Ecology of top predators in the North Sea – Baseline for and effects of offshore wind power developments
The North Sea is amongst the most intensively utilised sea areas worldwide and the marine habitat occupied by human uses substantially increased during the past years as numerous offshore wind farms (OWFs) were built. The construction of already approved OWFs and the consenting of additional areas for OWFs is still in progress. Due to the importance of apex predators for a healthy marine environment and their role as indicators for the state of the ecosystems it is of utmost importance to increase our knowledge on their habitat use and behaviour and to understand if and how they are affected by anthropogenic activities. In this thesis, fundamental knowledge on the ecology of several important top predator species is provided and applied as baseline information to investigate their reactions towards OWFs constructed in the southern North Sea. To provide a comprehensive view on the different aspects two methods were applied: Visual and digital surveys offer the opportunity to study the species abundance and distribution as well as changes of these on a large spatial and temporal scale. Telemetry of individual animals provides detailed information on their movements and behaviour during specific periods of their annual life cycle and enables to directly observe their reaction to changes in their environment. Five top predator species (including one species group) which differ substantially in their ecological requirements and constraints, e.g. due to their foraging behaviour, were chosen: black-legged kittiwakes (Rissa tridactyla), common guillemots (Uria aalge), the species group of loons (Gavia spp.), northern gannets (Morus bassanus), and grey seals (Halichoerus grypus). As wide ranging and abundant species they are prone to experience interference with OWFs at various locations. However, the reaction of seabirds towards large scale OWFs in the southern North Sea and towards OWFs located close to their breeding colonies, as well as their individual behaviour in response to OWFs were poorly studied so far. Furthermore, although grey seals have become an important part of the ecosystem in the southern North Sea over the last 50 years, information on their habitat use and their reaction towards OWFs was currently missing for the German North Sea. Based on an extensive, long-term and large scale dataset collected by means of visual and digital surveys before and after construction of the OWFs an effect analysis was conducted. Loon distribution was strongly affected by the OWFs located close to one of their most important resting sites during spring migration. The abundance of loons decreased highly significantly by 94.5% inside the OWFs. The associated ship traffic also had a significant negative impact on loons, indicating that OWFs deterred them through the combined effect of ship traffic and the wind turbines themselves. Strong effects of the OWFs located 23-35 km north of Helgoland were furthermore detected for guillemots in spring (63% reduced density in the OWF) and for guillemots and kittiwakes in the breeding season (44% and 45% reduced density in the OWF). Telemetry was applied on breeding gannets and for the first time in German waters on breeding guillemots and weaned grey seal pups. Strong effects were revealed for both seabird species during two breeding seasons. Most individuals strongly avoided the OWFs. Of the tagged guillemots 16% shortly entered the OWFs during two to three occasions and 10% of the gannets frequently entered the wind farms when foraging or commuting between colony and foraging areas. A point process modelling (PPM) approach revealed a significantly reduced resource selection inside the OWFs compared to outside. For gannets resource selection was reduced by 37% and for guillemots by 67%. For guillemots the OWF avoidance was stronger when turbine blades were rotating (76% reduced resource selection). For the grey seal pups a PPM approach revealed that they strongly increased their distance to Helgoland already in the first weeks and generally during the first 70 days at sea. The frequency of the foraging behaviour increased until week four, whereas fast travelling behaviour increased throughout the whole study period. The pups’ individual spatial overlap with OWFs under construction and in operation was revealed but no obvious behavioural response to OWFs was detected. This thesis strongly enhances our knowledge on the ecology of important top predators in the southern North Sea and provides new and detailed information on the effects of OWFs on the studied species. Different reactions towards OWFs were detected and for the seabird species substantial habitat losses were revealed. The here presented findings will contribute fundamentally to future studies on anthropogenic impacts, to environmental monitoring or to conservation and management implications of the respective species and need to be considered during the planning process of future OWFs.