1. Gemini offshore windfarm is located 55 km north of the island of Schiermonnikoog. The construction works for the 150 wind turbines and electrical infrastructure were carried out in 2015-2016. Pile-driving, which is generally considered to have largest expected immediate but short term impact on marine fauna, was carried out in July-October 2015. Installation of the turbines commenced in February 2016. Movements of both harbour and grey seals were studied during 2013-2014, before construction of the Gemini windfarm, a T0 period (Brasseur & Kirkwood 2015). For this study seals were tracked in 2015, the year of construction, called Tc, with the aim of studying possible effects of the construction activities on seals. 2. As stipulated in the Monitoring and Evaluation Plan the methods chosen for this study were in line with the seal tracking research carried out by IMARES (now: Wageningen Marine Research) in previous years. In April 2015 seven grey seals were deployed with trackers prior to the onset of the pile driving. In September 2015 (during constructions) nine grey seals and ten harbour seals were deployed with trackers. 3. The offshore construction activities in the Gemini windfarm area include a variety of activities such as preparation of the field, scour protection at monopile pads and cable crossings, cable installation, pile driving of monopile foundations and installation of the turbines on the monopile foundations. Data for many of these activities are presented in this report. However, the analysis concentrates on the pile driving activity for this study. 4. Out of the 26 tracked seals deployed for the Gemini Tc study, 23 trackers functioned well, collecting data for 56-208 days. The other three trackers – two on grey seals, one a harbour seal – performed only for a relative short time. Data collected in the framework of Luchterduinen windfarm is incorporated into the data analysis of this study. Grey seals tracked following both spring and autumn deployments, remained most of the time in Dutch waters, though three visited the UK and one visited Helgoland. Most trips of the grey seals at sea were relatively close to shore (< 60 km), though four animals travelled more broadly across the North Sea. Maximum dive depth for most seals was <60 m, though in the area around Gemini most dives ranged between 20 and 35 m. There was also a clear peak at approx. 3 m. The harbour seals (only autumn deployments) generally remained within 50 km of the coast and in the general area around Gemini. Two animals moved west, one even travelling to the Dutch delta. The harbour seals dived at depths 45km) compared to previous years. 8. Observed patterns in the seal’s distribution at sea depend on the current size of the population, environmental conditions and human activities. Increasing anthropogenic developments in the North Sea 6 of 64 Wageningen Marine Research C004/18 have potentially a significant effect on movement and habitat use of seals in North Sea. A study on cumulative effects based on existing data may provide for insight in underlying processes. Next to the population monitoring by aerial surveys, seal tracking (as part of regular monitoring) would provide the necessary data to monitor and eventually to understand long-term changes in the seals’ behaviour, in relation to the recent windfarm development. 9. Habitat models and individual based models can be used to help understand and describe the mechanism underlying seal distribution, and have the potential to make predictions in both space and time. With these models, scenarios can be tested to predict the seal population development under changing circumstances and human activities.

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