Wind 2012c). Of these sightings 38% were positively identified as harbour porpoise and 53% as ‘small cetacean’ (which could be harbour porpoise or patterned dolphin). It is likely many of those identified ‘small cetaceans’ were harbour porpoise (East Anglia Offshore Wind Limited 2012c).
174. During the 24 months of aerial surveys covering the East Anglia ONE site (November 2009 – October 2011) 181 cetaceans in total were recorded, 130 of which (72%) were positively identified as harbour porpoise, and a further 2.5% identified as either a porpoise or a dolphin.
175. Aerial surveys conducted for the East Anglia THREE site plus buffer commenced in September 2011 and were completed in August 2013 (Appendix 12.2). The survey area consisted of the East Anglia THREE site and a 4km buffer around it.
176. Peak sightings of harbour porpoise were in the autumn months (see Appendix 12.2). Over the 24 month survey period (including sightings at the surface and at depth) a total of 70 harbour porpoise were positively identified within the East Anglia THREE site and 180 harbour porpoise were positively identified within the East Anglia THREE site plus buffer.
177. The East Anglia THREE aerial surveys show harbour porpoise occur across the East Anglia THREE site plus buffer during both survey years (Figure A6.1 and Figure A6.2 in Appendix 12.2).
178. During aerial surveys of marine mammals, a proportion of the population will not be captured by the survey because they will be below the surface of the water. Traditional boat surveys typically only record marine mammals when they surface and therefore they account for availability (i.e. an index of the probability of the animals being at the surface given that they are present in the water column) by applying a correction factor to the counts. High resolution aerial stills capture marine mammals both above and just below the surface. However, there will still be an unknown proportion of the population submerged at depths which are not captured.
179. In order to calculate estimates of abundance and density from the surveys which are as close to absolute values as possible two approaches were taken in the analyses of the aerial survey data; using surface only sightings corrected for availability, and using all sightings (with no correction factor) as detailed in Appendix 12.2. Correction factors applied were based on availability data provided in the latest Joint Cetacean Protocol Phase II report (Paxton et al. 2011). Estimates of abundance and
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 55
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