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European shag (Phalacrocorax aristotelis)

The following has been adapted from original text by Sarah Wanless and Mike P. Harris in Seabird Populations of Britain and Ireland (with permission from A&C Black, London).

 

The European shag is endemic to the northeast Atlantic and the Mediterranean. An inshore species that is almost never observed out of sight of land, it takes a wide range of small fish that it catches on or near the seabed over both sandy and rocky substrates. The species nests on offshore islands or on cliffs and colonies range in size from a few to several thousand pairs.

Conservation status

European shag is currently identified as a conservation priority in the following:

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International importance

UK Population

Biogeographic Population

% World Population

26,600 AON*

38.3 (ssp. aristotelis)

34.1

*AON = Apparently Occupied Nests

The UK population figure (rounded to the nearest hundred) was derived from data in Mitchell, P.I., Newton, S.F., Ratcliffe, N. and Dunn, T.E. (eds.) 2004. Seabird Populations of Britain and Ireland. Poyser, London. This was also the source of figures for the Biogeographic and World populations.

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UK population estimates and change 1969–2002 (census data)

European shags build large conspicuous nests which superficially appear straightforward to count, but there are, however, several major problems associated with a widespread survey of this species: i) detection of nests – these can be hidden among boulders and in caves, or can easily be overlooked when present at low densities among other species in large cliff nesting colonies; ii) a prolonged and variable breeding season – in Britain eggs have been laid in every month of the year except September and October; iii) occasional years when many adults do not breed – however, such events tend to be localised and did not appear to be a problem during census years. Seabird 2000 aimed to overcome the second problem by conducting a single count in the period of maximum nest occupancy (1 May–25 June). Previous censuses probably suffered from similar problems, so these will all have led to the underestimation of the absolute size of the breeding population.

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

UK Population estimate (AON*)

29,956

36,276

26,565

% change since previous census   

n/a

+21

-27

*AON = Apparently Occupied Nests

For census results for individual countries and Ireland, the Channel Islands and the Isle of Man see under relevant sections below.

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Distribution/abundance

The Seabird 2000 census provides the most comprehensive recent assessment of the distribution and abundance of breeding seabirds. Numbers of European shag found in different regions, and a map showing the location and size of colonies, is provided in the Seabird 2000 European shag results page.

The locations sampled during the annual Seabird Monitoring Programme provide some information on distribution and are accessible via the Seabird Monitoring Programme online database.

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Annual abundance and productivity by geographical area

With reference to the regional accounts below please note the following:

Breeding abundance: graphs of abundance index with 95% confidence limits are only shown for a region where the trend produced has been deemed accurate (see methods of analysis). Where a trend was thought to be inaccurate, graphs of abundance at major colonies in a region may be shown instead, particularly if such colonies hold greater than 10% of the regional population, are monitored frequently and may thus help illustrate regional population fluctuations outwith national censuses. Occasionally, too few data have been collected regionally to produce either of these.

Productivity: graphs of productivity are only shown if analysis of breeding success data produced a significant result for regional and/or year effects (again see methods of analysis). If results were not significant, then a regional mean productivity value is given. However, on some occasions, too few data are available from which to provide a meaningful average. Furthermore, for 11 species where the quality of monitoring data available was considered high, population viability analysis was undertaken at the UK level and the results of this are also reported.

 

United Kingdom

Breeding abundance

Shag UK breeding abundance.jpg

Figure 1. Trend in UK abundance index (solid line) of European shag 1986-2018 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

 

The UK shag population increased slightly from 30,000 pairs in 1969-70 to 36,000 pairs 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 reduced persecution. However, numbers had fallen by 27% by the time of Seabird 2000. Annual sample data collected by the SMP also recorded an overall decrease between the mid-1980s and 2000 but the decline wasn’t consistent during that period with several fluctuations. At the beginning of the monitoring period, an initial steep rise in the index 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 trend also 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 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 no recovery before the 'wreck' during the winter of 2012/13, when over 650 corpses were found between Orkney and Suffolk2. It is, therefore, not surprising that index values for 2013 and 2014 (approximately 50% below the baseline) were the lowest yet recorded. By 2018, the index had increased slightly to 37% below the baseline. Measures of the return rates of shags to the Isle of May show the impact of such 'wrecks' (Figure 4, below). Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have significant effects on population size3. European shag has recently been added to the UK Red list4 because of severe population decline, hence continued (indeed enhanced) monitoring and ringing of the species is to be encouraged.

