Framework for Assessing Ecological and Cumulative Effects of Offshore Wind Farms: A First Approach to Deal with Cumulative Effects on Birds and Bats of Offshore Wind Farms and Other Human Activities in the Southern North Sea


Title: Framework for Assessing Ecological and Cumulative Effects of Offshore Wind Farms: A First Approach to Deal with Cumulative Effects on Birds and Bats of Offshore Wind Farms and Other Human Activities in the Southern North Sea
Publication Date:
January 15, 2015
Document Number: C166/14
Pages: 360

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Leopold, M.; Boonman, M.; Collier, M.; Davaasuren, N.; Fijn, R.; Gyimesi, A.; de Jong, J.; Jongbloed, R.; Poerink, B.; Krijgsveld, K.; Lagerveld, S.; Lensink, R.; Poot, M.; van der Wal, J.; Scholl, M. (2015). Framework for Assessing Ecological and Cumulative Effects of Offshore Wind Farms: A First Approach to Deal with Cumulative Effects on Birds and Bats of Offshore Wind Farms and Other Human Activities in the Southern North Sea. Report by IMARES - Wageningen UR. pp 360.

Renewable energy is an increasing demand, and governments of North Sea countries are looking at developing offshore wind farms to help meet sustainability demands. The first at-sea wind farms have become operational in several countries, or are under construction, but many more are on the drawing board. Altogether, around 100 offshore wind farms are scheduled to be operational by 2023 in the southern North Sea (51- 56°N) alone. There may be two sides to this development in environmental terms: on the one hand this will help reduce CO2 emissions, on the other hand protected North Sea biota may be negatively impacted. This report considers the cumulative impact of all projected wind farms in the southern North Sea (by 2023) on birds and bats.


Birds and bats have flight in common, and any animal flying over the North Sea may collide with rotor blades in (future) offshore wind farms. This will lead to increased mortality. Mortality rates will depend on the numbers of animals in the air, at rotor height, moving through the wind farms, and their behaviour while doing so. The impact of collisions on the population level will depend on the relative population size (to the number of casualties) and the regenerative power of the species concerned. North Sea seabirds also use the area as their habitat and may suffer additional mortality through habitat loss or habitat degradation. Space taken up by offshore wind farms may be avoided by seabirds, and this loss of habitat may lead to additional loss of fitness. It has been assumed that 10% of the seabirds that are displaced by offshore wind farms, will die. We note that loss of life through habitat loss may be structural, in that the carrying capacity of the southern North Sea will be permanently decreased, leading to higher stress on the seabirds that rely on this habitat.


The combined, cumulative effects of collisions and displacement, have been modelled for all wind farms considered operational in 2023 in the southern North Sea, using the method recently proposed by Bradbury et al. (2014). We have extended this method to be able to predict numbers of birds killed directly from collisions, and indirectly from displacement. Total numbers of birds estimated to die remain below PBR for all species of seabirds commonly occurring in the North Sea.


Predicted numbers of casualties, relative to these latter factors have been compared in Potential Biological Removal (PBR) models. It is assumed that if the cumulative number of casualties (of all wind farms) remain under PBR, the birds (or bats) killed will be replaced and the population size will not decrease because of offshore wind farm development. The results of this modelling exercise shows, that total predicted mortalities in all seabird species will remain within the limits of PBR. This would imply that no bird species will go extinct because of the development of offshore wind alone. Eight seabird species have predicted mortalities that are 10% or more of PBR: Lesser Black-backed Gull (52%), Great Black- backed gull (26%), Black-legged Kittiwake (24%), Herring Gull (22%), Northern Gannet (18%), Common Guillemot (15%), Great Skua (13%) and Red-throated Diver (10%), while two more come close to this figure: Black-throated Diver (9%) and Razorbill (8%). For all other species, predicted wind farm related mortality rates are below 5% of PBR (see Table 5.1 in this report).


The collision-part of the extended Bradbury-method was cross-checked. Cumulative numbers of collisions were also estimated by using the Band (2012) method. Model outcomes of this routine also largely predicted mortality rates below PBR. However, with this method, higher and in some cases much higher mortality rates were predicted, which in two species exceed PBR: Lesser Black-backed Gull (313% of PBR) and Great Black-backed Gull (131%), which would mean that collisions alone would lead to extinction of these species in the southern North Sea. Using the Band (2012) method also resulted in high predicted mortality rates from collisions in European Herring Gull (81% of PBR), Northern Gannet (50%), Black-legged Kittiwake (36%), and Great Skua (10%), while two more species had predicted mortality rates exceeding 5% of PBR: Common Eider (8%) and Sandwich Tern (6%; for the full list see Table 4.23 in this report).


Mortality rates, resulting from collisions with offshore wind farms were also estimated with the Band (2012) method for land- and waterbirds (from freshwater habitats) that commonly migrate across the southern North Sea (see Table 4.24 in this report), from known population sizes and migration routes, and compared to their specific PBR values. None of these species was predicted to suffer a cumulative mortality above PBR, but high values were found for Eurasian Curlew (60% of PBR), Black Tern (52%), and Tundra Swan (also known as Bewick’s Swan: 44%) , while notably high figures were also found for Sanderling (21%), Common Starling (12%), Red Knot (11%) and Bar-tailed Godwit (6%).


The Band model appears to be highly sensitive to the numbers of birds assumed to be flying through the wind farms. For some of the seabirds, unrealistically high numbers were possibly generated for some future wind farms, on the basis of at-sea count data. These had to be extrapolated over wider areas and peak counts, from e.g. concentrations of gulls and Northern Gannets around fishing vessels were a possible cause. Similarly a count of a flock of migrating Common Eiders over a spot that was chosen for a future wind farm, generated a high local density for this species in that particular future wind farm, leading to a high predicted mortality rate. This explanation, however, is not valid for migrant birds, such as Black Tern, Tundra Swan, the waders mentioned above and the Common Starling, for which no at-sea survey data were used as model input. We also note that wind farm related mortality should be seen in concert with other mortality factors, and high mortality rates from collisions alone, in comparison with PBR, are worrying.


All predicted mortality rates, at this stage, are only model predictions. The displacement part of predicted seabird mortality is still highly uncertain and could not be cross-checked by another model. We could only model displacement in relation to wind farm configuration in Common Murre, the species for which comparable data are available from several offshore wind farm impact studies. Displacement varied between wind farms, in relation to turbine density. For many future offshore wind farms, the turbine density is not yet known, and more data will be needed to explore this in full, also for other species of seabirds.


Our modelling exercise did, however, identify species that would seem to be at risk. Most of these are already closely monitored: the seabirds in their breeding colonies, the Tundra Swan at its wintering and staging quarters, and the waders in tidal basins around the North Sea. We recommend that populations remain closely followed, now also in the light of offshore wind farm development. Given that the circa 100 wind farms considered in this report will not be built overnight, population trends of the various bird species identified here as vulnerable can be followed and compared to increasing mortality rates from offshore wind farms, as progressively more projects become operational. Following developments closely would allow adjustment of the development of offshore wind, should mortality rates become unacceptably high.


For bats at sea, far less information is available than for birds, but the same general rules apply. We note that several species have been regularly identified flying over the North Sea, or may be expected to do so, by extrapolation. However, the sizes of populations likely to be impacted are very imperfectly known, as are bat numbers at sea and their offshore behaviour. Several species may be impacted negatively by offshore wind farm development in the southern North Sea, most notably the Nathusius’ Pipistrelle, while Particoloured Bat and Noctule would also seem to be vulnerable. For bats, increased monitoring, particularly at sea, is required to get to grips with the possible problem at hand.

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