Great Cormorant Phalacrocorax carbo

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

 

Description

Great Cormorant vignette

The following has been adapted from original text by Robin M. Sellers in Seabird Populations of Britain and Ireland (with permission from A&C Black, London).

 

Historically, great cormorants have been regarded as primarily coastal birds in Britain and Ireland, but during the last 40 years there has been a gradual shift of wintering quarters inland, to the extent that almost every lowland lake and river has some. In England, the number nesting inland in trees has increased from just 151 pairs at one colony in 1986 to 1,334 pairs at 35 colonies in 1999-2002. This growth of inland colonies has been fuelled by the immigration of the sub-species P. c. sinensis from continental Europe. P. c. carbo nests predominantly on the coast and constitutes most of the UK population, which accounts for 13% of the world population that is restricted to the northern Atlantic coasts.

 


Conservation status

 

Great cormorant is currently identified as a conservation priority in the following:

EC Birds Directive - migratory species

Not 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
8,884 AON* 2.4 (ssp. carbo/sinensis) 1.5

 

*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.

Note: The UK population figure above includes data from both inland and coastal colonies hence differs from that tabled below.

 


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

 

Great cormorants build large conspicuous nests with coastal colonies normally situated on stacks, rocky islets, cliffs or rocky promontories. Many colonies persist in the same place for long periods, but others come and go or suddenly shift location – the presence of a colony in one year is no guarantee that there will be one there the following year. This introduces uncertainty in population size when counts from a number of years have to be combined, as was the case during all three national censuses. To limit this problem, an effort was made during Seabird 2000 to reduce the number of years over which counts were obtained. However, the timing of breeding by different pairs of great cormorants within the same colony is not always synchronous resulting in no guarantee that a single count of the nests will reflect precisely the true number of breeding attempts. Seabird 2000, like previous censuses, conducted a single count at an optimum time within a given year (1 May-25 June), so population estimates are comparable although the absolute size of the breeding population is probably underestimated.

 

 

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

UK Population estimate (AON*) 6,071 6,610 7,238
% change since previous census N/a +9 +10

 

*AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the two earlier national censuses so to enable direct comparison the Seabird 2000 figure refers to the coastal population only.

 

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 great cormorant found in different regions, and a map showing where colonies are found and how large they are, is provided in the Seabird 2000 great cormorant 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 Great Cormorant index 2012

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

 

The abundance index for cormorant (which includes inland and coastal breeders) increased between 1986 and 1995 but has mostly been in decline since then, bar a short period of increase from 2000-2004. The index in 2012 has fallen back to levels recorded at the start of the SMP. Prior to this, census results indicate that the coastal population increased slightly between 1969/70 (6,100 pairs) and 1985-88 (6,600 pairs). In the UK, inland breeding cormorants are largely confined to England and increased markedly during the 1990s (helped in part by immigration of birds from continental Europe of subspecies P. c. sinensis) but have declined since 2002.  At the last census, in 1999-2002, inland breeders in the UK comprised about 18% of the total breeding population (c.1,646 of 8,884 AON). There is pronounced regional variation in the trends of abundance in great cormorant; populations in northern Scotland have declined particularly severely. Increases in abundance up to 1995 are likely to have been facilitated by increased legal protection instigated under the Wildlife and Countryside Act 1981. Factors responsible for recent declines are likely to include increased mortality from licensed and unlicensed shooting1 2, as well as possible changes in food availability.

 

Productivity

 

UK cormorant breeding success 2012

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

 

Average productivity has declined over the monitored period and has been relatively low since 2005; the causes of this are unknown.