Table 1 shows how numbers have changed at some of the most important UK colonies (those in the SPA network) in the period since they were surveyed for Seabird 2000. Numbers have fallen in most SPAs, except on Mingulay and Berneray, and Puffin Island, with some particularly large declines recorded on Foula and Fair Isle.

 

Table 1. Recent counts of the number of European shag (AON) recorded in SPAs in Britain and Ireland compared to the number recorded in them during Seabird 2000. The percentage that each colony has changed by, and the per annum change, is also provided. (Note: data for St Abb's Head relates to only part of the SPA).

Area

SPA Name

Seabird 2000

Count (Year)

Change (%)

% per annum

Shetland

Foula

2,300 2000

48 2015

 -98

-22.7

Shetland

Fair Isle

663 2001

204 2013

-69

-9.4

East Coast

Buchan Ness to Collieston Coast

408 2001

363 2013

-11

-1.0

East Coast

Forth Islands

1,289 2001

824 2018

-36

-2.6

East Coast

St Abb's Head NNR

233 2000

95 2018

-59

-4.9

East Coast

Farne Islands

1,287 2000

688 2016

-47

-3.9

The Minch

Canna and Sanday

740 1999

282 2018

-62

-5.0

The Minch

Mingulay and Berneray

2811998

294 2014

+5

+0.3

Irish Sea

Lambay Island

1,122 1999

469 2017

-58

-4.7

Irish Sea

Calf of Man

220 2000

107 2017

-51

-4.2

Irish Sea

Puffin Island

220 1999

356 2018

+62

+2.7

South-West Coast

Isles of Scilly

1,092 1999

998 2015

-9

-0.5

 

Productivity

Shag UK productivity.jpg

Figure 2. Trend in UK productivity (no. of chicks fledged per pair) of European shag 1986–2018. Based on SMP data; view the methods of analysis.

 

Sandeels are a component of shag diet5 and shortages may have contributed to periodic low productivity (Figure 2) at some colonies. However, on the Isle of May, although sandeel abundance has declined in recent years6,7, 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 temperature8 leading to changes in the abundance and composition of plankton,9,10 and reduced sandeel recruitment11,12,13. 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 specialists14,15. In addition to the steep increase in 2014 (1.52 chick fledged per pair), productivity increased marginally in 2016 to 1.56. In 2018, an average of 1.16 shag chicks fledged per pair in the UK.

Analysis of the SMP dataset found mean breeding success of European shags at monitored colonies was 1.21 chicks per nest per year between 1986 and 2008, and was relatively stable throughout the period16. The quality of the dataset meant a decline in breeding success of 5% or greater could be detected with confidence. Where existing levels of breeding success were maintained, population viability analysis (using available life history information such as population size, clutch size, age at first breeding and survival rates of different age classes) suggests the national population may decline by a 9% over 25 years. Breeding success would need to fall to 1.10 for a decline in abundance of 25% over 25 years to occur. For the UK population to decline by over 50% in 25 years, breeding success would need to fall to 0.90. It should be noted that the population viability analysis did not consider climate change and predicted increases in severe weather events which can effect European shag survival rates.

 

Scotland

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register 

(1985-88)

Seabird 2000

(1998–2002)

Population estimate (AON*)

27,077

31,560

21,487

% change since previous census   

n/a

+17

-32

*AON = Apparently Occupied Nests

 

Breeding abundance

Shag Scotland breeding abundance.jpg

Figure 3. Trend in abundance index (solid line) of European shag in Scotland, 1986-2018 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, so it is not surprising that the post-1986 trends shown for both are similar. Common features are the initial steep rise to 1987 due to many adults choosing not to breed in 1986 (e.g. on Canna and the Isle of May) and dips in abundance 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. Fourteen east coast colonies between Orkney and Berwickshire counted in 2013 held a total of 1,872 AON compared to 3,433 AON in 2004, when abundance was at its highest during the last 20 years (Figure 3), representing a fall of 45% over that period. As a result of these 'wrecks' and non-breeding events, the abundance index value for Scotland in 2014 was almost 100% lower than its peak in 1987. The index has remained relatively stable since 2014.

