Abstract
The planned large-scale development of offshore wind farms in the North Sea has potential consequences for many marine organisms, including seabirds. Seabirds may suffer from collisions with offshore wind farms during migratory or foraging flights at sea and this additional mortality may in turn negatively affect the populations of seabirds using the Dutch continental shelf.
In Environmental Impact Assessments and Appropriate Assessments for future offshore wind farms the effects of these initiatives on birds are assessed with relatively simple criteria and thresholds like the ORNIS 1% criterion and the Potential Biological Removal (PBR). These methods are easy to apply and understand but come with many limitations. On the other hand, population models can predict population dynamics of species based on the latest, species-specific scientific knowledge, and will probably give better estimates of bird mortality associated with wind farms.
In this study species-specific (Leslie-Matrix) population models were developed and applied to create a tool to better assess the impacts of future offshore wind farms on bird populations. These population models take parameter uncertainty into account, resulting in a different outcome for each simulation. The impact assessment is based on a comparison of the distribution of outcomes for different scenarios.
This study looked at collision victims from all offshore wind farms planned pre-2030 in the North Sea. Species of interest were selected based on the Framework for the Assessment of Ecological and Cumulative Effects (KEC 1.0 - KEC 3.0; Rijkswaterstaat 2015; 2019), and include five gulls, two skuas, two terns, three species of wildfowl and one wader. Based on best available data, population models were constructed for the current situation (without additional mortality).
This study only calculated effect sizes of additional mortality directly due to collisions with turbines of offshore wind farms. There are many more causes of direct and indirect anthropogenic mortality for the species at hand, some of which may even turn out to be indirect effects of offshore wind farms (e.g. knock-on effects caused by ecosystem effects of shifts in hydromorphology and/or foodweb relationships). In this report we attempted to quantify this additional mortality but available data were not of such quality that they were considered valid enough to be included in this study.
This report shows how population models can be used for a population-level impact assessment of mortality due to collisions of birds with turbines. Population models provide a better picture than other methods of the possible effects of offshore wind farms on these species. However, before the models can be used to inform the permit 11 process, thresholds are needed for the metrics they can produce. This is a policy decision rather than a scientific one.