Abstract
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 <40 m, most ranged between 20 and 35 m, and there was a clear peak at 3 m, and also around 10 m depth.
5. While in years prior to the construction, several seals were observed within and near the Gemini
area, only one grey seal was tracked through the wind park. Though this occurred in the pile driving
period, there was no pile driving activity on-going. Seven harbour seals and six grey seals were tracked within 50 km of Gemini during pile driving, some individuals at multiple occasions. Three harbour seals and five grey seals were tracked within 40km of the wind farm during pile driving.
6. During pile driving seals swam significantly more often away from than towards the pile driving
activity. This was the case for grey seals that were within 30 and 40 km from active pile driving site (45
events in total). Harbour seals hardly approached the pile driving thus sample size was low (12 events)
and no significant effect in swimming direction could be determined. Dives during pile driving for both
species were shallower.
7. Aside from to the Gemini windfarm, 10 other windfarms were built in the study area in recent
years (2007-2015). The construction activities have been almost continuous. For harbour seals the usage further offshore seems to have diminished since the first tracking in the area with high-resolution GPS trackers in 2009. Also during the year of construction (2015) there was less usage further offshore (> 45km) compared to previous years.
7. 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 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.
8. 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.