In 2015, shag breeding numbers at three major SPA colonies 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 201417.

 

Productivity

Shag Scotland productivity.jpg

Figure 4. Trend in productivity (no. of chicks fledged per pair) of European shag in Scotland, 1986–2018. 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 (1986-2018), which is lower than in England (1.32) and Wales (1.93). Productivity peaked at Scottish colonies at 1.60 chicks fledged per pair in 2016 but then declined to 1.18 chicks per pair in 2018.

Productivity at Sumburgh Head in 2018 (1.07) was higher than in 2017 (0.85) but was below the average for all monitored years (1.12 for 1988–2017). 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. The long-term trend since 1986 has been a gradual decline in productivity, with particular poor productivity recorded in 2008, 2009, 2011, 2017 and 2018. In 2017 and 2018, on average European shags in Scotland fledged 1.29 and 1.18 chicks per pair respectively.

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 study showed resident shag pairs produced an average of 0.7 chicks per year more than those that contained one or two migrants. Resident individuals and pairs will therefore have higher breeding success and may contribute more recruits to the population per year, depending on whether survival of immatures is related to the migration strategy of their parents. If extreme events effect that produces the majority of young18.

 

England

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

2,111

3,491

3,863

% change since previous census   

n/a

+65

+11

*AON = Apparently Occupied Nests

 

Breeding abundance

Shag England breeding abundance.jpg

Figure 5. Trend in abundance index (solid line) of European shag in England, 1986-2018 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.

 

National census data indicate a large increase in European shag breeding numbers occurred between 1969-70 and 1985-88 but numbers were then stable between 1985-88 and 1998–2002. Detail provided by data collected for the SMP shows the effects of winter 'wrecks' in the abundance indices for England; 'wrecks' occurred in the winter preceding the 1994, 2005 and in 2013 breeding seasons. Prior to the first two events, the index had been rising, although had remained stable in the years immediately before the most recent 'wreck'. However, recovery from the 1994 crash was slow and still incomplete before the next crash in 2005. For example, on the Farne Islands, the largest and most frequently monitored colony in England, numbers fell from 1,948 to 771 AON between 1993 and 1994. Partial recovery saw numbers increase to 1,678 AON in 2003 before another sharp decline to 937 AON by 2005. There had been no further recovery up to 2012 when 965 AON were recorded. Since then, numbers have been fluctuating but decreasing. The net result is that the English index value for 2018 lies 53% below that of 1986, its lowest ever value. 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 of since 1979 (1986–2018 average is 1,126 AON).

 

Productivity

Shag England productivity.jpg

Figure 6. Trend in productivity (no. of chicks fledged per pair) of European shag in England, 1986–2018. Based on SMP data; view the methods of analysis.

 

Productivity of European shags at English colonies declined after recording began in 1986 and reached a low point in 2004, when an average of only 0.35 chicks were fledged per pair. Since 2004, productivity has increased in most years, with 2010 and 2014 being particularly successful breeding seasons. Almost all data have been collected from the Farne Islands, with few contributions from other colonies. European shags breeding at this colony have an average productivity of 1.12 chicks fledged per nest between 1987–2015. The low productivity recorded on the Farne Islands in 2004 (the effects of which are evident in the graph above) was in response to poor weather and low food availability during the chick rearing stage, combined with predation of nest contents by gulls early in the season, after which few replacement clutches were laid. 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 at 1.2 chicks fledged per pair, probably assisted by the dry and warm weather that followed in July of that year.

 

Wales

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000  

(1998–2002)

Population estimate (AON*)

550

785

914

% change since previous census   

n/a

+43

+16

*AON = Apparently Occupied Sites

 

Breeding abundance

Shag Wales breeding abundance.jpg

Figure 7. Trend in abundance index (solid line) of European shag in Wales, 1986–2018 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.

 

European shag populations in Wales, in common with those in England and Scotland, increased between the first two national censuses. In contrast to England, the abundance index of the Welsh population underwent a steady decline until the early 1990s, as was the case in Scotland. It reached its lowest level in 1993 and, thereafter, the trend was generally upward until 2001, coinciding with the increased numbers recorded by Seabird 2000. Since the last census 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. Since then, the index increased to 43% above the baseline in 2017, the highest index value since monitoring began, although declined in 2018 to almost the same level as 1986.

