Assessing Northern Gannet Avoidance of Offshore Windfarms


Title: Assessing Northern Gannet Avoidance of Offshore Windfarms
Publication Date:
June 01, 2014
Document Number: APEM Report 512775
Pages: 26

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Rehfisch, M.; Barrett, Z.; Brown, L.; Buisson, R.; Perez-Dominguez, R.; Clough, S. (2014). Assessing Northern Gannet Avoidance of Offshore Windfarms. Report by APEM Ltd. pp 26.
  1. A novel approach is presented for estimating northern gannet Morus bassanus macro- and micro-avoidance of offshore windfarms from high resolution digital images gathered from aerial survey. This approach calculates macro- and micro-avoidance based on the measured change in gannet density at a distance from the windfarm and inside the windfarm.
  2. Four aerial surveys of the built Greater Gabbard offshore windfarm (GGOWF) were carried out between 30 October 2014 and 23 November 2014, a period of high gannet autumn passage off the East Anglian coast and in the southern North Sea.
  3. Digital images were collected by planes flying at over 300 m leading to no observable disturbance to the birds and thus minimising any bias in the data. Each survey consisted of between 14 and 20 pseudo-randomly generated transects, with the caveat that each transect had to either cross or abut the windfarm. Each transect started and ended 10 km before and after the GGOWF, respectively. In total the four surveys covered 320% and 75% of the windfarm footprint and buffer areas, respectively, with 570 m wide transects.
  4. In total 336 gannets were recorded in the images during the four autumn passage surveys of which eight and 328 were recorded within and outside the GGOWF footprint, respectively. The gannets had a minimum recorded approach distance of 443 m and 359 m away from the nearest turbine within and outside the footprint, respectively.
  5. A zero-inflated negative binomial model is used to describe the relationship between the distance to the nearest turbine and gannet counts outside of the GGOWF footprint.
  6. The model suggests that gannet numbers change with distance to the built windfarm (P=0.0518). Gannet counts increase from zero close to the turbines to reach a “background at sea” plateau two kilometres away from the nearest turbine. The lower density within two kilometres of the windfarm is likely to reflect gannets avoiding the vicinity of the GGOWF.
  7. A macro-avoidance value of 95.02% has been calculated for gannets as the percentage change from their background at sea density 4 km or more outside of the GGOWF compared to their density within the GGOWF footprint. A distance of 4 km outside the GGOWF is used rather than 2 km to ensure that a robust background gannet density is used. Observing no birds closer than 359 m to a turbine suggests 100% micro-avoidance and an overall avoidance value of 100%.
  8. In conclusion, the results of this study strongly suggest that northern gannets avoid the close proximity of built windfarms, at least during the autumn passage period and they support previous studies that also showed strong avoidance. A 95.02% macro-avoidance value, a 100% micro-avoidance value and a 100% total avoidance value is indicated by the data. Based on the published offshore micro-avoidance value of 97.6% we estimate total avoidance to be 99.9%. It is therefore not unreasonable from the evidence of this study, taken together with previous studies, to suggest that an avoidance rate of 99.5%, at least for autumn passage gannets, may be appropriately precautionary for use in collision risk modelling.
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