 

Analysis of the SMP dataset by Cook and Robinson3 found the average breeding success in the great cormorant to be 1.89 between 1986 and 2008 but was declining at a rate of 0.027 chicks per nest per year. This equates to a decline in breeding success of 47% between 1986 and 2008. The quality of the dataset meant a change greater than 5% over 25 years would be detected with confidence. Although breeding success has shown a significant decline over this time period, because the number of nests monitored each year fluctuated widely from 48 in 1989 to 1,095 in 2002, this trend may not be representative of the population as a whole. If this level of average breeding success was 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 a large population increase, greater than 200%, may be expected over the next two decades. For the population to decline by 25% over 25 years, breeding success would have to fall to 0.70 chicks per nest per year.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 3,438 2,986 3,626
% change since previous census    N/a -13 +21

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the two earlier national censuses so to enable direct comparison the Seabird 2000 figure refers to the coastal population only.

 

Breeding Abundance

 

Between Operation Seafarer and the Seabird Colony Register the great cormorant population declined by 13% although numbers had subsequently recovered by Seabird 2000, increasing by 21% since the previous census. Although the SMP annual sample is not thought to be representative of the national population (e.g. the derived trend from SMP data indicates a decline between SCR and Seabird 2000 in contrast to the above table) the number of breeding pairs has probably declined since Seabird 2000. For example, 39 colonies counted in 2012 held 1,028 pairs, 29% fewer than the 1,457 pairs they held in 2000, so numbers may well be on the wane. Cormorant colonies can move location quite quickly so apparent declines at those monitored regularly may be due to birds moving to nest elsewhere. These new colonies may not be detected during routine monitoring by the SMP but will be detected by national censuses, when coverage is complete.

 

Productivity

 

Great cormorants at Scottish colonies on average fledged approximately 1.81 chicks per pair per year between 1991 and 2012; there was no statistically significant variation over time.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 1,057 1,220 1,315
% change since previous census    N/a +13 +8

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the two earlier national censuses so to enable direct comparison the Seabird 2000 figure refers to the coastal population only.

 

Breeding Abundance

 

England Great cormorant index 2012

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

 

The abundance of great cormorants at colonies in England peaked in 2003 but has fallen since, although there has been some fluctuation in the trend in recent years. National census data also indicates that numbers at coastal colonies have increased since 1969/70. The current population would appear to be on a par with the 1,315 AON recorded during Seabird 2000.

 

In the UK, inland breeding cormorants are largely confined to England and increased markedly during the 1990s (helped in part by immigration of birds from continental Europe of subspecies P. c. sinensis), but are thought to declined since 2002.  At the last census, in 1999-2002, the inland breeding population totalled 1,332 pairs - slightly more than recorded on the coast.  

 

Productivity

 

Great cormorants at colonies in England on average fledged approximately 1.65 chicks per pair per year between 1986 and 2012; there was no statistically significant variation over time.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 1,468 1,668 1,634
% change since previous census    N/a +14 -2

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the two earlier national censuses so to enable direct comparison the Seabird 2000 figure refers to the coastal population only.

 

Breeding Abundance

 

Wales Great cormorant index 2012

Figure 1: Trend in abundance index (solid line) of great cormorant in Wales, 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).

 

National census data, together with the trend calculated from colonies sampled by the SMP, indicate that great cormorant abundance in Wales has remained fairly stable since 1969/70 although there has been some fluctuation. The trend peaked in 1989 and 1994 with troughs in 1991, 1999 and 2002. Few inland colonies exist in Wales. 

 

Productivity

 

Wales cormorant breeding success 2012

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

 

Collection of productivity data at colonies in Wales has been sporadic, especially in the first half of the recording period, such that no clear trend is evident. However, recently there appears to have been a decline in the number of chicks fledged each year although the cause of this is unknown. Unfortunately, no data was collected in 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*) 108 736 663
% change since previous census    N/a +581 -10

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the two earlier national censuses so to enable direct comparison the Seabird 2000 figure refers to the coastal population only.

 

Breeding Abundance

 

In Northern Ireland, where there are only a few great cormorant colonies, numbers underwent a large increase between Operation Seafarer and the Seabird Colony Register, multiplying almost six-fold. This was followed by a decline, with numbers during Seabird 2000 being 10% lower than in 1985-88. Since then, the most frequently surveyed colony, at Strangford Lough (the second largest colony in the country), increased from 278 pairs in 2000 to 403 pairs in 2012 (with a peak of 490 pairs in 2005). However, a count of Sheep Island (the largest colony in Seabird 2000, holding 344 pairs in 1999) in 2009 recorded only 182 AON. Without data from any other colonies, nor data from Sheep Island since 2009, it is difficult to comment exactly on the likely current population size of great cormorants in Northern Ireland but it would seem unlikely to be greater than that recorded by Seabird 2000.

