360. The confidence in the data underpinning the assessment for harbour porpoise is medium. The confidence in the data underpinning the assessment is considered to be high for grey seal, and medium for harbour seal.
12.6.1.5 Impact 4: Vessel interactions – ship strikes 361. During the construction of East Anglia THREE there will be an increase in vessel traffic in the region. Vessel use of the site, and potential increases have been summarised in relation to marine mammals in section 12.6.1.2 using data presented in Chapter 15 Shipping and Navigation and associated Appendices. The construction phase will use mostly large vessels, which are likely to travel at slow speeds, whilst only small workboats and crew transfer vessels are likely to operate at greater speed.
12.6.1.5.1 Sensitivity 362. Despite the potential for marine mammals to detect and avoid vessels, ship strikes are known to occur in cetaceans and cause injury and death (Wilson et al. 2007). Distraction whilst undertaking other activities such as foraging and social interactions are possible reasons why collisions occur (Wilson et al. 2007). Marine mammals can also be inquisitive, which may increase the risk of collision. It is not possible to fully quantify strike rates, as it is believed that a number go unnoticed. It is also possible that collisions which are non-fatal can leave the animal vulnerable to secondary infection, other complications or predation (Wilson et al. 2007).
363. However, marine mammals are relatively robust to potential collision, as they have a thick sub-dermal layer of blubber, which defends their vital organs from the worst of the impact (Wilson et al. 2007).
364. Harbour porpoise and small are highly mobile, and given their observed responses to noise (section 12.6.1.2), are expected to largely avoid vessel collision. However, harbour porpoise have been observed with signs of physical trauma (blunt trauma or propeller cuts) indicating vessel strike (4% of all post mortem examinations within the ASCOBANS area; Evans et al. 2011). It is possible that harbour porpoise in this region are habituated to vessel noise and may not respond to or avoid vessels. Furthermore it is possible the masking from other noise during construction (e.g. pile driving noise) may limit the ability of harbour porpoise to detect approaching vessels. Given this evidence harbour porpoise are considered to have high sensitivity to this impact.
365. Ship strikes involving species of seal are not widely reported, and seals are considered to have negligible sensitivity to this impact. Impacts related to ducted propellers are assessed in section 12.6.16, below.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 88
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