NatureScot Research Report 1417 - Digital Aerial Survey of Northern Gannet (Morus bassanus) colonies off North-west Scotland in 2024.

Introduction

NatureScot commissioned APEM Ltd. (APEM) to conduct a 100% coverage digital aerial survey (DAS) to provide a repeated colony count of the Northern gannet (Morus bassanus, hereafter gannet) breeding colonies at the north-west of Scotland, specifically Sula Sgeir, Sule Stack, Sule Skerry, Flannan Isles, and Rockall. 

The main aim of the survey was to assess the status of the different colonies, and any changes the populations have undergone since the previous standardised survey undertaken in 2013 (Wanless et al. 2015a), focussing on the numbers of gannets recorded as Apparently Occupied Site (AOS) as this is the breeding population metric recommended in the Seabird Monitoring Handbook (Walsh et al. 1995). 

The methodology and data collection process for the counts presented herein followed those used in the 2013 Scottish gannet surveys (Wanless et al. 2015a) with the additional benefit of collection of vertical imagery using APEM’s bespoke high-resolution digital camera system, allowing accurate data collection and comparisons in colony extents and AOS numbers. Comparisons to the 2004 standardised survey (Wanless et al. 2015b) and other available data sources are provided where applicable for completeness. The data collected as part of this project will further be incorporated into the production of a GB and Ireland wide breeding population estimate and provide a baseline against which future monitoring can be compared to.

Colony Details

Sula Sgeir

Location: 59º06´N 06º09´W, HW620305, 64km north of Lewis in the Western Isles (Figure 1).

Originally established as a gannet colony before 1549 (Murray and Wanless, 1986). Throughout the series of standardised imagery colony assessments (1985, 1994, 2004, 2013 and 2024) the colony was initially divided into seven counting sections, and increased to eight sections in 1994, which has been maintained for the current assessment (Annex 2, Figure a).

Sule Stack

Location: 59º02´N 04º30´W, HX565177, 64km west of the Orkney Islands (Figure 1).

Originally established as a colony before 1710 (Murray and Wanless, 1986). In the first standardised colony assessment (1985) four counting sections were drawn on the colony, however, from 1994 eight sections have been used instead. Additionally, in both 2013 and 2024, the same subsections were used (Annex 2, Figure A).

Sule Skerry

Location: 59º05´N 04º24´W, HX621244, 60km west of the Orkney Islands (Figure 1).

Originally established as colony in 2003 (Wanless et al. 2015a,b). During the 2004 colony counts, this colony was assessed from pictures taken from land (Wanless et al. 2015b). In 2013 however, two counting sections and three subsections for aerial imagery were defined (Wanless et al. 2015a). For this survey, two sections were kept, but two extra subsections, Sule Skerry East North and Sule Skerry East South, were defined due to colony expansion (Annex 2, Figure a).

Flannan Isles

Location: 58º18´N 07º36´W, NA690465, 27km west of Lewis in the Western Isles (Figure 1).

Originally established as a colony in 1969 (Murray and Wanless, 1986). The colony count sections and subsections changed three times between the first standardised survey in 1985 and the last standardised survey in 2013 due to colony expansion (Wanless et al. 2015a). In this survey the previously established subsection Roareim South B was divided into Roareim South B1 and B2 due to colony expansion (Annex 2, Figure a).

Rockall

Location: 57º35´N 13º41´W, MC035165, 370km west of North Uist in the Outer Hebrides (Figure 1).

This survey represents the first attempt of including Rockall as part of the standardised Scottish colony counts. Anecdotal records of nests and gannets attempting breeding have been recorded since 1992, however productive breeding is considered unlikely due to the environmental conditions of the site (Murray et al. 2015).

Avian Flu

Methodology

Digital Aerial Survey Methods

A program of five colony aerial surveys took place between June and July 2024. 

Surveys were carried out by a twin-engine fixed-wing aircraft that orbited the target area capturing data at a safe distance using a handheld oblique camera positioned at the aircraft side window. The aircraft stand-off distance was selected by the crew on the day of survey which ranged between 1,300 and 1,500 foot (ft), this was managed through a dynamic risk assessment of target survey area topology, weather conditions, and colony activity. APEM maintained a set target resolution through this range by utilising interchangeable lenses (200 mm, 100 mm, and 85 mm) with differing focal length which were focused pre-flight and changed whilst airborne. The focal length was selected to achieve a Ground Sampling Distance (GSD) on the surface of the area of interest of between 1.5-2.0 centimetres (cm) which was sufficient to capture any distinguishing features. APEM additionally vertically mapped the colonies with its state of-the-art mapping camera system which was installed in the aircraft at the same time as the oblique camera equipment. For the oblique imagery, images were collected via a handheld Canon 5D camera with a 200mm prime lens following Wanless et al. (2015a). A metadata recorder was fitted to the camera for exchangeable image file format (Exif) encoding. For the vertical survey camera, APEM used a 110 mm lens fitted with a global navigation satellite system (GNSS) and inertial measurement unit (IMU) sensor. Further details of flight operations are detailed below.