 

Productivity

 

No systematic data on the productivity of great cormorants 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*) 1,842 3,981 4,073
% change since previous census    N/a +116 +2

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the two earlier national censuses so to enable direct comparison the Seabird 2000 figure refers to the coastal population only.

 

Breeding Abundance

 

Great cormorant numbers in the Republic of Ireland more than doubled between Operation Seafarer and the Seabird Colony Register to 3,981 pairs. Seabird 2000 found numbers had been stable since the SCR having increased by fewer than 100 pairs. Data are received from very few colonies each year but the three largest colonies during Seabird 2000, Lambay, Ireland's Eye and St. Patrick's Island, were all counted in 2009 and 2010 and held totals of 1,706 and 1,385 pairs in these years respectively (c.f. 1,539 pairs in 1999). The current status of the national population is, however, largely unknown as cormorant colonies may move between years and therefore increases at some colonies may be offset by decreases elsewhere. Only three colonies of smaller stature (each holding less than 200 AON) have been counted since 2010 so add little to our knowledge of current trends.   

 

Productivity

 

Data submitted to the SMP on the productivity of great cormorants in the Republic of Ireland 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*) 1,950 4,717 4,736
% change since previous census    N/a +142 <+1

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the two earlier national censuses so to enable direct comparison the Seabird 2000 figure refers to the coastal population only.

 

Breeding Abundance

 

For the whole of Ireland combined, national censuses show an increase between Operation Seafarer and the Seabird Colony Register, but numbers between the SCR and Seabird 2000 were similar. Since then, data from a few of the largest colonies suggest numbers have been at best stable in the interim. However, cormorant colonies may move between years and therefore increases at some colonies may be offset by decreases elsewhere, so without recent comprehensive census data the current population status is uncertain.   

 

Productivity

 

No systematic data on the productivity of great cormorants in Northern Ireland have been submitted to the SMP, while data submitted from colonies in the Republic of Ireland 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*) 35 102 134
% change since previous census    N/a +191 +40

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

Great cormorant numbers on the Isle of Man almost trebled between Operation Seafarer and the Seabird Colony Register to 102 pairs. By the time of Seabird 2000 a further increase had occurred with numbers rising to 134 pairs. Colonies are few; all were surveyed in 2008 and 2011 when totals of 232 and 206 pairs were counted respectively. The 2011 data indicates an increase of 54% since 1999 when all were counted for Seabird 2000.

 

Productivity

 

Data submitted to the SMP on the productivity of great cormorants in 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*) 62 113 115
% change since previous census    N/a +82 +2

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

National census data indicate numbers of great cormorants have doubled on the Channel Islands since Operation Seafarer in 1969/70. However, this increase occurred before the 1985-88 census as numbers recorded during the Seabird Colony Register and Seabird 2000 were virtually identical. No data have been collected since the 1998-2002 census. 

 

Productivity

 

No systematic data on the productivity of great cormorants on the Channel Islands have been submitted to the SMP.

 

 


UK phenology, diet, survival rates

 

No data have been collected as part of the Seabird Monitoring Programme.

 


References

1 Carss, D.N. 1994. Killing of piscivorous birds at Scottish finfish farms, 1984-1987. Biological Conservation 68, 181-8.

2 Wernham, C.V., Armitage, M., Holloway, S.J., Hughes, B., Hughes, R., Kershaw, M., Madden, J.R., Marchant, J.H., Peach, W.J. and Rehfisch, M.M. 1999. Population, Distribution, Movements and Survival of Fish-eating Birds in Great Britain. Department of the Environment, Transport and the Regions Report, London.

3 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.

 


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 >>

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