 

Productivity

Shag Wales productivity.jpg

Figure 8. Trend in productivity (no. of chicks fledged per pair) of European shag in Wales, 1986–2018. Based on SMP data; view the methods of analysis.

 

The productivity of European shags breeding in Wales has consistently been high, 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 colonies are Middleholm, Bardsey and Puffin Island. In 2018, productivity on Bardsey (2.08 chicks fledged per nest) and on Puffin Island (1.80 chicks fledged per nest) were slightly above average (Bardsey, 2.05 between 1987-2018 and Puffin Island 1.72 between 2010–2018); no data were available for Middleholm.

 

Northern Ireland

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

218

440

301

% change since previous census   

n/a

+102

-32

*AON = Apparently Occupied Nests

 

Breeding abundance

Census data show numbers of European shags in Northern Ireland doubled between Operation Seafarer and the Seabird Colony Register but then declined by approximately one-third by Seabird 2000. Very few sites holding European shags in Northern Ireland are monitored regularly, so it is difficult to assess the species' current status. Counts on the Isle of Muck on the west coast of the Larne Lough peninsula, the most frequently monitored colony, show a steady increase from 1987 to 2018 from 3 to 34 AON. On Rathlin Island, only 46 and 47 AON were recorded in 2007 and 2011 respectively, compared to over 100 during the Seabird Colony Register. In 2018, 79 AON were counted at three colonies (Muck Island, Gobbins and The Maidens) which held 115 AON during Seabird 2000. Based on this limited information, it is difficult to predict whether the national population has increased, declined or has been stable since the previous census.

 

Productivity

Data submitted to the SMP on the productivity of European shags in Northern Ireland are sparse; thus, no meaningful average productivity value can be given.

 

Republic of Ireland

Population estimates and change 1969–2018 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000  

(1998–2002)

Republic of Ireland Census

(2015-18)

Population estimate (AON*)

2,783

4,676

3,426

4,980

% change since previous census   

n/a

+68

-36

+45

*AON = Apparently Occupied Nests

 

Breeding abundance

Over the long-term, national census data collected during Seabird 2000 indicated numbers had increased since Operation Seafarer in 1969-70, although fewer were found than during the Seabird Colony Register census in 1985-88. Few colonies are monitored frequently, or in any one year. Between 2015 and 2018, the Republic of Ireland Seabird Census recorded 4,980 AON, an increase of 45% since 3,426 AON were recorded during Seabird 2000. The majority of AON (37%) were recorded on Lambay Island, one of the largest colonies in the country, where the population had declined by 58% since Seabird 2000 (1,122 AON)19. However, the large decline at Lambay was offset by increases at other nearby east coast sites (e.g. Howth Head and Ireland’s Eye) and a significant increase at Inishmurray. There were insufficient data from the Republic of Ireland to allow a trend to be generated for the period 1986 to 2018.

 

Productivity

Few data on the productivity of European shags at colonies in the Republic of Ireland have been submitted to SMP. On average, productivity was 1.17 chicks per pair per year between 1996 and 2011 (the most recent period when productivity data have been provided).

 

All Ireland

Population estimates and change 1969–2002 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998–2002)

Population estimate (AON*)

3,001

5,116

3,727

% change since previous census   

n/a

+70

-37

*AON = Apparently Occupied Sites

 

Breeding abundance

Over the long-term, national census data collected during Seabird 2000 indicated numbers had increased since Operation Seafarer in 1969-70, although fewer were found than during the Seabird Colony Register census in 1985-88. Very few sites holding European shags in Ireland are monitored regularly. Low numbers of shags breed in Northern Ireland compared to the Republic of Ireland, which holds c. 90% of the All Ireland population. Based on limited information from Northern Ireland, it is difficult to predict whether the national population has increased, declined or has been stable since the previous census. Between 2015 and 2018, the Republic of Irealnd Seabird Census recorded 4,980 AON, an increase of 45% since 3,426 AON were recorded during Seabird 2000. The majority of AON (37%) were recorded on Lambay Island, one of the largest colonies in the country, where the population had declined by 58% since Seabird 2000 (1,122 AON)19. However, the large decline at Lambay was offset by increases at other nearby east coast sites (e.g. Howth Head and Ireland’s Eye) and a significant increase at Inishmurray.

