Great Cormorant Phalacrocorax carbo

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

 

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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

Green listed in Birds of Conservation Concern 4 (2015 update)

(further information on Conservation Designations for UK Taxa)

Amber listed in  Birds of Conservation Concern in Ireland 2014-2019 (2014 update)


International importance

 

UK Population % Biogeographic Population % World Population
8,900 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 at the same location 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*)

Inland

Total

6,071 6,610

7,238

1,646

8,884

% change since previous census N/a +9 +10

 

*AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies 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 the location and size of colonies, is provided on 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 cormorant abundance trend

Figure 1: Trend in UK abundance index (solid line) of great cormorant 1986-2015 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, apart from a short period of increase between 2000 and 2004. The index in 2015 lies close to that 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).  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 severely. In England, inland colonies at least have increased with 2,362 pairs nesting in 20121. In Wales, numbers have been more stable. 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 shooting2, 3, as well as possible changes in food availability.

 

Productivity

 

Great cormorants at colonies in the United Kingdom fledged approximately 1.84 chicks per nest per year between 1991 (no breeding success data were submitted before this year) and 2015; there was no statistically significant variation over time.

 

However, analysis of the SMP dataset 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 year4. 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*)

Inland

Total

3,438 2,986

3,626

        0

3,626

% change since previous census    N/a -13 +21

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies 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 Scottish 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 the last national census. For example, compared with data collected during Seabird 2000, 17 colonies counted in 2014 held 519 AON (cf.927 AON), and the same 17 colonies counted in 2015 held 612 AOS, 44% and 34% fewer, respectively, suggesting numbers have fallen. However, 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 geographical coverage is complete.

 

Productivity

 

Great cormorants at Scottish colonies on average fledged approximately 1.89 chicks per nest per year between 1991 (no breeding success data were submitted before this) and 2015; 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*)

Inland

Total

1,057 1,220

1,315

1,581

2,896

% change since previous census    N/a +13 +8

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies only.

 

Breeding Abundance

 

England cormorant abundance trend

Figure 1: Trend in abundance index (solid line) of great cormorant in England, 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

National census data indicate that numbers at coastal colonies in England have increased between 1969-70 and 1998-2002. In conjunction with this, inland breeding cormorants (in the UK largely confined to England) increased markedly during the 1990s, helped in part by immigration of birds from continental Europe of the subspecies P. c. sinensis. During Seabird 2000, the inland breeding population totalled 1,581 AON - slightly more than recorded on the coast. In 2012, this inland population was estimated at 2,362 pairs breeding at 48 sites4. Figure 1 shows how abundance at colonies (both coastal and inland) has increased since 1986. The trend climbs until 2003, after which there is some fluctuation, although index values remain high compared to the 1986 baseline. This contrasts markedly with the UK trend which has been falling during the last decade due to declines recorded in Scotland and Northern Ireland. A total of 13 smaller colonies counted in 2014 held 812 AON, and the same colonies surveyed in 2015 held 815 AOS, suggesting a stable population at these colonies.

 

Productivity

 

The productivity of great cormorants at colonies in England showed no statistically significant variation over time; approximately 1.63 chicks were fledged per pair per year between 1986 and 2015.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*)

Inland

Total

1,468 1,668

1,634

      65

1,699

% change since previous census    N/a +14 -2

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies only.

 

Breeding Abundance

 

Wales cormorant abundance trend

Figure 1: Trend in abundance index (solid line) of great cormorant in Wales, 1986-2015 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, albeit with some fluctuation. The trend peaks in 1989 and 1994 with troughs in 1991, 1999 and 2002. Although the index has declined in recent years it suggests numbers may still be on a par with that recorded during the Seabird Colony Register and Seabird 2000. Few inland colonies exist in Wales; no updated information is available for these. The largest two colonies of great cormorant in Wales, at Puffin Island and Little Orme, have increased between 2010 to 2014 by 16% and 41%, respectively.

 

Productivity

 

Wales cormorant productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of great cormorant in Wales, 1986-2015. 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, there appears to have been a decline in the number of chicks fledged each year; the cause of this is unknown. No data were collected in 2012 or 2013. Aberdaron Coast and Bardsey Island SPA (Gwynedd) recorded a slight increase in productivity from 1.44 to 1.56 chicks fledged per pair between 2014 and 2015, while productivity on Skomer declined from 1.50 to 1.14 over the same period.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*)

Inland

Total

108 736

663

     0

663

% change since previous census    N/a +581 -10

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies only.