During each survey, APEM’s on-board camera technician continually monitored the imagery as it was collected to ensure the data collected were fit for purpose. The camera technician would make the decision to cease data collection should the conditions become unsuitable for surveying or data collection.

Imagery was captured in raw format and post-processed to ensure optimal quality for the subsequent stage of image analysis, to extract information on gannets or other notable occurrences. When a survey was completed, data were checked to ensure the coverage and quality of the imagery was acceptable. Once the image analysis was completed, further quality assurance (QA) processes took place (see Section 2.3).

No health or safety issues were reported during the surveys.

Four out of the five colonies were surveyed in the ideal survey window (as defined by Walsh et al. 1995). However, Rockall had to be postponed until mid-July due to inclement weather conditions and military usage of the area. The dates, start, and end times for survey time for each colony are provided in Table 1 with the corresponding weather conditions provided in Table 2.

Weather conditions during all surveys were conducive to collecting and analysing imagery for the purpose of providing data on the identification, distribution, and abundance of gannets within the five colonies. Favourable conditions for surveying were defined as: a cloud base of at least 1,300 ft, visibility of greater than 10 km, and wind speed of less than 30 knots. For safety reasons, no surveying took place in icing conditions.

Measures were taken to minimise glint and glare, such as avoiding surveying when the sun angle had the greatest potential to impact image quality. Furthermore, additional vertical imagery was collected throughout the survey, providing an alternative set of images for analysis to ensure that sufficient coverage was achieved in the case that hand-held imagery was affected strongly by glint or glare, oblique angle or difficulty in capturing gannets located behind rock features/within crevices. 

Gannet Identification and Counts

Section boundaries within colonies were drawn on images using appropriate image editing software. The most representative image for each section was selected for counts, ensuring alignment with historical sections. In some cases (e.g. ‘Top’ & ‘Centre’ sections in Sule Stack) vertical imagery was analysed, which was comparable to previously counted reference sections. All section boundaries were checked by multiple analysts to ensure full colony coverage and accuracy with respect to previous survey reference sections. Additional new sections were created in areas where colony expansion had occurred.

Demarcated imagery was loaded into GIS software and a dedicated team of experienced analysts recorded gannet Apparently Occupied Sites (AOS) as vector points in GIS shapefiles. Using GIS ensured exact numbers could be tracked with high precision, avoiding the need for repeat counts. Gannet AOS were determined based on the position of birds amongst the colony (either one or two birds may be present). Gannets typically nest at relative distances from one another and spacing between AOS can be readily defined in aerial imagery. In some cases, AOS were partly or completely obscured by terrain, and additional vertical imagery covering that section was checked ensuring an accurate count. Any out of view AOS in the specific image that were counted were indicated separately. Immature birds and areas containing loafing, or ‘club’ birds were omitted from AOS counts.

A colony tracker sheet was used detailing the number of sections to be counted for each colony alongside the filename of the respective image(s) containing the given colony section. Any loafing/immature or deceased gannets were also noted for each section. Image quality scores were assigned to each section counted and recorded on the tracker sheet. Quality scores for image resolution and sharpness were as follows:

1 – Very High 

2 – High 

3 – Average 

4 – Low 

5 – Very Low

Summary of Quality Assurance

Following completion of colony counts by analysts, the imagery and associated section shapefile counts detailing AOS assignment were quality assured (QA) by Technical Specialists for accuracy. Checks for potential missed AOS and correct AOS status allocation were carried out as well as annotation of section boundaries, ensuring no double counting had occurred and that birds were assigned to the correct section. Multiple images covering a given section were referred to, ensuring that obscured AOS were included and that the AOS counts were accurate. Following QA, gannet counts were totalled to form an overall count for a given colony. Quality scores were checked to ensure that they were representative and consistent.

