European shag (Phalacrocorax aristotelis)

Figure 1. Trends in abundance index of European shag 1986–2019 for the UK (red), Scotland (blue) and Wales (orange). Further details, including 95% confidence limits, can be found in the sections below. Based on SMP data; see the methods of analysis.

There has been pronounced regional variation in European shag abundance trends since monitoring began in 1986 (Figure 1). Because Scotland comprises most of the monitored nests/sites in the UK, their trend lines match closely. In Scotland, the European shag abundance index declined markedly in the early 1990s. It then increased gradually between 1994 and 2003 but has declined again since then, reaching its lowest ever value, at 47% below the baseline, in 2019. The index in Wales increased gradually after a considerable decline during the late 1980s but, since 2010, has declined again, albeit with a short-lived increase which peaked in 2017. European shag abundance data submitted to the SMP for England, Northern Ireland, Republic of Ireland, the Channel Islands and the Isle of Man are sparse; thus, no meaningful trend can be provided.

Productivity

Figure 2. Trends in breeding productivity (no. of chicks fledged per pair) of European shag 1986–2019 for the UK (red), Scotland (blue) and Wales (orange). Further details can be found in the sections below. Based on SMP data; view the methods of analysis.

The number of European shag sites monitored for productivity in Scotland comprises the majority of the UK sample and both have, therefore, followed a similar, relatively stable, trend since monitoring began in 1986. There is greater fluctuation in the Welsh productivity trend across the monitoring period, although this has remained above Scotland and UK throughout. European shag numbers in Northern Ireland, Republic of Ireland, the Channel Islands and the Isle of Man are sparse; thus, no meaningful trend can be provided.

Figure 3. Trend in UK abundance index (solid line) of European shag 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

Census data show that the UK shag population increased from 30,000 AON in 1969-70 to 36,000 AON in 1985-88, possibly as a result of better coverage of previously inaccessible coastlines through the use of inflatable boats; increased legal protection (e.g. under the Wildlife and Countryside Act 1981); and less persecution. However, the population had declined 27% by the time of Seabird 2000 (1998–2002). At the beginning of the SMP annual monitoring period, an initial steep rise in the abundance index (Figure 3) up to 1987 was probably due to many adults choosing not to breed in 1986 (e.g. on Canna and the Isle of May); thus, counts at many colonies were low that year.

The index shows how breeding abundance is heavily affected by the incidence of mass mortality events – or ‘wrecks’ – which occur during prolonged periods of onshore gales, when species such as shag find it hard to forage. Severe weather events, such as those in the winters of 1993/1994¹ (also preceded by a year when many pairs skipped breeding) and 2004/2005, considerably affected populations on the east coast of the UK. Subsequent recovery from the 1993/1994 'wreck' was slow, with breeding numbers not fully restored before the next 'wreck' occurred in 2004/2005. There appears to have been little recovery of the index before the next 'wreck', during the winter of 2012/13, when over 650 corpses were found between Orkney and Suffolk². However, by 2017, the index had recovered slightly to 32% below the baseline. Since then, it has declined again with the lowest value to date being recorded in 2019, at 49% below the 1986 baseline.

Shag return rates on the Isle of May show the impact of such 'wrecks' (Figure 13, below). Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have significant effects on population size³. In addition, declines may be related to a reduction in the availability of the shag’s preferred prey species, the sandeel (Ammodytes marinus)^4,5. European shag has recently been added to the UK Red list⁶ because of severe population decline; hence, continued (indeed enhanced) monitoring and ringing of the species is to be encouraged.

Table 1 shows how shag numbers have changed at colonies within the Special Protection Area (SPA) network in the period since they were surveyed for Seabird 2000. Numbers have fallen in most SPAs, except Mingulay and Berneray, with particularly large declines recorded on Foula, Fair Isle, and the Forth Islands.

Figure 4. Trend in UK productivity (no. of chicks fledged per pair) of European shag 1986–2019. Based on SMP data; view the methods of analysis.

Sandeels are a component of shag diet⁷ and shortages may have contributed to periodic low productivity (Figure 4) at some colonies. However, at one of the most-studied colonies, the Isle of May, although sandeel abundance has declined in recent years^4,5, the link between sandeel abundance and breeding success may not be as strong as previously thought (see ‘Diet’ section below for more information). Sandeel abundance is thought to have declined as a result of increases in sea surface temperature (SST)⁸ leading to changes in the abundance and composition of plankton^9,10 and reduced sandeel recruitment^11,12,13. Shag productivity in 2009, 2010, 2014, 2015 and 2016, which were comparatively better years for sandeels than many during the last decade, was high, in common with other sandeel specialists^14,15. In addition to the steep increase in 2014 (1.52 chick fledged per pair), productivity increased marginally in 2016 to 1.56. In 2019, an average of 1.46 shag chicks were fledged per pair in the UK.

