3.2. Potential Biological Removal
3.2.4. Sensitivity of the PBR estimate
Dillingham & Fletcher (2008) discuss the sensitivity of the PBR estimate in rela-tion to variability in survival rates and age of first breeding. It is generally the case that survival estimates are derived in non-optimal conditions or estimates have not been adjusted for possible emigration from the study area. When so,
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consideration of the impact of changes in different survival estimates on the PBR by Dillingham & Fletcher (2008) has led to the recommendation that conservative (i.e. high) survival estimates should be used to avoid over-estimation of λmax and PBR. As such, it is not considered inappropriate to use the survival esti-mates as published by Horswill & Robinson (2015) in the current analysis.
For seabirds with delayed fecundity and high survival, Dillingham & Fletcher (2008) stated changes in α lead to only small changes in λmax Fecundity and age-specific breeding success of seabirds increases in the initial two or three years of breeding. Mid-point values for α are usually appropriate, while high values lead to conservative estimates of λmax and PBR (Dillingham & Fletcher 2008). The current analysis uses the typical age of first breeding (α) as published by Horswill
& Robinson (2015).
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DISPLACEMENT ANALYSES FOR SEJERØ BUGT OWF 4.
The numbers of Common Scoters estimated to be displaced by the worst case scenarios of the two design options for Sejerø Bugt windfarm + 5 km buffer are tabulated in Table 2. In addition, the mean estimated number of birds per survey for the survey area and season is tabulated.
Table 2: The estimated number of displaced Common Scoters from each of the two design scenarios for Sejerø Bugt Offshore Windfarm assuming a 70% reduc-tion of density within the windfarm footprint and a linear reducreduc-tion of impact out to a distance of 5 km from the windfarm periphery.
Season
Number of birds displaced by each wind-farm design
4.1. Assessment against the regional and flyway populations
The displacement matrices for Common Scoters during the four seasons using each of two design scenarios + 5 km buffer population estimates as calculated from modelled density surfaces of aerial digital surveys are shown in Tables 3 and 4.
The numbers of Common Scoter at risk of displacement do not surpass a 1%
threshold of the national population for any period of the annual cycle irrespec-tive of the selected level of up to and including 20% mortality. This holds true at the predicted levels of displacement and extreme worst case of 20% mortality when summing the predicted displacement mortality across each season. These observations are equally applicable to a 1% threshold of the flyway population at its lower limit. It should be noted that calculating a total annual mortality by sum-ming the predicted displacement mortality across each season is considered overly precautionary with a more realistic expectation being to only consider the displacement impact in the “worst case” season.
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Table 3: Common Scoter predicted mortality as a result of displacement from a 150 MW windfarm at Sejerø Bugt for each season with respect to the Danish wintering population estimate and lower range of the flyway population estimate.
Season % mortality
1 5 10 15 20
Summer 8 38 76 113 151
Autumn 3 17 34 52 69
Winter 10 51 102 153 204
Spring 81 404 807 1,211 1,615
Input data: Estimated number of birds displaced for the respective season;
Summer (755), Autumn (344), Winter (1,020) and Spring (8,074) Reference population 1: Denmark = 600,000 individuals
Reference population 2: Flyway population = 600,000 individuals (minimum estimate)
Table 4: Common Scoter predicted mortality as a result of displacement from a 200 MW windfarm at Sejerø Bugt for each season with respect to the Danish wintering population estimate and lower range of the flyway population estimate.
Season % mortality
1 5 10 15 20
Summer 8 38 76 115 153
Autumn 3 17 35 52 70
Winter 10 52 103 155 207
Spring 82 410 821 1,231 1,642
Input data: Estimated number of birds displaced for the respective season;
Summer (764), Autumn (348), Winter (1,033) and Spring (8,209) Reference population 1: Denmark = 600,000 individuals
Reference population 2: Flyway population = 600,000 individuals (minimum estimate)
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DISPLACEMENT ANALYSES FOR SMÅLANDSFARVANDET OWF 5.
