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
8,900 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 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*)

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

 

Cormorant abundance in UK 2013

Figure 1: Trend in UK abundance index (solid line) of great cormorant 1986-2013 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 2013 has fallen below 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).  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. In England, inland colonies at least have increased with 2,362 pairs nesting in 20124. In Wales, numbers have been altogether 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 shooting1 2, as well as possible changes in food availability.

 

Productivity

 

Great cormorants at colonies in the United Kingdom fledged approximately 1.83 chicks per nest per year between 1991 (no breeding success data were submitted before this year) and 2013; there was no statistically significant variation over time.

 

However, 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*)

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, 39 colonies counted in 2012 held 1,028 pairs (cf. 1,457 pairs) and 32 colonies counted in 2013 held 715 pairs (cf. 1,167 pairs), 29% and 38% 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 1.82 chicks per nest per year between 1991 (no breeding success data were submitted before this) and 2013; 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

 

Cormorant abundance in England 2013

Figure 1: Trend in abundance index (solid line) of great cormorant in England, 1986-2013 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. Partly 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, reaching a peak in 2003. There has been an apparent decline during the last decade albeit with some fluctuation in the trend in recent years. However, much data from inland colonies collected between 2006 and 2012 have yet to feed in to the SMP which may alter the trend for the later years. The current population would appear to be at least on a par with that recorded during Seabird 2000.

 

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 2012. No data were submitted for 2013.

 

 

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

 

Cormorant abundance in Wales 2013

Figure 1: Trend in abundance index (solid line) of great cormorant in Wales, 1986-2013 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 numbers are probably still on a par with that recorded during Seabird 2000. Few inland colonies exist in Wales with no updates on these provided since Seabird 2000. 

 

Productivity

 

Cormorant productivity in Wales 2013

Figure 2: Trend in productivity (no. of chicks fledged per pair) of great cormorant in Wales, 1986-2013. 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 were collected in 2012 or 2013.

 

 

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

 

Cormorant abundance in NI 2013

Figure 1: Abundance of great cormorant at Strangford Lough, 1986-2013.

 

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 the SCR, the most frequently surveyed colony, at Strangford Lough (also one of the largest colonies in the country), increased substantially from 125 AON to a peak of 490 AON in 2005 (Figure 1), although numbers had fallen to 306 AON as of 2013. Over the same period numbers on Sheep Island have fallen from 380 to 112 AON. Two other colonies monitored in 2013, Gobbins (11 AON) and Little Skerries (98 AON), also appear to be in decline (peak counts of 46 AON in 1992 at Gobbins and 163 AON in 2010 at Little Skerries).  The reasons for these declines are unknown. No data have been received recently from other extant colonies at Burial Island (67 AON in 2009) or North Rock (72 AON in 1997).

 

In summary, four of six known colonies held 527 AON in 2013, but apparently all are declining. Thus, given the general decline, which may also be occurring at two colonies not surveyed recently, it would seem unlikely that the Northern Ireland cormorant population is currently greater than that recorded during the last national census. However, as recently as 2010, just three colonies (Strangford, Sheep and Little Skerries) held 755 AON - almost 14% higher than during Seabird 2000.

 

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 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 respectively held totals of 1,706 and 1,385 pairs in these years (cf. 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*)

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 at best stable in the interim. For example, in Northern Ireland, four of six known colonies held 527 AON in 2013, but apparently all are declining. In the Republic of Ireland, 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 (cf. 1,539 pairs in 1999). However, 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. Colonies are few; all 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 were counted for Seabird 2000. No more recent data are available.

 

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. No data have apparently 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.

4 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.

 


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 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.