 

Productivity

Very few sites are monitored for productivity of European shags in Northern Ireland and few data have been submitted to the SMP from colonies in the Republic of Ireland. On average, monitored colonies fledged 1.75 chicks per pair per year between 1996 and 2018.

 

Isle of Man

Population estimates and change 1969–2018 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000

(1998–2002)

Isle of Man Census

(2017-18)

Population estimate (AON*)

567

575

912

376

% change since previous census   

n/a

+1

+59

-41

* AON = Apparently Occupied Nests

 

Breeding abundance

Shag IOM breeding abundance.jpg

Figure 9. Abundance of European shag on the Calf of Man, 1986–2017. Data was not collected in 2003, 2004, 2007, 2008, 2016 and 2018.

 

Long-term 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% thereafter by the time of Seabird 2000 (1998–2002). Few colonies are monitored regularly as part of the SMP. During 2017-18, a seabird census was carried out on the Isle of Man and recorded 376 AON, a 41% decline since Seabird 2000 (912 AON)20. The most frequently counted and largest colony is on the Calf of Man (Figure 9), where 225 AON were recorded during the SCR in 1986. Numbers increased up to 1994 when 352 AON were counted. During Seabird 2000, 218 AON were recorded, although counts were only done from land (land and sea during 1986 and 1994). Other land-based counts immediately after Seabird 2000 suggested numbers remained stable at this colony; 220 AON counted in 2000, 208 AON in 2001 and 205 AON in 2002. Similar numbers were recorded between 2011 and 2013 but either side of this period counts in 2005, 2010 and 2014 were far lower, in the range of 115-132 AON (with the lowest count recorded since 1986, 58 AON in 2015). In 2017, the European shag population at the Calf of Man colony was 107 AON. Clearly, such fluctuations mask any apparent trend at the colony.

 

Productivity

Data submitted to the SMP on the productivity of European shags on the Isle of Man are sparse; thus, no meaningful average productivity value can be given.

 

Channel Islands

Population estimates and change 1969–2016 (census data)

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000  

(1998–2002)

Channel Islands Census

(2015-16)

Population estimate (AON*)

570

1,443

1,403

707

% change since previous census   

n/a

+153

-3

-50

*AON = Apparently Occupied Nests

 

Breeding abundance

Seabird 2000 census data (1998–2002) showed that European shags numbers in the Channel Islands had more than doubled since the 1969-70 census, from 570 AON to 1,403, although there was almost no change over the latter half of this period (since the Seabird Colony Register census in 1985-88). During the Channel Islands Seabird Census (2015-16), 707 AON were recorded, a decrease of 14% since Seabird 2000 (818 AON)21. In the winter of 2013/14, the islands were struck by a sequence of severe storms and a total of 50,000 seabird deaths were recorded in the Channel Islands and along the coasts of Cornwall, Brittany and Normandy22. Shag was one of the worst affected species and declines in breeding numbers were reported during the 2014 season.

 

Productivity

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

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UK phenology, diet, survival rates

Phenology

No systematic data on phenology (timing of life-cycle events) have been collected as part of the SMP.

 

Diet

Shag percentage of sandeels in diet.jpg

Figure 10. Percentage of sandeels (by weight) in the diet of young shags at the Isle of May, 1987–2018.

 

Figure 10 shows a marked decline in the proportion by weight of sandeels in the diet of European shag chicks on the Isle of May14,6,7, 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 (see Figure 4 above). Until the early 2000s, European shags on the Isle of May fed their young mainly sandeels5,23,24 but, since then, chick diet has changed substantially, with a decline in sandeels and an increase in diet diversity5. 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 range25,26,27,28. 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

Shag annual retunr rate Isle of May.jpg

Figure 11. Annual return rate of European shag breeding on the Isle of May, 1987–2018.

 

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 2018, the return rate was estimated to be 68.0%, which was below the long-term average at the colony (79.6%, 95% CI = 73.0–86.2) and the lowest estimate since 2012. In 2018, the breeding season on the Isle of May commenced late for all species, including European shags, presumably as a consequence of the 'Beast from the East', which brought cold and windy conditions through much of March14.

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 11). 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. Measurements of shag return rates on the Isle of May show the impact of such 'wrecks' (Figure 11)14.

Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have important impacts on population size2,29.

