Detailed visual seabed survey at drilling site 7218/11-1

It is clear that Cochraneadopted a "biologicallyintuitive" approach,the reasoningbeing that since
spongesare round or irregularlyshaped,evenat maximalcoverage,there still will be someareasof
sedimentvisible.Duringmobilisationfor the current surveyat Darwin,to two different Akvaplan-niva
observers,Palerudand Hansenconductedthe survey. In the interveningtime betweensurveys,we
were ableto givethe matter considerablygreaterattention. Duringefforts to standardiserecording
strategies, they adopted a more precise "mathematical" approach. A guidance scheme was
developed,andthe precise%coverof spongerelativeto sedimentwasrecordedandprinted out asa
template. It is clear that the schemeabovegivesfar higher % cover figures than the mathematical
approach.
Figure18. Mathematicallycalculated%coverschemeadoptedby Palerudand Hansenduringthe presentvisual
surveyat Darwin,August,2012.
Bell& Smith(2004)adopteda mathematicalcoveragesystemto quantify spongesfrom still images
of sponges. Theyprojectedthe photographsonto a grid of 400dots,andcountedthe numberof dots
which landed on a sponge,and from this calculateda % cover figure. Thisis compatiblewith the
current approach, but in-situ visual surveyspresent the challengethat the operators must make
continual, qualitative decisionson estimated % cover. There is a clear potential optical illusion –
actual23%coverasshownaboveintuitively seemsdenser.Trainingis required to ensureconsistent
recordings.
Whateverthe classificationmethod used,it alsois veryclearthat we facean urgentchallenge– how
to classifyspongedensitieswith correctmeaning?Thephrase5 %covertendsto provokereactions
of – "that's not much", whereasat the samedensity,a trawl might pick up more than one tonne of
spongesin a singlehaul. If another observeris more 'intuitively' biased,that samedensitymight be
reportedat higherclassifications.
We recommenda standardisationof approachesbetweenoperatorsat leastin Norway,to makesure
we observeandreport in a similarmanner.Fromthis starting-point, we needfurther studyto be able
to translate%coverinto vulnerabilityandprotectionstatusat an ecosystemscale.
Forthe currentassessment,we placeemphasison the following:
Visualimagesof spongeassemblages, to illustratethe variousabundancecategories;
Holistic and integrated assessmentof the impacts of drilling mud/cuttings on the
environment
o on the sea floor, but also other impacts of the related pressuressuch as energy
consumption(emissionsto air);
o interpretation of environmentalcostsandbenefitsof variousdischargescenarios.
Assessmentof overallvulnerabilityandsensitivity– canwe answerthe necessary questions?
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