 

Breeding Abundance

 

Great Cormorant abundance at Strangford Lough

Figure 1: Abundance of great cormorant at Strangford Lough (Co. Down), 1986-2015.

 

In Northern Ireland, there are only six known great cormorant colonies. These held 663 AON during Seabird 2000, which was 10% fewer than that recorded during the SCR Census (736 AON) but six-times more than recorded by Operation Seafarer (108 AON). However, as recently as 2010, just three colonies (Strangford (Co. Down), Sheep and Little Skerries (both Co. Antrim)) held 755 AON - almost 14% higher than during Seabird 2000. At Strangford Lough, the most frequently surveyed colony (and also one of the largest colonies in the country), numbers increased substantially post Seabird 2000 from 125 AON to a peak of 490 AON in 2005 (Figure 1), although numbers have since declined; only 245 AON were recorded in 2015. Numbers of cormorants at Sheep Island have fluctuated annually but show and overall decrease since 1985 (380 AON) to just 66 AON in 2015.

The colony at The Gobbins cliffs have shown big changes in numbers recently, dropping as low as two AON in 2007, returning to 33 AON in 2008, but have fallen again to nine AON in 2015. No recent data have been received from the other extant colonies in County Down at Burial Island (67 AON in 2009) or North Rock (72 AON in 1997). In summary, four of six known colonies held 501 AON in 2014, but all are apparently in decline. Thus, it would seem likely that the Northern Ireland cormorant population is currently lower than during the last national census unless numbers at Burial Island and North Rock have remained stable.

 

Productivity

 

Data submitted to the SMP on the productivity of great cormorants in Northern 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*)

Inland

Total

1,842 3,981

4,073

   475

4,548

% change since previous census    N/a +116 +2

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies 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 relatively stable since the SCR, having increased by fewer than 100 pairs. Colonies monitored in the Republic of Ireland since Seabird 2000 are few, however a census at 24 colonies was carried out in 20155. At these a total of 810 AON were recorded compared to 989 AON during Seabird 2000, a decrease of 18%. However, this survey only covered a small proportion of the total national population of European shag in the Republic of Ireland. Little information exists as to population trends and no sizable colonies have been re-surveyed since Seabird 2000.   

 

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*)

Inland

Total

1,950 4,717

4,736

   475

5,211

% 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 stable in the interim. For example, in Northern Ireland, four of six known colonies held 509 AON in 2014, but numbers at all are apparently declining. In the Republic of Ireland, data are received from very few colonies each year, but the two largest colonies during Seabird 2000 (Lambay and Ireland's Eye both Co. Dublin) were counted in 2015. They respectively held totals of 282 and 424 pairs, a decrease of 58% and an increase of 39% in comparison to the Seabird 2000 counts. Cormorant colonies may move between years, therefore decreases at some colonies may be offset by increases elsewhere, so without comprehensive coverage the true status of the All-Ireland population is uncertain.

 

Productivity

 

Data submitted from colonies in Northern Ireland and the Republic of Ireland are sparse. Thus, no meaningful average productivity value can be given for 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*)

Inland

Total

35 102

134

     0

134

% change since previous census    N/a +191 +40

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies only.

 

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. All colonies were surveyed in 2008 and 2011 when totals of 232 and 206 pairs were counted, respectively. The 2011 data indicate an increase of 54% since 1999 when all colonies were counted for Seabird 2000. In 2014, only nine (of a potential 16 colonies) were surveyed, seven of which were empty (and have been for a number of years now) with the other two holding 45 AON, although little information can be deduced from this as to the current trend as some major colonies were not visited. No data were submitted to the SMP for 2015.

 

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*)

Inland

Total

62 113

115

     0

115

% change since previous census    N/a +82 +2

 

* AON = Apparently Occupied Nests

Note: Inland colonies were not counted during the first two national censuses, so, to enable direct comparison, the percentage change refers to coastal colonies only.

 

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. A number of small colonies are counted each year sporadically. In 2015, Les Amfroques held 34 AON compared to 20 AON during Seabird 2000 and Lihou Island was counted for the first time with 16 AON. 

 

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 Newson, S., Marchant, J., Sellers, R., Ekins, G., Hearn, R. and Burton, N. 2013. Colonisation and range expansion of inland-breeding cormorants in England. British Birds 106: 737-743.

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

3 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.

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 Newton, S., Lewis, L. and Trewby, M. 2015. Results of a Breeding Survey of Important Cliff‐Nesting Seabird Colonies in Ireland 2015. National Parks and Wildlife Service, Ireland.

 


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; 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 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.