The numbers of Common Scoters estimated to be displaced by the worst case scenarios of the two design options for Smålandsfarvandet windfarm + 5 km buffer are tabulated in Table 5. In addition, the mean estimated number of birds per survey for the survey area and season is tabulated.
Table 5: The estimated number of displaced Common Scoters from each of the two design scenarios of Smålandsfarvandet Offshore Windfarm assuming a 70
% reduction of density within the wind farm footprint and a linear reduction of impact out to a distance of 5 km from the windfarm periphery.
Season
Estimated number of birds in the
Smålandsfarvandet survey area
Number of birds displaced by each wind-farm design
5.1. Assessment against the regional and biogeographic migratory flyway populations
The displacement matrices for Common Scoters during the four seasons using each of two design scenarios + 5 km buffer population estimates as calculated from modelled density surfaces of aerial digital surveys are shown in Tables 6 and 7.
The numbers of Common Scoter at risk of displacement do not surpass a 1%
threshold of the national population for any period of the annual cycle irrespec-tive of the selected level of up to and including 20% mortality. This holds true at the predicted levels of displacement and extreme worst case of 20% mortality when summing (1) the predicted displacement mortality across each season and (2) both windfarms are considered. These observations are equally applicable to a 1% threshold of the biogeographic migratory flyway population at its lower limit.
It should be noted that calculating a total annual mortality by summing the pre-dicted displacement mortality across each season is considered overly precau-tionary with a more realistic expectation being to only consider the displacement impact in the “worst case” season.
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Table 6: Common Scoter predicted mortality as a result of displacement from a 150 MW windfarm at Smålandsfarvandet for each season with respect to the Danish wintering population estimate and lower range of the biogeographic mi-gratory flyway population estimate.
Season % mortality
1 5 10 15 20
Summer 11 55 109 164 218
Autumn 7 35 69 104 139
Winter 59 297 593 890 1,186
Spring 39 193 386 579 772
Input data: Estimated number of birds displaced for the respective season;
Summer (1,090), Autumn (694), Winter (5,932) and Spring (3,860) Reference population 1: Denmark = 600,000 individuals
Reference population 2: Biogeographic migratory flyway population = 600,000 individuals (minimum estimate)
Table 7: Common Scoter predicted mortality as a result of displacement from a 200 MW windfarm at Smålandsfarvandet for each season with respect to the Danish wintering population estimate and lower range of the biogeographic mi-gratory flyway population estimate.
Season % mortality
1 5 10 15 20
Summer 11 56 112 168 225
Autumn 7 36 72 107 143
Winter 61 307 614 921 1,229
Spring 40 200 399 599 798
Input data: Estimated number of birds displaced for the respective season;
Summer (1,123), Autumn (715), Winter (6,143) and Spring (3,992) Reference population 1: Denmark = 600,000 individuals
Reference population 2: Biogeographic migratory flyway population = 600,000 individuals (minimum estimate)
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6.1. Selecting the recovery factor f
For Common Scoter, an increasing trend in the short-term and fluctuations in the long-term winter population estimates in Denmark (BirdLife International 2015) would suggest a recovery factor (f) of 0.5 may be appropriate (see Section 1.1.8.). At a European level, the overall trend emerging from national trend esti-mates for wintering birds in shows a stable population. Considering the evidence underpinning the selection of recovery factors in this report, it is deemed appro-priate that the analysis considers the implications of a range of recovery factor for Common Scoter.
6.2. Potential Biological Removal
Table 8 presents the PBR values for the national and biogeographic migratory flyway populations of results for Common Scoter predicted to interact with the two Projects for a range of recovery factors.
Table 8. PBR values for the national and biogeographic migratory flyway popula-tions of Common Scoter predicted to interact with the Projects.
Population Population size Age of First Breeding4 (α) Annual Adult Survival5 (s) Growth Rate (λmax) Population Trend6 f = 0.1 f = 0.5 f = 1.0
Denmark
600,000 3 0.783 1.20617 Increase short-term;
600,000 (minimum) 3 0.783 1.20617 Increasing short- and
long-term 6185 30926 61852
6.3. Predicted mortality rates from displacement in terms of PBR