European Shag Phalacrocorax aristotelis

Status;  International importance;  Population estimates;  Distribution;  Annual abundance/ productivity; Phenology/diet/survival

 

Description

European Shag vignette

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:

EC Birds Directive - migratory species

Amber listed in Birds of Conservation Concern 3 (2009 update)

(further information on Conservation Designations for UK Taxa)

Amber listed in Birds of Conservation Concern in Ireland 2008-2013 (2013 update)

 


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.

 


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.

 


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 where colonies are found and how large they are, is provided in the Seabird 2000 European shag results page (PDF, 2.0 mb).

 

An interactive map is available on the NBN Gateway, where you can filter to display only the Seabird 2000 data.  For more recent, but less comprehensive, coverage view the distribution on the NBN with all available contributing datasets

 

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

 


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% CLs 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.   

 



 

Breeding abundance

 

UK Shag index 2012

Figure 1: Trend in UK abundance index (solid line) of European shag 1986-2012 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

The UK shag population increased slightly from 1969/70 (30,000 pairs) to 1985-88 (36,000 pairs), possibly as a result of better coverage of previously inaccessible coastlines through the use of inflatable craft, increased legal protection (e.g. under the Wildlife and Countryside Act 1981) and reduced persecution. The initial steep rise in the index up to 1987 is 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 19941 (also preceded by a year when many pairs took a year off from breeding) and 2005 (see under Survival), considerably affected populations on the east coast of the UK. Subsequent recovery from the 1994 wreck was slow with breeding numbers not fully restored before the wreck in 2005 occurred; there appears to have been no recovery since. Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have significant effects on population size3.  Predation of eggs and chicks by invasive mammalian predators is known to have localised effects on shag population size.

 

Productivity

 

UK shag breeding success 2012

Figure 2: Trend in UK productivity (no. of chicks fledged per pair) of European shag 1986-2012. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Shags are heavily reliant on sandeels2 and food shortages are likely to have contributed to periodic low productivity and population decline. Sandeel abundance is thought to have declined as a result of increases in sea surface temperature that changed the abundance and composition of plankton and reduced sandeel recruitment. However shag productivity in 2009, 2010 and 2011, which were better years for sandeels, was comparatively high, in common with other sandeel specialists.

 

Analysis of the SMP dataset by Cook and Robinson4 found mean breeding success of European shags at monitored nests was 1.21 chicks per nest per year between 1986 and 2008, and was relatively stable throughout the period. The quality of the dataset meant a decline in breeding success of 5% or greater could be detected with confidence. Were existing levels of breeding success to be 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 modest 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 success would need to fall to 0.90.

 

 

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

 

Scot Shag index 2012

Figure 1: Trend in abundance index (solid line) of European shag in Scotland, 1986-2012 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Long-term census data show an increase of 17% between Operation Seafarer and the Seabird Colony Register after which numbers fell by nearly one third by Seabird 2000. Scotland holds about 80% of the European shags found nesting in the UK so it is unsurprising 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 the 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 not have recovered at all (2005 to present). The result is the index value in 2012 is now almost 60% lower than the peak in 1987. In Shetland, non-breeding was recorded in 2011 and 2012 which resulted in low numbers of nests at some colonies. Between 2010 and 2012, numbers at Sumburgh Head fell from 290, to 100 then 79 AON and at Noness nest counts declined from 204, to 134 then 54 AON over the same period5. On Fair Isle, numbers have also crashed e.g. a whole colony count in 2003 recorded 732 AON compared to 235 AON in 2008, a year when breeding success also reached a nadir of just 0.03 chicks fledged per pair. Distinguishing non-breeding from early failure is not straightforward but may be indicated by paired adults standing at empty nest sites, or declining nest counts, over successive visits to a colony during the breeding season5. 

