375. In the East Anglia region eleven grey seals were discovered on the north Norfolk coast in the vicinity of Blakeney Point between October 2009 and March 2010. A total of 24 harbour seal and five unidentified seals (though most likely to have been harbour seal based on their description), were found in the same area between April and September 2010 Thompson et al. 2013). Two unidentified seals with similar injuries had also been reported at Blakeney in March 2009. However, an absence of any reports from East Anglia between the end of 2010 and early 2013 suggests that it was specific boat activity during 2010 that was the most probable cause and that all of the observed mortality may have been due to a small number of vessels.
376. Throughout the UK, both harbour and grey seal have been found with these characteristic wounds, but perceived risk is higher in juvenile grey seal and adult female harbour seal (Thompson et al. 2013; SNCAs 2012). The greatest numbers of recorded affected carcasses have been found in relatively close proximity to haul out sites.
377. There is a large amount of uncertainty around the mechanism for impact, which is part of an ongoing Marine Scotland funded research project at SMRU (Thompson et al. 2013). However, two possible mechanisms have been proposed 1) attraction to concentrations of food associated with the vessel or, 2) an inappropriate response to an acoustic signal from the motor/ship/propeller.
378. Given the available evidence, sensitivity of harbour and grey seal to this impact is assessed as high.
12.6.1.6.2Magnitude
379. There is a large amount of uncertainty as to the potential magnitude of effect. However, the majority of vessel activity during construction will be in the offshore site; an area of extremely low density for both species. The distance between the site and the nearest harbour or grey seal breeding colony is greater than 30nm (~55km), and therefore both seal species are considered to be at low risk (SNCAs 2012).
380. The impact will be spatially limited, although could result in permanent loss of individuals from the population. Despite the extremely low density of both species of seal in the East Anglia THREE site increased vessel activity may occur closer to haul out sites, but this will be largely dependent on the choice of port during construction.
381. Based on the densities of individuals in the East Anglia THREE site and offshore cable corridor, the magnitude of effect is likely to be negligible. In the case of harbour seal
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 90
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