Figure 5. Trend in abundance index (solid line) of European shag in Scotland, 1986–2019 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

In Scotland, long-term census data show an increase in shag abundance of 17% between Operation Seafarer and the Seabird Colony Register, after which numbers fell by nearly one-third by Seabird 2000. Scotland holds approximately 80% of European shags found nesting in the UK, hence the similarity of the two trends. The annual SMP abundance index (Figure 5), shows long term decline in the Scottish shag population, such that by 2014 the index was at its lowest level (52% below the 1986 baseline) to date. In 2019, it had recovered slightly to 47% below the baseline.

The initial steep rise in abundance between 1986 and 1987 was due to many adults choosing not to breed in 1986 (e.g. on Canna and the Isle of May) and dips in 1994 and 2005 due to 'wrecks' along the east coast in the preceding winters, from which numbers have been slow to recover (1994–2004) or appear to have not recovered at all (2005 to present). Yet another 'wreck' during the 2012-13 winter, centred on Aberdeenshire and the Firth of Forth, further lowered numbers breeding in east coast colonies.

By 2015, shag breeding numbers at three SPAs in Shetland (Fair Isle, Foula, and Sumburgh Head) had declined by c. -87% since Seabird 2000. A study of the European shag population and breeding dynamics at these colonies suggested that the majority of the decline could be accounted for by high mortality associated with prolonged gales in the late winters of 2003, 2011 and 2014¹⁶.

Productivity

Figure 6. Trend in productivity (no. of chicks fledged per pair) of European shag in Scotland, 1986–2019. Based on SMP data; view the methods of analysis.

Shag productivity in Scotland has averaged 1.28 chicks fledged per pair over the study period (1986–2019). The productivity trend peaked at 1.59 chicks fledged per pair in 2016 but had declined to 1.46 chicks per pair by 2019.

At Sumburgh Head, breeding success was 1.54 chicks fledged per incubated nest in 2019, the fifth highest measure on record and 44% higher than in 2018, when breeding success was 1.07. In 2018, many pairs failed to lay but those that did were successful in fledging young. For a high proportion of adults, severe storms and low temperatures in March that year may have reduced body condition to a level where laying was not possible. In 2019, a comparatively high proportion of the incubated nests at Sumburgh Head progressed to hatch (80.6%); mean brood size was unusually high (2.06); and there was a high proportion of fledged broods of three chicks (21.4%), indicating that food availability during the incubation and chick-rearing periods was not greatly constrained that year¹⁷.

European shags of both sexes can be partially migratory, meaning that a proportion of individuals remain resident at the breeding colony throughout the year, while the remainder migrate to other locations in the non-breeding season. A recent study¹⁸ found that resident shag pairs produced an average of 0.7 chicks per year more than those containing one or two migrant birds. Resident individuals and pairs may, therefore, contribute more recruits to the population per year, although this depends on whether survival of immatures is related to the migration strategy of their parents. If extreme events effect resident shags more on average, that produces the majority of young¹⁸.

Figure 7. Number of European shag AON on the Farne Islands, 1986–2019.  

National census data for England show a large increase in European shag breeding numbers between 1969-70 and 1985-88 but numbers were relatively stable between 1985-88 and 1998–2002. In England, European shag abundance data submissions to the SMP have fallen in recent years, therefore, a valid annual abundance trend could not be produced.

Severe weather events, causing mass mortality such as those in the winters of 1993/1994¹ and 2004/2005, considerably affected populations on the east coast of the UK. For example, on the Farne Islands (Figure 7), the largest and most frequently monitored colony in England, numbers fell from 1,948 to 771 AON between 1993 and 1994. Partial recovery there saw numbers increase to 1,678 AON by 2003 before another sharp decline to 937 AON by 2005. Since then, numbers have been fluctuating but decreasing. On the Farne Islands in 2018, poor weather conditions due to the ‘Beast from the East’ and subsequent storms in June had taken their toll on European shags, with just 476 AON being recorded, the lowest number since 1979 (1986–2018 average is 1,126 AON). Numbers were similar in 2019, with 484 AON being recorded.

Productivity

Figure 8. Trend in productivity (no. of chicks fledged per pair) of European shag at the Farne Islands, 1986–2019. Based on SMP data; view the methods of analysis.

Productivity of European shags at the Farne Islands (the most frequently monitored colony in England) declined after recording began in 1986, reaching a low point in 2004 when only 0.27 chicks were fledged per pair. Since then, productivity has increased in most years, with 2010, 2014 and 2015 being particularly successful. European shags breeding at this colony had an average productivity of 1.12 chicks fledged per pair between 1987 and 2015. Poor weather conditions on the Farne Islands due to the ‘Beast from the East’ in February 2018, and subsequent storms in June of that year, led to the lowest number of AON (476) being recorded since 1979. Despite these losses in AON, productivity was above average that year at 1.2 chicks fledged per pair, probably assisted by the dry and warm weather that followed in July.

