Northern gannet (Morus bassanus)

Figure 1. Trend in UK abundance index of northern gannet 1986–2018.

Given the logistical problems of surveying gannet colonies, annual sampling of all but the smaller colonies is impractical. The few colonies counted annually tend to be the smaller and increase at a greater rate than larger colonies (see Table 1), leading to bias in any estimated trends. Instead, decadal censuses have been undertaken since the 1980s^1,2,3. The last census to cover all UK gannetries was carried out over two breeding seasons in 2003 and 2004⁴. Figure 1 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 to 218,000 AON in 2003/04 and reached 293,161 AON between 2013–2015.

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 the gannetries at Sule Stack and Scar Rocks, and new gannetries continue to be formed (Berneray, Western Isles) or re-colonised (Rockall, Western Isles). Since the 2013-15 gannet census, a few colonies have been surveyed again resulting in an additional 2,117 AOS (the dominant survey unit). Most colonies recorded an increase, however, at Troup Head numbers reduced from 6,456 AOS in 2014 to 3,847 AOS in 2016.

Figure 2. Trend in UK breeding productivity (no. of chicks fledged per pair) of northern gannet 1986-2018. Based on SMP data; view the methods of analysis.

Gannet productivity has varied very little since monitoring started in 1986. At monitored colonies, productivity generally lies between 0.60-0.90 chicks per breeding pair over the study period (Figure 2). 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.

Analysis of the SMP dataset found mean productivity of northern gannets to be 0.69 chicks per nest per year between 1986 and 2008¹⁰. The quality of the dataset meant a change in productivity of 10% or more would be detected with confidence. Using available life history information (population size, clutch size, age at first breeding and survival rates of different age classes), it was predicted that if this level of productivity were maintained then northern gannet populations may decline by 59% within 25 years. Results from population viability analysis further suggested that if productivity is less than one, such a decline is likely. Because populations of northern gannets are currently increasing and productivity can never be above one for a species that lays a single egg this suggests that survival (which is not currently measured as part of the SMP), or some other parameter, may be underestimated.

The new EU Common Fisheries Policy came into effect in 2014. It included the obligation that all catches of regulated commercial species had to be landed and counted against quota. From 2015 to 2019, the landing obligation was phased in across the majority of EU fisheries. This is likely to impact on 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 gannet¹¹. It is, therefore, possible that this decline in discards may affect the growth rate of the UK gannet population in the future.

Figure 3. Trend in breeding productivity (no. of chicks fledged per pair) of northern gannet in Scotland, 1986-2018. 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 low 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 gannet 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 low productivity recorded that year (0.73 chicks fledged per pair).

Figure 4. 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 appears to have escalated. A steep increase can be seen between counts in 2004 and 2017, although there is 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. Predictably, the high rate of increase was sustained in 2017, when 13,392 nests were counted. On average, approximately 700 AON have been added each year since 2009. Gannets at Bempton Cliffs were not counted in 2018.

Productivity

The productivity of gannets breeding at Bempton Cliffs shows no statistically significant variation over time; on average 0.80 chicks were fledged per nest between 1986 and 2018.

Figure 5. 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 have been collected on Grassholm since 2002. Analysis shows no statistically significant variation over time; an average of 0.71 chicks are fledged per nest each year.

Figure 6. Abundance of northern gannet on Ortac and Les Etacs, 1986–2015.

There are two gannetries in the Channel Islands, on Ortac and Les Etacs, which became established during 1940-45. Small in size compared to some other gannet colonies around the British and Irish coastlines, they nevertheless also show the familiar upward trend in numbers since Operation Seafarer visible at other gannetries. However, the most recent surveys, in 2015, recorded 2,777 AON on Ortac with 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 period¹³.

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.