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Great cormorant (Phalacrocorax carbo)

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:

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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.

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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)

Population estimate (AON*)

6,071

6,610

7,238

Inland numbers

-

-

1,646

Total Figure

-

-

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.

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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 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.

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

great-cormorant-uk-ab.jpg

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

In the UK, first-year survival and productivity is higher for inland breeding cormorants which is linked to earlier and greater food availability at inland sites4. Great cormorants at colonies in the United Kingdom fledged approximately 1.81 chicks per nest per year between 1991 and 2018; there was no statistically significant variation over time.


However, analysis of the SMP dataset found great cormorant average productivity to be 1.89 between 1986 and 2008 but declining at a rate of 0.027 chicks per nest per year5. This equates to a decline in productivity 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 productivity 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 productivity 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, productivity would have to fall to 0.70 chicks per nest per year.

 

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*)

3,438

2,986

3,626

 

Inland numbers

-

-

0

Total Figure

-

-

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 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, 15 colonies counted in 2018 held 932 AOS, 29% fewer, 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 SMP monitoring but will be detected by national censuses, when geographical coverage is complete.

 

Productivity

Great cormorants at Scottish colonies on average fledged approximately 1.81 chicks per nest per year between 1991 (no productivity data were submitted before this) and 2018; there was no statistically significant variation over time.

 

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*)

1,057

1,220

1,315

Inland numbers

-

-

1,581

Total Figure

-

-

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

great-cormorant-england-ab.jpg

Figure 2. Trend in abundance index (solid line) of great cormorant in England, 1986–2018 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis.

 

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 sites5. Figure 2 shows how abundance at colonies (both coastal and inland) has overall increased since 1986. The trend climbs until 2003, after which there is some slight fluctuation and then a steep climb between 2017 and 2018, where the highest index (200) since monitoring began was reached. This contrasts markedly with the UK trend which fell to its lowest point in 2013, although has recovered since to then.



Productivity

The productivity of great cormorants at colonies in England showed no statistically significant variation over time; approximately 1.65 chicks were fledged per pair per year between 1986 and 2018.

 

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*)

1,468

1,668

1,634

Inland numbers

-

-

65

Total Figure

-

-

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

great-cormorant-wales-ab.jpg

Figure 3. Trend in abundance index (solid line) of great cormorant 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.

 

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 1988 and 1994 with troughs in 1991, 1999, 2002 and in 2018. Although the index has declined in recent years, reaching its lowest value in 2018, 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 and no recent information is available for these. The largest two colonies of great cormorant in Wales have had contrasting fortunes between 2010 and 2018; at Puffin Island numbers have increased by 2% and at Little Orme they have decreased by -58%.

 

Productivity

great-cormorant-wales-prod.jpg

Figure 4. Trend in productivity (no. of chicks fledged per pair) of great cormorant in Wales, 1986–2018. Based on SMP data; view the methods of analysis.

 

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 although the cause is unknown. Great cormorants no longer breed on Skomer, with the colony moving from the Mew Stone to Middleholm in 2017, where 11 AON were counted in 2018.

 

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*)

108

736

663

Inland numbers

-

-

0

Total Figure

-

-

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-ni-ab.jpg

Figure 5. Abundance of great cormorant at Strangford Lough (Co. Down), 1986–2018.

 

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, from 2017 to 2018, five colonies (Strangford Lough, Burial Island, Gobbins, Little Skerries and Sheep Island) held 673 AON, a very similar number to the Seabird 2000 count. 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 5). Since then numbers have fallen back to 314 AONs in 20186. Numbers of cormorants at Sheep Island have fluctuated annually but has shown an overall decrease in size, from 380 AON in 1985 to 88 AON in 2018. The colony at Gobbins has fluctuated in numbers recently, dropping as low as two AON in 2007, increasing to 33 AON in 2008, but falling again to 12 AON in 2018. In summary, it would seem likely that the Northern Ireland cormorant population is currently 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.

