A population assessment of the Northern Gannet Morus bassanus on Alderney, Channel Islands, in June 2015 estimated a total of 8,737 Apparently Occupied Sites (AOS), split between the two gannetries with 5,960 AOS on Les Etacs and 2,777 AOS on Ortac. This was an annual increase of 2.3% and 0.9% for Les Etacs and Ortac respectively over the decade since the last count in 2005. However, both growth rates were below the average rate experienced by other UK Northern Gannet colonies during this same time period. Results show the reduced growth rates of the colonies may be due to them reaching carrying capacity with little room for further expansion.

The islets of Les Etacs (49°42’N, 2°14’W) and Ortac (49°43’N, 2°17’W), located approximately 0.35 km west and 4.5 km northwest of Alderney respectively, support two of the most southerly Northern Gannet Morus bassanus (hereafter ‘Gannet’) breeding colonies. Les Etacs covers an area of 1.2 ha and is comprised of a series of rocks and outer stacks rising steeply from the sea to a maximum height of 37 m. Ortac is an isolated rock 4.2 km northwest of Les Etacs with an approximate area of 0.3 ha, rising to 22 m above sea level with steep sides and a relatively shallow sloped top. Ortac is regularly topped by waves in severe weather conditions. Both islets are within Alderney’s Ramsar site.

Gannets have a large breeding range extending north from Brittany in France to Norway in the east Atlantic, westward through Iceland to Labrador in northwest Canada. Despite the large range, there are in fact relatively few breeding colonies; as of 2010, there were just 20 gannetries in the British Isles (JNCC 2016). Breeding Gannets were first recorded in Alderney in 1940. A single pair incubating an egg was observed on Ortac during a trip to monitor the Black-legged Kittiwake Rissa tridactyla colony (Lockley 1948). In the years following, there were no recordings of Gannets due to the evacuation of Alderney during the Second World War. However, upon returning to the island in 1945, a fisherman described the eastern side of Ortac as covered in Gannets and noticed another smaller colony on Les Etacs. The exact number was unknown until 1946 when a survey counted 250 pairs on Ortac and 200 on Les Etacs (Dobson & Lockley 1946). Since these first observations, the number of breeding Gannets on Les Etacs and Ortac has increased, following global population trends. Gannets are one of only a few seabird species experiencing population increases (BirdLife International 2016), unlike the majority of seabird species which have shown drastic population declines in recent years (Paleczny et al. 2015).

Regular monitoring is important as, similar to other seabird colonies, Alderney’s Gannets may face increasing pressure from anthropogenic activities, particularly changes in fisheries practice and offshore wind farms (Croxall et al. 2012). Tracking studies have shown that Alderney’s Gannets forage in two main areas. The first is an area northwest from the colonies to the south coast of the UK, between Devon and Hampshire. The second is an area to the east, from the northern tip of the Normandy peninsula to Le Havre, France. In these areas, there are a number of proposals for offshore wind farm developments, of which when built, Gannets may come into contact with the turbines (Warwick-Evans et al. 2015a, 2016a, 2016b), resulting in low levels of mortality, likely to increase as wind farm area increases (Warwick-Evans et al. 2018). Alderney’s Gannets have been regularly monitored since they were first recorded in 1940 (Table 1) including during the four Britain and Ireland Gannet censuses: Operation Seafarer, the Seabird Colony Register, Seabird 2000 and the most recent census in 2005 (Cramp et al. 1974; Hill 1989; Sanders & Harris 2005; Wanless et al. 2005). Subsequent aerial population counts have been, and are planned to be, carried out every five years as part of the management of Alderney’s Ramsar site, with the most recent count undertaken in 2011.

The aerial counts on Alderney have been complemented with annual productivity counts (Alderney Wildlife Trust 2016) and the mass-ringing of chicks (Veron & Lawlor 2009) to measure annual survival and gain insight into the distribution of these birds during the non-breeding season. Both juvenile and adult survival rates have increased over time, most likely due to their use of discards from fishing vessels (Votier et al. 2013; Warwick-Evans et al. 2016b). With changing climate combined with the changes to the fisheries policy, which will ban the discard of unwanted fish (European Union Parliament and Council 2013), Alderney’s Gannet population may be particularly vulnerable due to its position near the southern limit of the species’ range. It is essential that regular monitoring of these Gannet colonies continues in order to monitor population trends, given the declines in other seabird populations.

The last published census of Alderney’s Gannet population was in 2005 (Sanders & Harris 2005) and the last census of all UK Gannet colonies was carried out in 2004 (Wanless et al. 2005). During these studies, Alderney’s Gannet population was estimated at 4,862 Apparently Occupied Sites (AOS) on Les Etacs and 2,547 AOS on Ortac. Sanders & Harris (2005) estimated that since 1979, the total population had grown at an average rate of 2.8% per annum (p.a.), with a growth rate of 3.3% p.a. on Les Etacs and 1.2% p.a. on Ortac. This was greater than the average rate of increase of the total British and Irish population which was only 1.2% p.a. over the same time period. This study provides a decadal update on the estimated number of pairs of Gannets breeding on Alderney.

We would like to thank Rod Paris, the pilot and owner of the aircraft and Bill Black the photographer, without the help from both, this survey would not have been possible. Our thanks also go to Mike Harris and Simon Murray for their help performing the counts.

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