Northern gannet (Morus bassanus)

Figure 1. Trends in breeding productivity (no. of chicks fledged per pair) of northern gannet 1986–2019 for UK (red) and Scotland (blue). Further details can be found in the sections below. Based on SMP data; view the methods of analysis.

A large proportion of nests monitored for northern gannet productivity are located in Scotland and, as such, the UK and Scottish trends are closely matched. Both trends show lower productivity values between 1986 and 2001 compared with 2002 to 2019.

An insufficient number of northern gannet colonies are monitored in England, Wales, Northern Ireland, Republic of Ireland, Isle of Man and the Channel Islands, to allow the production of valid annual productivity trends.

Figure 2. Trend in UK abundance index of northern gannet 1986–2019.

Given the logistical difficulties of surveying large offshore gannetries, annual sampling usually takes place at smaller more accessible colonies. These colonies tend to increase at a greater rate than the larger colonies (see Table 1), leading to bias in any estimated trends. Instead, decadal censuses have been undertaken since the 1980s^1,2,3,4,5. Figure 2 shows interpolated and extrapolated values from these complete censuses, expressed as an index. Over the long-term, the UK gannet population increased from 113,000 pairs in 1969/70 to 175,000 in 1984/85, then from 218,000 AON in 2003/04 to 293,161 AON by the time of the 2013–2015 census.

In 2013 and 2014 all Scottish colonies were surveyed^5,6,7,8, while Bempton Cliffs (England) and Grassholm (Wales) were counted in 2015 (Table 1). Most have continued to increase, except for the gannetries at Sule Stack and Scar Rocks, and new gannetries continue to be formed (e.g. Berneray, Western Isles) or re-colonised (e.g. Rockall, Western Isles).

Figure 3. Trend in UK breeding productivity (no. of chicks fledged per pair) of northern gannet 1986–2019. Based on SMP data; view the methods of analysis.

Gannet productivity has varied little since monitoring began in 1986. At monitored colonies, productivity was between 0.60–0.90 chicks fledged per breeding pair over the study period (Figure 3). It is thought that because gannets can travel great distances (up to 500 km) from their nest site to forage⁹ and they are adaptable in what they eat (including live fish of various species and discards from the fishing industry), they rarely encounter food shortages. This and high adult survival rates, may be the main factors behind observed population increases.

From 2015 to 2019, the obligation that all catches of regulated commercial species had to be landed and counted against quota was phased in across the majority of EU fisheries. Following departure from the EU, the UK gained competency to manage fishing activities in its waters. This is regulated by the UK Fisheries Act 2020 which includes a 'Discard prevention charging scheme', although this has not yet been implemented. This is likely to have impacted seabird populations that feed on discards. Species native to the north-east Atlantic that are currently extensively exploiting fishery discards are kittiwake, herring gull, lesser black-backed gull, great black-backed gull, great skua, northern fulmar and the northern gannet¹⁰. It is, therefore, possible that this decline in discards may affect the future growth rate of the UK gannet population.

Figure 4. Trend in breeding productivity (no. of chicks fledged per pair) of northern gannet in Scotland, 1986–2019. Based on SMP data; view the methods of analysis.

Data on northern gannet productivity have been collected each year at several gannetries in Scotland. Productivity at these monitored colonies has generally been high, seldom lower than 0.60 chicks fledged per pair, and appears to have increased over the study period. Productivity was lower in the years 1987, 1990 and 1993, but the reasons for this are unclear. However, in 1993, productivity was severely reduced on Ailsa Craig due to poor weather (including snow and ice) in May. In 2015, northern gannets again had a successful breeding season on Ailsa Craig fledging 0.90 chicks per pair. During 2018, gannets in Scotland suffered a freezing spring followed by a very warm summer which may have contributed to the relatively low productivity recorded that year (0.73 chicks fledged per pair). In 2019, productivity increased slightly to 0.74 fledged per pair.

Figure 5. Abundance of northern gannet at Bempton Cliffs, England, 1986–2017.

Only one northern gannet colony exists in England, at Bempton Cliffs in Humberside. This colony has been increasing steadily since its formation in the 1960s, although in recent years the growth rate has escalated. A steep increase can be seen between counts in 2004 and 2017, although there was an uncharacteristic decrease in 2005. Between 2008 and 2009 numbers increased by just over 900 AON from 6,954 to 7,859. During the survey in 2009, a further 1,470 non-breeding immature birds were counted in 'club' areas of the gannetry which suggested further increases could be expected. The high rate of increase was sustained in 2017, when 13,392 nests were recorded. On average, approximately 700 AON have been added each year since 2009. Gannets at Bempton Cliffs were not counted in 2019.

Productivity

Gannet productivity at Bempton Cliffs between 1986 and 2019 has averaged 0.79 chicks fledged per pair.

Figure 6. Abundance of northern gannet on Grassholm, Wales, 1986–2015.

There is just a single gannet colony in Wales, although occasionally single pairs have set-up territories elsewhere without colonies becoming established. Changes in the size of the Grassholm gannetry have been documented since its foundation around 1820. Since the 1940s, when 6,000 apparently occupied sites (AOS) were estimated, the colony has grown rapidly, with 15,500 AOS estimated by aerial survey in 1964. Since 1984, counts have been made from aerial photographs, varying in quality of resolution and coverage. In 2009, using high resolution digital images, 39,282 AOS were recorded – making it the third largest gannetry in the UK and Ireland. The most recent survey in 2015 counted 36,011 AOS.

Directly comparable with counts in 1984, 1999 and 2004, in terms of image quality and coverage, the 2015 count suggests that the colony has decreased by 8% since 2009.

Productivity

Productivity data collected on Grassholm between 2002 and 2019 averaged 0.70 chicks fledged per pair.

Figure 7. Abundance of northern gannet on Ortac and Les Etacs, 1986–2016.

There are two gannetries in the Channel Islands, on Ortac and Les Etacs, which became established between 1940 and 1945. Small in size compared to some other gannet colonies around the British and Irish coastlines, they nevertheless show the familiar upward trend in numbers since Operation Seafarer. The most recent surveys in 2015 and 2016 recorded 2,777 AON on Ortac and 5,909 AON on Les Etacs. In summary, Les Etacs has apparently been increasing at a steady rate since 1986 while numbers on Ortac appear relatively stable over the same time period¹². No northern gannet abundance data from the Channel Islands have been submitted to the SMP since 2016.

Productivity

Data submitted to the SMP on the productivity of northern gannets on the Channel Islands are sparse; thus, no meaningful average productivity value can be given.