187. In agreement with Natural England, the higher site specific estimate from these combined sightings, based on corrected surface counts for the East Anglia THREE site plus buffer will be used in the impact assessment (0.284 individuals per km2).
12.5.3.1.4 Reference population for assessment
188. The reference population used in the assessment is the NS MU (IAMMWG, 2013) with an estimated abundance of 227,298 (CV 0.13, 95% CI 176,360 – 292,948) based on the Hammond et al. (2013) analysis of the SCANS II data. This reference population has been agreed with Natural England in consultation (Table 12.1).
189. Given the transboundary nature of the IAMMWG management units, the NS MU comprises ICES area IV, VIId and Division IIIa (Skagerrak and north Kattegat). This could be a conservative approach as guidance from Marine Scotland (Northridge 2012) suggests that considering large stock areas for harbour porpoise is appropriate; with open borders existing between the North Sea and the Kattegat, the North Sea and Norwegian Sea and between western Channel and Celtic Shelf/Irish Sea.
12.5.3.1.5Current stressors 190.
Since 2000, the most common sources of mortality of stranded harbour porpoise are by-catch, attack from bottlenose dolphins, starvation and infectious disease (JNCC 2013).
191. Harbour porpoise in the southern North Sea are effectively apex predators as key species that prey on them in other geographies, such as killer whales, are absent. Some studies have shown that bottlenose dolphins will attack and kill harbour porpoise but do not actively prey on them for food (CMS 2012).
192. Harbour porpoise are under threat from anthropogenic pressures, in particular incidental fisheries by-catch and marine water pollution. The principal area of concern for by-catch is the south-western waters of the western English Channel and Celtic Sea; and in the UK there is on-going research on mitigation measures. However, it is likely that, based on assessment including estimated levels of by- catch, the harbour porpoise population in the North Sea has relatively low rates of potential increase (Winship 2009).
12.5.3.2 White-beaked dolphin 12.5.3.2.1Population structure
193. White-beaked dolphin are widespread across the northern European continental shelf. This species is cited as the most abundant cetacean after the harbour
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 57
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