Population comparison

Results

Sula Sgeir

Colony status: 10,200 AOS in 2024, 11,230 AOS in 2013, 9,230 AOS in 2004. The breeding population has declined by 9.17% since 2013 (Table 3, Table 7). This decrease is driven by the decrease in Sula Sgeir Sections 3 (-13.6%), 4a (-17.9%), 4b (-4.4%) and 7 (-21.6%). However, select sub-sections within these sections specifically proved to be a challenge imagery-wise (Annex 3 Table A), with the terrain making it difficult to achieve full coverage, even with the addition of vertical imagery. Both sections 3 and 4b contained groups of immature/non-breeding birds (Annex 3 Table A). The remaining sections exhibited increases in numbers of AOS since 2013; Section 1 + 10.5%, Section 2 + 8.2%, and Section 6 + 12.5%. Both Section 5 (0 AOS) and the helipad (1 AOS) have shown no material change from previous usage of the area, both recorded as club sites, with virtually all gannets captured in the imagery recorded as loafing, which is in line with the previous colony assessment, though these areas still represent potential space for colony expansion.   

Sule Stack

Colony status: 2,950 AOS in 2024, 4,550 AOS in 2013, 4,620 AOS in 2004. AOS numbers have decreased by over 30% in comparison to both 2013 and 2004 (Table 4, Table 7), meaning that Sule Stack had the biggest recorded decrease in AOS of all the colonies surveyed. AOS numbers decreased in all sections since 2013, ranging from 19.1%, in the South East section, to 100% in the South Rock section, with zero AOS recorded compared to 96 recorded in 2013. The North Rock section remained unoccupied by breeding birds, as was the case in 2013. From the imagery it appears that the section is subject to wash from incoming waves which would prevent birds from nesting. Additionally, it must be noted that various sections such as South Rock had immature/non-breeding birds recorded, as did sections North East and North West A, where large numbers of immature gannets were recorded among AOS (Annex 3 Table B). Other sections such as the Top and Centre, had smaller numbers of immature/potentially non-breeding gannets, with only Sule Stack East and North Rock Sections having no immature/loafing birds. 

Sule Skerry

Colony status: 3,470 AOS in 2024, 1,870 AOS in 2013, 55-60 in 2004. The numbers of AOS have experienced the largest % increases of any of the colonies surveyed; 85.56% since 2013 and 5783.33% since 2004 (Table 5, Table 7).

The growth of the breeding colony has occurred in both the East and West sections. All the colony sections and subsections showed increases in numbers, ranging from 63% (5.7% pa) in the West A section, to 96.5% (8.8% pa) in the West B section. Even though the West section included the higher subsection increase in total birds (Annex 3 Table C), the East section showed geographical expansion, with two new subsections being colonised, one with 111 AOS (East South), and a second (East North) that appears to be a loafing area/club site on the periphery of the main East section. Originally, in 2004 the colony only had gannets on the west side, however section edge expansion was observed in 2013 (Wanless et al. 2015a). Interestingly, despite the almost doubling in size of the colony, both sections remain separated by a grass patch.

*Difference between original Sule Skerry East, and current East 1 section

**Difference between original Sule Skerry East, and current complete east section

Flannan Isles

Colony status: 7,650 AOS in 2024, 5,280 AOS in 2013, 2,760 AOS in 2004. Numbers of AOS have increased by 44.89% since 2013 and 177.17% since 2004 (Table 6, Table 7).

AOS numbers in the Flannan Isles colony showed large increases in all sections and subsections except for Main Stack A (-11.76%), ranging from 17.39% (Arch Stack) to 1630.30% (subsection Roareim A). Peak increases in AOS per total section were recorded in West Stack (189.91%), and Roareim (80.65%), this latter almost doubling in number since 2013, with subsection Roareim South A having developed a new sub-colony, and sub-section Roareim South B expanding sufficiently to be divided into Roareim South B1 and B2. The increase in Roareim AOS numbers matches the historical increase described in 2013, when compared to 2004. Also in line with the previous survey, the Roareim East section contained a large number of gannets, most of which were immature, non-breeding birds. The adults present here appeared to be loafing on a grassy area and not sitting on definite occupied sites or nests.  Some adult birds appeared to be pair bonding, and some appeared to be pulling at the grass, potentially gathering nest material. This area appears to still be a clubbing or loafing area as previous surveys of the island concluded (Wanless et al. 2015a) (Annex 3 Table D). The remaining sections except for Main Stack A exhibited more moderate AOS increases. 

Rockall

Even though there are previous records of gannets breeding in Rockall (Wanless et al. 2015a), only one gannet, deceased, was observed among the birds recorded in our images. The gannet seemed to have recently perished (within the 2024 breeding season), and was amongst ropes, indicating that entanglement from gear that had been left from an independent expedition to the island, was the cause of death for this individual. 

Deceased gannet observations