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References

1 Harris, M.P. and Wanless, S. 1996. Differential responses of guillemot Uria aalge and shag Phalacrocorax aristotelis to a late winter wreck. Bird Study, 43, 220–230.

2 Newell, M., Wanless, S., Harris M.P. and Daunt, F .2015. Effects of an extreme weather event on seabird breeding success at a North Sea colony. Marine Ecology Progress Series, 532, 257–268.

3 Frederiksen, M., Daunt, F., Harris, M.P. and Wanless, S. 2008. The demographic impact of extreme events: stochastic weather drives survival and population dynamics in a long-lived seabird. Journal of Animal Ecology, 77(5), 1020–1029.

4 Eaton, M.A., Aebischer, N.J., Brown, A.F., Hearn, R.D., Lock, L., Musgrove, A.J., Noble, D.G., Stroud, D.A. and Gregory, R.D. 2015. Birds of Conservation Concern 4: the population status of birds in the United Kingdom, Channel Islands and Isle of Man. British Birds, 108, 708–746.

5 Harris, M.P. and Wanless, S. 1991. The importance of the lesser sandeel Ammodytes marinus in the diet of the shag Phalacrocorax aristotelis. Ornis Scandinavica, 22, 375–82.

6 Howells, R.J., Burthe, S., Green, J.A., Harris, M.P., Newell, M.A., Butler, A., Johns, D.G., Carnell, E.J., Wanless, S. and Daunt, F. 2017. From days to decades: short- and long-term variation in environmental conditions affect diet composition of a marine top-predator. Marine Ecology Progress Series, 583, 227–242.

7 Howells, R.J., Burthe, S., Green, J.A., Harris, M.P., Newell, M.A., Butler, A., Wanless, S. and Daunt, F. 2018. Pronounced long-term trends in year-round diet composition of the European shag Phalacrocorax aristotelis. Marine Biology, 65, 188.

8 Dye, S.R., Hughes, S.L., Tinker, J., Berry, D.I., Holliday, N.P., Kent, E.C., Kennington, K., Inall, M., Smyth, T., Nolan, G., Lyons, K., Andres, O. and Beszczynska-Möller, A. 2013. Impacts of climate change on temperature (air and sea). MCCIP Science Review 2013, 1–12.

9 Luczak, C., Beaugrand, G., Lindley, J.A., Dewarumez, J.M., Dubois, P.J. and Kirby, R.R. 2012. North Sea ecosystem change from swimming crabs to seabirds. Biological Letters, 8, 821–824.

10 Frederiksen, M., Anker-Nilssen, T., Beaugrand, G. and Wanless, S. 2013. Climate, copepods and seabirds in the boreal Northeast Atlantic – current state and future outlook. Global Change Biology, 19, 364–372.

11 van Deurs, M., van Hal, R., Tomczak, M. T., Jónasdóttir, S. H., and Dolmer, P. 2009. Recruitment of lesser sandeel Ammodytes marinus in relation to density dependence and zooplankton composition. Marine Ecology Progress Series, 381, 249–258.

12 MacDonald, A., Heath, M. R., Edwards, M., Furness, R. W., Pinnegar, J. K., Wanless, S., Speirs, D.C. and Greenstreet, S.P.R. 2015. Climate driven trophic cascades affecting seabirds around the British Isles. Oceanography and Marine Biology. An Annual Review 53, 55–80.

13 Régnier, T., Gibb, F.M. and Wright, P.J. 2019. Understanding temperature effects on recruitment in the context of trophic mismatch. Scientific Reports, 9(1), Article number: 15179.

14 Newell, M, Harris, M.P., Burthe, S., Bennett, S., Gunn, C.M., Wanless S. and Daunt, F. 2019. Isle of May seabird studies in 2018. Unpublished JNCC Report, Peterborough.

15 Wanless, S., Harris, M.P., Newell, M.A., Speakman, J.R. and Daunt, F. 2018. Community-wide decline in the occurrence of lesser sandeels Ammodytes marinus in seabird chick diets at a North Sea colony. Marine Ecology Progress Series, 600, 193–206.

16 Cook, A.S.C.P. and Robinson, R.A. 2010. How representative is the current monitoring of breeding success in the UK? BTO Research Report, No. 573, British Trust for Ornithology, Thetford.