 

Productivity

 

Scot shag breeding success 2012

Figure 2: Trend in productivity (no. of chicks fledged per pair) of European shag in Scotland, 1986-2012. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Data on productivity collected at Scottish shag colonies has generally been higher than one chick fledged per pair each year but with no real trend. Productivity was at its lowest in 1990 for a variety of reasons. For example, detailed monitoring on the Isle of May in 1990 revealed over-winter survival (from 1989) was low, the breeding season was incredibly late, that many shags did not breed or even construct nests and only two-thirds that did have nests laid eggs plus adults also appeared to have trouble finding sufficient quality food during the season with few sandeels of any size available. Various combinations of these factors, together with inclement summer weather, have reduced overall productivity in other years e.g. 2004. However, regional variation exists within years. In 2012, shags in north Scotland had their most successful breeding season on record (1.95 chicks per nest) with those in north-west (1.55) and south-west (1.58) Scotland also quite productive. Colonies in Orkney had their least productive season (0.74) while colonies on Shetland, where it was a late season with extensive non-breeding in the south and south-east Mainland5, fledged the fewest chicks in 2012 (0.68) although success there has been lower in several other years. In south-east Scotland, success was 1.08 which was the lowest recorded since 2004 and 2005 in this relatively productive region.

 

 

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

 

Eng Shag index 2012

Figure 1: Trend in abundance index (solid line) of European shag in England, 1986-2012 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 (PDF 158 kb).

 

As for the UK, the affects of winter 'wrecks' can be seen in the abundance index for England in 1994 and 2005. Prior to both these events indices had generally been increasing. Population recovery from the 1994 crash was slow and still incomplete before the 2005 mortality event. For example, on the Farne Islands (Northumberland), numbers fell from 1,948 to 771 AON between 1993 and 1994. Partial recovery then saw numbers increase to 1,678 AON in 2003 before another sharp decline to 937 AON by 2005. There appears to have been no real recovery since either on the Farne Islands, where 965 AON were recorded in 2012, nor in the national trend; indeed the national index value for 2012 lies 12% below that for 1986 although the confidence limits are wide. Prior to this, national census data indicate a large increase had occurred between 1969/70 and 1985-88.

 

Productivity

 

Eng shag breeding success 2012

Figure 2: Trend in productivity (no. of chicks fledged per pair) of European shag in England, 1986-2012. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Productivity of European shags at English colonies had generally been falling since recording began but reached a nadir in 2004 when an average of only 0.41 chicks were fledged per pair. Since 2004, productivity has improved most years, with 2010 the most successful year on record. Most data has been collected on the Farne Islands, where low levels of breeding success 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 stage, combined with predation of nest contents by gulls early in the season, after which few replacement clutches were laid. Good (though unquantified) feeding conditions around the Farne Islands in 2012 ensured adequate numbers of chicks fledged despite inclement weather throughout much of the breeding season. 

 

 

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 Nests

 

Breeding Abundance

 

Wales shag index 2012

Figure 1: Trend in abundance index (solid line) of European shag in Wales, 1986-2012 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

European shag populations in Wales, in common with those in England and Scotland, increased between the first two national censuses. The abundance index indicates the Welsh population underwent a steady decline until the early 1990s, in contrast to that seen in England, although in Scotland this also occurred. Thereafter the trend was generally upward until 2001 but with no discernable pattern thereafter. The 'wrecks' of 1994 and 2005, common to both the Scottish and English indices, do not feature in the Welsh index. Currently, the index is on a par with that recorded in 1986.

 

Productivity

 

Analysis of productivity data from European shags collected at colonies in Wales revealed there was no statistically significant variation over time. Overall, shags in Wales were the most productive in the UK, fledging an average of 1.76 chicks per pair per year between 1987 and 2012.

 

 

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 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 at Strangford Lough, the most frequently monitored colony, ranged from eight to 22 between the Seabird Colony Register and Seabird 2000. However, recently this colony has declined with none breeding by 2007. On Rathlin Island, only 46 and 47 pairs were found in 2007 and 2011 respectively, compared with over 100 during the Seabird Colony Register. Based on this information it seems likely that the national population will have fallen since Seabird 2000.   

 

Productivity

 

No systematic data on the productivity of European shags in Northern Ireland have been submitted to the SMP.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 2,783 4,676 3,426
% change since previous census    N/a +68 -36

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

ROI Shag abundance 2012

Figure 1: Abundance of European shag on Lambay, 1986-2012.

 

Over the longer-term, national census data collected during Seabird 2000 indicated numbers have increased since Operation Seafarer in 1969/70, although numbers are currently lower than was recorded by the Seabird Colony Register in 1985-88. Few data have been collected at colonies since Seabird 2000, so it is difficult to assess the species' current status. Numbers at Lambay (Figure 1), the largest colony (holding about one-third of the national population) and most frequently monitored, have fluctuated. Surveys in 2009 and 2010 (no data was collected there in 2011 and 2012) recorded several hundred fewer nests compared to those in 2004 and 2007, although figures were on a par with counts during the 1990s. The cause of these changes is unknown.

