Bass Rock Colony Counts

Many amazing seabird species have shown major population declines in recent years. In contrast, the Northern gannet has been increasing in breeding numbers for over a century (Jeglinski et al. 2022). Nowhere has this increase been demonstrated better than the population on the Bass Rock. Here, numbers increased from around 3000 pairs in 1904 to over 75,000 Apparently Occupied Sites (AOS) in 2014, when it became the world’s largest colony of Northern gannets (Nelson 2002, Murray et al. 2014). An Apparently Occupied Site in the colony is an area occupied by a single bird or pair of birds, irrespective of whether nest material is present.

However, things changed abruptly in 2022 when Highly Pathogenic Avian Influenza (Avian Flu) resulted in unprecedented mortality in gannet populations throughout the North Atlantic (Lane et al. 2023). The disease was confirmed on the Bass Rock on 4 June 2022 and 5035 dead birds were identified in images taken by a drone on 30 June 2022 (Glen Tyler in Lane et al. 2023). The drone images and observations made in the colony throughout the 2022 season indicated that the number of gannets present in the colony had dramatically reduced and was consistent with a significant population decline.

Carrying out a counts of the colony in 2023 and in 2024 was a high priority to assess the impact of the Avian Flu outbreak. Advances in the technology now available to monitor breeding gannets and interpret survey results also brought with it opportunities to undertake drone surveys and machine learning trials.

To complete this essential research, The Scottish Seabird Centre worked in close collaboration with The University of Edinburgh School of Geosciences, The University of Edinburgh Airborne Research and Innovation Facility, UK Centre for Ecology & Hydrology, and Edinburgh Napier Centre for Conservation and Restoration Science. We are very grateful to all the individuals and organisations involved for bringing their extensive expertise and commitment to this project.

An arial view of gannets on the Bass Rock from drone imagery

Summary of Count Results: 2023

A drone survey of Northern Gannets on the Bass Rock was carried out on 27 June 2023. The survey covered approximately 85% of the colony and three independent counts of the drone imagery were made. The total Bass Rock population was estimated to be 51,844 AOS. Comparison of the 2023 count with the same count areas in 2014 indicated a decrease of 31%.

To find out more about how this assessment was completed and the methods and assumptions that underpin these results, please read the 2023 Bass Rock Count Report:

2023 Bass Rock Count Report

Summary of Count Results: 2024

A drone survey of Northern Gannets on the Bass Rock was carried out on 29 July 2024, following the protocol of the earlier surveys in 2023 and 2022. The resulting orthomosaic vertical image covered c.97% of the colony. The total Bass Rock population in 2024 was estimated to be 46,045 AOS. Comparison of the areas counted in both 2023 and 2024 indicated a 6.7% decline in AOS, while comparison of the estimated total population indicated a decrease of around 11%. Thus, there was no evidence of a population recovery after the high pathogenicity avian influenza (HPAI) outbreak in 2022.

A detailed analysis of inter-observer variation in counts of AOS was also carried out. There was no statistical support that counters differed in how they counted areas varying in size and aspect, suggesting appreciable savings in time and/or expense needed to obtain population counts of gannetries may be possible by reducing the need for replicate counts.

To find out more about how this assessment was completed and the methods and assumptions that underpin these results, please read the 2024 Bass Rock Count Report:

2024 Bass Rock Count Report

A view of the Bass Rock from the air, showing the white lighthouse, the mainland in the far distance, and many gannets making the rock look almost white

Find out more

Discover more about how the development of a deep learning neural network fed into our research on Bass Rock during 2023 in this research paper:

Quantifying the Impact of Avian Influenza on the Northern Gannet Colony of Bass Rock Using Ultra-High-Resolution Drone Imagery and Deep Learning

Tyndall AA, Nichol CJ, Wade T, Pirrie S, Harris MP, Wanless S, Burton E. Drones. 2024; 8(2):40.