabed survey at drilling site 7218/11-1
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Figure16. Imagefrom Salina,2010,classifiedashavinga high densityof sponges.Thegeneralseafloorpicture<br />
issimilaralsofor both DarwinandJuksa.<br />
When comparingimages,it is clear th<strong>at</strong> scaleplaysa very large part in how rel<strong>at</strong>ive abundanceis<br />
perceived(comparelower two imagesin Figure15 with Figure16). Subjectively,the round Geodia<br />
(?barretti) spongesare estim<strong>at</strong>ed to have a maximum diameter of around 10 cm, but without<br />
consistentlaserscale,we cannotsayfor certain.If the imageabovehadbeentaken<strong>at</strong> a lower zoom<br />
(or higher flight distance),it might havebeen difficult to distinguishbetween "regular abundance"<br />
and"high abundance".<br />
Anothersourceof biasis th<strong>at</strong> there hasbeenno <strong>at</strong>tempt to comparedensitiesreportedby trawl (see<br />
Figure7) with imagesfrom the south-westernpart of the BarentsSea. Intuitively, the phrase'high<br />
density' conjuresimagesof tonnes of sponges(capturedalong an entire trawl route), whereasthe<br />
reality in termsof individualcoloniesper m 2 (or km 2 ) is quite different.<br />
The baseline<strong>survey</strong>scarried out <strong>at</strong> the three fields were not intended to provide a quantit<strong>at</strong>ive<br />
assessmentof sponges,andthereforethe abundancescalesare only of useto comparebetweenthe<br />
five planned <strong>drilling</strong> <strong>site</strong>s documented. Subjectiveabundancescalesalso are notoriously userspecific,and<br />
so it also is not guaranteedth<strong>at</strong> another user would have classedthese imagesin<br />
exactlythe sameway(seealsoour methodologicalexperiencesoutlinedin Section4.2).<br />
4.2 Current approach es – assessing coverage<br />
DuringJuly,2012,CochraneandGeraudiefrom Akvaplan-niva conductedsimilar<strong>site</strong> assessments<strong>at</strong><br />
the nearbyfields Salina(PL533) and Juksa(PL490). Theneed for thesemissionsarose<strong>at</strong> very short<br />
notice,andwe <strong>at</strong>tempted to devisea methodologybasedon practicalpossibility,but asobjective as<br />
possible, and with reference to the intern<strong>at</strong>ional Europeanstandard for <strong>visual</strong> assessmentsof<br />
biodiversity(CENFprEN16260). Coverageof organismsis recommendedto be recordedin terms of<br />
numbersof individuals/coloniesper m 2 , or %coverfor encrustingorganisms(Table3).<br />
Akvaplan-niva AS, 9296 Tromsø<br />
22 www.akvaplan.niva.no
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