 

Productivity

 

Few data on the productivity of European shags at colonies in the Republic of Ireland have been collected as part of the SMP. On average, six monitored colonies fledged 1.17 chicks per pair per year between 1996 and 2011 (although no data was submitted in several years including 2012).

 

 

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 Nests

 

Breeding Abundance

 

Few data have been collected throughout the whole of Ireland as part of the SMP. Therefore it is difficult to present a coherent picture on the current status of the European shag there. Please see entries for Northern Ireland and the Republic of Ireland instead. Over the longer-term, national census data collected during Seabird 2000 indicated numbers had increased since Operation Seafarer in 1969/70, although a decline was recorded since the Seabird Colony Register in 1985-88.

 

Productivity

 

No data on the productivity of European shags has been collected at colonies in Northern Ireland and few data has been submitted from colonies in the Republic of Ireland as part of the SMP. On average, monitored colonies fledged 1.17 chicks per pair per year between 1996 and 2011 (although no data was submitted in several years including 2012).

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 567 575 912
% change since previous census    N/a +1 +59

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

IOM shag abundance 2012

Figure 1: Abundance of European shag on the Calf of Man, 1986-2012.

 

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 during 1998-2002. Few colonies are monitored regularly as part of the SMP. The most frequently counted and largest colony is on the Calf of Man where close to 40% of the Manx population nests. Numbers of European shags breeding there have fluctuated over the recording period with no clear trend until the late 1990s, after which there is evidence of a decline. It is unknown if this decline is representative of numbers on the Isle of Man as a whole. No counts were carried out at this colony in 2011 and 2012.   

 

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.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 570 1,443 1,403
% change since previous census    N/a +153 -3

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

National census data show that the numbers of European shags in the Channel Islands recorded during 1998-2002 had more than doubled since 1969/70 from 570 pairs to 1,403, although there had been almost no change over the latter half of this period since the Seabird Colony Register in 1985-88. No data has been collected since Seabird 2000 so the current status is unknown.  

 

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.

 

 


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

 

Figure 3 shows a marked decline in the proportion by weight of sandeels in the diet of European shag chicks on the Isle of May, especially between 2004 and 2009, though this had no clear negative effect on productivity; in fact, 2008 and 2009 were two of the most productive breeding seasons on record. In these years, although sandeels still comprised the greatest single constituent, the remainder comprised gadids (members of the cod family, including whiting and rockling), clupeids (such as sprat), goby, butterfish and wrasse. In 2012, the most frequent prey (by occurrence in a regurgitate) in the 26 regurgitations collected was sandeel which occurred in 76.9% of samples and constituted 68.2% of the biomass, a large fall from the previous two years but still higher than the proportions recorded in 2004-2009. The remains of other items found were dragonet (present in 6 samples), flatfish (5), butterfish (3), wrasse (2), goby (2), eelpout (2) and clupeid (2).

 

IOM shag diet 2012

Figure 3: Percentage of sandeels (by weight) in the diet of young shags at the Isle of May (SE Scotland), 1987-2012.

 

Return rates and survival rates

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 in the Firth of Forth, SE Scotland. 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 is seen in the following year. Because not every adult alive is seen each year, return rates for 2012 presented here for Isle of May, although close to the highest on record, need to be treated as a minimum estimate of survival of birds seen alive in 2011. In contrast, survival estimates - (not currently undertaken for shag) do take into account 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 although the return rate for 2012 (92.9%) was markedly above the long-term average (79.4%, 95% CI = 71.9-86.8). 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 in early 1994 and in early 2005, reflected in the very low return rates in those years. Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have important impacts on population size3. In addition, European shags often take a year off breeding, which is manifested in return rates because, although they have not died, they are not seen at the breeding colony in the second year of a pair.

 

IOM shag return rate 2012

Figure 4: Annual return rate of European shag breeding on the Isle of May (SE Scotland), 1987-2012.

 


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

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 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, BTO, Thetford.

5 Heubeck, M. and Mellor, M. 2013. SOTEAG Ornithological Monitoring Programme: 2012 Summary Report. Aberdeen Institute of Coastal Science and Management, University of Aberdeen.

 


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; Centre for Ecology and Hydrology; Countryside Council for Wales; Department of Agriculture, Fisheries and Forestry (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 Environment Agency; 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 >>

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