density were also calculated over the East Anglia THREE site and the East Anglia THREE site plus buffer independently.
180. Appendix 12.2 provides all of the estimates of abundance and density, however, in the interest of taking a precautionary approach to the assessment, only the approach to analysis which provides the highest estimates of harbour porpoise density are summarised below. These estimates will be taken forward in the impact assessment, as agreed with Natural England during consultation (Table 12.1).
181. The highest mean estimates of relative density were generated from the East Anglia THREE site plus buffer using surface counts corrected for availability. Mean relative densities of animals within the East Anglia THREE site plus buffer across the full 24 month survey period were 0.187 individuals per km2.
182. For a large number of above and below sightings during the surveys (44% within the East Anglia THREE site, and 47% within the East Anglia THREE site plus buffer) it was not possible to identify the marine mammals to species level when analysing aerial images (Appendix 12.2).
183. Survey data suggest that harbour porpoise are the most commonly occurring species of marine mammal, therefore all sightings classified as ‘Unidentified dolphin / porpoise’ have been assumed to be harbour porpoise, and used to generate a maximum density for harbour porpoise (Appendix 12.2).
184. The East Anglia THREE aerial surveys show sightings of unidentified dolphin and unidentified porpoise occur across the East Anglia THREE site plus buffer during both survey years (Figure A6.3 and Figure A6.4 Appendix 12.2).
185. Within the East Anglia THREE site plus buffer the greatest estimate of mean density using the combined harbour porpoise and unidentified individuals was generated using surface counts corrected for availability. Mean relative densities of animals within the East Anglia THREE site plus buffer across the full 24 month survey period were 0.284 individuals per km2.
186. Under the assumption that any unidentified dolphin or porpoise were all harbour porpoise, this grouping is considered to represent a more precautionary abundance estimate (i.e. worst case scenario) for harbour porpoise. The density of individuals within the East Anglia THREE site plus buffer for the combined species group is also more comparable to the 2005 SCANS II survey (density estimate of 0.331 (CV 0.38) for survey block B), than the harbour porpoise only density estimate.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 56
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