17 Heubeck, M., Mellor, R.M., Gear, S and Miles, W.T.S. 2015. Population and breeding dynamics of European Shags Phalacrocorax aristotelis at three major colonies in Shetland, 2001-15. Seabird, 28, 55–77.

18 Grist, H., F. Daunt, S. Wanless, S. J. Burthe, M. A. Newell, M. P. Harris and J. M. Reid. 2017. Reproductive performance of resident and migrant males, females and pairs in a partially migratory bird. Journal of Animal Ecology, 86, 1010–1021.

19 Cummins, S., Lauder, C., Lauder, A. and Tierney, T. D. 2019. The Status of Ireland’s Breeding Seabirds: Birds Directive Article 12 Reporting 2013 – 2018. Irish Wildlife Manuals, No. 114. National Parks and Wildlife Service, Department of Culture, Heritage and the Gaeltacht, Ireland.

20 Hill, R.W., Morris, N. G., Bowman, K. A. and Wright, D. 2019. The Isle of Man Seabird Census: Report on the census of breeding seabirds in the Isle of Man 2017-18. Manx BirdLife. Laxey, Isle of Man.

21 Veron, M. and Veron, C. 2016. Seabird Count 2015; monitoring the status of Guernsey’s Seabirds. La Société Transactions, Channel Islands.

22 Morley, T.I., Fayet, A.L., Jessop, H., Veron, P., Veron, M., Clark, J. and Wood, M.J. 2016. The seabird wreck in the Bay of Biscay and South-Western Approaches in 2014: A review of reported mortality. Seabird, 29, 22–38.

23 Harris, M.P. and Wanless, S. 1993. The diet of Shags Phalacrocorax aristotelis during the chick-rearing period assessed by three methods, Bird Study, 40(2), 135–139

24 Daunt, F., Wanless, S., Greenstreet, S.P.R., Jensen, H., Hamer, K.C., Harris, M.P. 2008. The impact of the sandeel fishery closure in the northwestern North Sea on seabird food consumption, distribution and productivity. Canadian Journal of Fish Aquatic Science, 65, 362−381.

25 Wanless, S., Gremillet, D. and Harris, M.P., 1998. Foraging activity and performance of shags Phalacrocorax aristotelis in relation to environmental characteristics. Journal of Avian Biology, 29, 49-54.

26 Daunt, F., Wanless, S., Harris, M.P., Money, L. and Monaghan, P. 2007. Older and wiser: improvements in breeding success are linked to better foraging performance in European shags. Functional Ecology, 21, 561−567.

27 Bogdanova, M.I., Wanless, S., Harris, M.P., Lindström, J., Butler, A., Newell, M.A., Sato, K., Watanuki, Y., Parsons, M. and Daunt, F., 2014. Among-year and within-population variation in foraging distribution of European shags Phalacrocorax aristotelis over two decades: implications for marine spatial planning. Biological Conservation, 170, 292–299.

28 Michelot, C., Pinaud, D., Fortin, M., Maes, P., Callard, B., Leicher, M., Barbraud, C. 2017. Seasonal variation in coastal marine habitat use by the European shag: Insights from fine scale habitat selection modeling and diet. Deep Sea Research Part II: Topical Studies in Oceanography, 141, 224–236.

29 Rahmstorf, S. and Coumou, D. 2011. Increase of extreme events in a warming world. Proceedings of the National Academy of Sciences of the United States of America, 108, 17905-17909.

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Partners

Data have been provided to the SMP by the generous contributions of its partners, other organisations and volunteers throughout Britain and Ireland. Partners to the SMP are: BirdWatch Ireland; The British Trust for Ornithology; UK Centre for Ecology and Hydrology; Natural Resources Wales; Department of Environment, Food and Agriculture (Isle of Man); Department of Environment, Heritage and Local Government (Republic of Ireland); States of Guernsey Government; JNCC; Manx Birdlife; Manx National Heritage; The National Trust; National Trust for Scotland; Natural England; Northern Ireland Department of Agriculture, Environment and Rural Affairs; The Royal Society for the Protection of Birds; Scottish Natural Heritage; Seabird Group; Shetland Oil Terminal Environmental Advisory Group; Scottish Wildlife Trust. More about the SMP partners >>

 

Image of European shag appears courtesy of Ian Rendall ©, is subject to international copyright law and may not be reproduced in any form whatsoever.

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