 

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*)

1.842

3,981

4,073

4,688**

Inland numbers

-

-

475

-

Total Figure

-

-

4,548

-

% change since previous census

n/a

+116

+2

+15

*AON = Apparently Occupied Nests

** Combination of inland and coastal. Information on the exact split was not provided7.

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 (SCR) to 3,981 pairs. Seabird 2000 found numbers had been relatively stable since the SCR, having increased by fewer than 100 pairs. A total of 65 sites were counted during the recent Republic of Ireland Seabird Census (2015–2018), 21 more sites compared to Seabird 2000. At these a total of 4,688 AON were recorded compared to 4,548 AON during Seabird 2000, an increase of 3%. Less than 10% of the estimated breeding population of great cormorant occurred at inland sites5. There were insufficient data from the Republic of Ireland to allow trends to be generated for the period 2015 to 2018.

A 56% decline at Lambay Island has coincided with an increase at Ireland’s Eye, St. Patrick’s Island and a new colony at Bray Head (established 2009)8. No breeding great cormorants were recorded in 2017 at Lough Cutra, which held 150 AON during Seabird 2000. However, the population on Deer Island increased by 6% from 200 AON during Seabird 2000 to 212 AON in 2017 when surveyed using a drone9.

 

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.

 

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*)

1,950

4,717

4,736

Inland numbers

-

-

475

Total Figure

-

-

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

In summary, it appears that the Northern Ireland cormorant population is currently stable.

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, five known colonies held 673 AON in 2017/18, very similar to the count (663 AON) during Seabird 2000. The recent Republic of Ireland Seabird Census (2015–2018) recorded 4,688 AON, a 3% increase to the 4,548 AON counted during Seabird 2000, indicating that the all-Ireland great cormorant population of is currently stable. Less than 10% of the estimated breeding population was counted at inland sites7. Data from the Republic of Ireland are received from very few colonies each year, but the two largest colonies during Seabird 2000 (Lambay Island and Ireland's Eye) were counted in 20158. They held totals of 299 and 424 pairs respectively, a decrease of -56% and an increase of +39% compared to the Seabird 2000 counts. Cormorant colonies may move between years, therefore decreases at some colonies may be offset by increases elsewhere.

 

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.

 

Isle of Man

Population estimates and change 1969–2018 (census data)

 

Operation Seafarer

(1969-70)

Seabird Colony Register

(1984-88)

Seabird 2000

(1998–2002)

Isle of Man Census

(2017-18)

Population estimate (AON*)

35

102

134

251

Inland

-

-

0

0

Total Figure

-

-

134

251

% change since previous census

n/a

+191

+40

+87

*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 2011 and 2017 when totals of 206 and 251 pairs were counted, respectively. The 2017 data indicate an increase of 87% since 1999 when all colonies were counted for Seabird 2000. The increase does not reflect that for Britain and Ireland as a whole, but does reflect the St Bee's Head trend, indicating a possible regional increase in the north east of the Irish Sea10.

 

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.

 

Channel Islands

Population estimates and change 1969–2015 (census data)

 

Operation Seafarer

(1969-70)

Seabird Colony Register

(1984-85)

Seabird 2000

(1998–2002)

Channel Islands Census

(2015)

Population estimate (AON*)

62

113

115

185

Inland numbers

-

-

0

0

Total Figure

-

-

115

185

% change since previous census

n/a

+82

+2

+61

*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 almost 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. Only the small colony at Burhou was counted for great cormorants during 2017, indicating an increase from 5 AON during Seabird 2000 to 15 AON. The most recent seabird census in 2015 recorded 185 AON, an increase of 61% compared to Seabird 200011.

 

Productivity

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

 

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UK phenology, diet, survival rates

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

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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, 181–188.

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 Newson, S. E., Hughes, B., Hearn, R. and Bregnballe T. 2005. Breeding performance and timing of breeding of inland and coastal breeding Cormorants Phalacrocorax carbo in England and Wales. Bird Study, 52(1), 10–17.

5 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.

6 Booth Jones, K. and Wolsey, S. 2019. Northern Ireland Seabird Report 2018. British Trust for Ornithology, Thetford.

7 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.

8 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.

9 O'Connor, I., Walsh, A. and Tierney T.D. 2017. An assessment of the use of UAV for Great Cormorant (Phalacorcorax carbo) breeding colony census on Deer Island in Galway Bay. Unpublished GMIT report.

10 Hill, R.W., Morris, N. G., Bowman, and K. A., 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.

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

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

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SMP Report 1986–2018

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