Figure 9. Trend in abundance index (solid line) of European shag in Wales, 1986–2019 with 95% confidence limits (dotted lines; drawing of upper limit restricted to preserve detail in the abundance index). Based on SMP data; view the methods of analysis.

The European shag population in Wales, in common with England and Scotland, increased between the first two national censuses. In contrast to England, the abundance index of the Welsh population declined until the early 1990s, as was also the case in Scotland. It reached its lowest level in 1993 and, thereafter, the trend was generally upward until 2001. Between the last census and 2012 the trend has been relatively stable. The 'wrecks' preceding the 1994 and 2005 breeding seasons, which reduced breeding numbers and are common to both the Scottish and English indices, do not feature in the Welsh index. However, the 'wreck' during the 2012–2013 winter is likely to have effected numbers, as the index fell to 47% below the baseline in 2015. The index then climbed to its highest value, at 43% above the baseline, in 2017, but had declined again to 23% below the baseline in 2019.

Productivity

Figure 10. Trend in productivity (no. of chicks fledged per pair) of European shag in Wales, 1986–2019. Based on SMP data; view the methods of analysis.

Productivity of European shags in Wales has consistently been high (averaging 1.95 chicks fledged per pair between 1987 and 2019), with only two years when productivity has fallen below 1.6 chicks fledged per pair. Several colonies have been monitored over the years, although the most frequently monitored are Midland, Bardsey, and Puffin Island. In 2019, productivity on Bardsey (1.62 chicks fledged per nest) was below the long-time average (2.04; 1987–2019), while Midland (2.33 chicks fledged per nest) was above average (2.21; 1996–2019); no data were available for Puffin Island in 2019.

Figure 11. Abundance of European shag on the Calf of Man, 1986–2017. Data was not collected in 2003, 2004, 2007, 2008, 2016, 2018 and 2019.

National census data show little change in numbers of European shags on the Isle of Man between 1969-70 and 1985-88 but an increase of 60% by the time of Seabird 2000 (1998–2002). Few colonies on the Isle of Man are monitored regularly as part of the SMP, although during 2017-18 a seabird census was conducted which recorded 376 AON, a 41% decline since Seabird 2000 (912 AON) and the lowest total for the island since the late 1960s²¹.

The most frequently counted and largest colony is on the Calf of Man (Figure 11). Numbers there peaked in 1994 when 352 AON were recorded. During Seabird 2000, 218 AON were recorded on the Calf of Man, although counts were only done from land (land and sea during 1986 and 1994). In 2017, the European shag breeding population on the Calf of Man was 107 AON. No data on European shag were submitted to the SMP in 2019.

Productivity

Figure 12. Percentage of sandeels (by weight) in the diet of young shags at the Isle of May, 1987–2019.

Figure 12 shows a marked decline in the proportion by weight of sandeels in the diet of European shag chicks on the Isle of May^4,5,14, although during most of this period there was no clear negative effect on productivity; in fact, 2008 and 2009 were two of the most productive breeding seasons on record. Until the early 2000s, European shags on the Isle of May fed their young mainly sandeels^7,24 but, since then, chick diet has changed substantially, with a decline in sandeels and an increase in diet diversity⁷. European shags appear to be able to adjust their foraging behaviour in response to a change in the availability of different prey types within their foraging range^25,26,27,28. Therefore, although sandeel abundance may still be important, it may be less so when alternative prey are available during poor sandeel years, as appears to be the case around the Isle of May.

Return rates and survival rates

Figure 13. Annual return rate of European shag breeding on the Isle of May, 1987–2019.

Important notes on interpretation: Estimation of European shag adult return rate is currently only undertaken at one site within the Seabird Monitoring Programme – the Isle of May. Return rates are based on sightings of individually colour-ringed birds and are calculated as the proportion of marked birds present in year one that are seen in the following year. Because not every adult alive is seen each year, the return rates for 2018 presented need to be treated as a minimum estimate of survival of birds seen alive in 2017. In contrast, survival estimates do consider birds that are not seen one year but which re-appear in following years.

No clear trend in return rate is evident from the Isle of May data. In 2019, the return rate was estimated to be 77.6%, which was below the long-term average at the colony (79.0%, 95% CI = 72.8–85.7).

A notable feature of European shag biology is their susceptibility to die from periods of low food availability caused by unusually prolonged periods of strong onshore winds, which makes foraging difficult. Such 'wrecks' occurred around eastern Britain during the winters of 1994/95, 2004/05 and again in 2012/13 and are reflected in the very low return rates in the following breeding seasons (Figure 13). The 'wreck' during winter 2012/13 resulted in over 650 corpses being recovered from beaches between Orkney and Suffolk². It is, therefore, not surprising that the return rate for the Isle of May fell in 2013, although not all corpses recovered would have been from this colony. Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have important impacts on population size^2,29,30.