* Great Cormorant (Phalacrocorax carbo)

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Great Cormorant. Photo: gynti 46



SUMMARY INFORMATION

GENERAL ECOLOGY: Throughout its range the species is sedentary or locally dispersive with northerly populations also making strong migratory movements [7]. The timing of breeding varies geographically, occurring all year round [7] or coinciding with the rains in the tropics [17] and peaking between April and June in the temperate regions of the Northern Hemisphere [7]. The species breeds in mixed-species colonies [7] of 10-500 pairs [25] (occasionally up to 1,000 pairs) [4], the size of the colony depending upon the extent of nearby feeding areas [25]. It is usually a solitary feeder [4] but may form large fishing flocks in some areas [7]. It also roosts communally at nesting sites or in major feeding areas and flies in flocks of varying sizes [4]. The species frequents both coastal and inland habitats [4, 7, 17, 25, 34]. In marine environments it occurs in sheltered coastal areas on estuaries [7], saltpans, coastal lagoons [7, 17], mangrove swamps, deltas [17] and coastal bays [4], requiring rocky shores, cliffs and islets for nesting [7] but generally avoiding deep water and rarely extending far offshore [34]. It also inhabits fresh, brackish or saline inland wetlands [25] including lakes, reservoirs, wide rivers, flood waters [7], deep marshes with open water, swamps and oxbow lakes [17], requiring trees, bushes, reedbeds or bare ground for nesting [7] and avoiding overgrown, small, very shallow or very deep waters [25]. Its diet consists predominantly of fish as well as crustacean, amphibians [7], molluscs and nestling birds [4]. At sea the species preys mostly on bottom-dwelling fish from bare or vegetated seabeds, occasionally also taking shoaling fish in deeper waters [7]. The nest varies from a depression [25] to a platform of sticks, reeds and seaweed [7]. On the coast the species nests on inshore islands, cliffs, stacks, amongst boulders and occasionally on artificial structures [7], also nesting inland on trees or bushes, in reedbeds or on bare ground [7]. The species usually nests in mixed-species colonies, often re-using sites and nests from year to year [7]. It may be possible to alleviate conflicts between this species and fisheries by using such strategies as preventing birds from landing on fish ponds through disturbance, or creating unsuitable feeding conditions [19].

FORAGING RANGE: Max 50 km, mean max 31.67 km, mean 8.46 km.

FORAGING DEPTH: Max 35 m, mean 12.07 m.

FORAGING TRIP DURATION: Mean 3.36 (n=5)

KEY HABITATS: Marine populations use sandy areas, rocky and vegetated substrates; non-marine populations concentrate on lakes, reservoirs and marshes with standing open water.

KEY PREY ITEMS: Fish (mostly benthic), some crustaceans.

KEY FORAGING ASSOCIATIONS: None significant.

DETAILED INFORMATION

General feeding behaviour
The Great Cormorant normally feeds on fish, obtained by diving from the surface with or (especially when fed) without a forward leap [6]. Normally only their feet are used for underwater propulsion with wings held close to the body. However, cormorants can make tight manoeuvres, as when trying to dislodge fish from hiding places, and they then use partially extended wings as well as splayed feet. Cormorant plumage is partially wettable, which constrains marine populations to foraging inshore. Foraging bouts are typically short (c.15min), interspersed with long periods ashore for digestion. There is little fat deposition in cormorants and so they have to feed very regularly, possibly daily throughout the year. Great Cormorants can sometimes form large feeding flocks of up to several hundred birds in western Europe (foraging flocks are not conspicuous in North America and are limited to a few individuals). Such flock-feeding is associated with schooling prey and (in some areas) with shallow, often turbid, water; the flock move slowly forwards with ranks of birds diving almost synchronously in successive waves, driving fish before them towards the surface.

Foraging habitat – breeding season, migration and wintering period
Great Cormorants are foot-propelled divers that feed mostly on benthic prey, mainly during the day. In clear waters they may use pursuit-diving after individual prey but in turbid waters probably forage by disturbing prey from the substrate or from hiding places which are grabbed at short range [36]. The depth of water in which they can forage is limited, and so they have a largely neritic distribution, extending from the low tide mark to around 30m depth. The carbo race of the Great Cormorant is markedly marine (though they may also forage in estuaries), whereas sinensis occurs mainly inland (frequenting inland wetlands including lakes, reservoirs, flood waters, and deep marshes with open water). At sea, cormorants rarely wander far from the coast preferring sheltered areas and estuaries, and normally feed in shallow water. Great Cormorants tend to feed on benthic prey to a greater extent than European shags Phalacrocorax aristotelis and, in general, are found in estuaries and freshwater more often than shags. The major findings on the use of feeding areas are that there are: (1) seasonal changes in the use of feeding grounds in relation to possible changes in food availability in these areas during the breeding season, and (2) increased energy costs in relation to foraging trip distances [28]. One study found that Great Cormorants in Japan eat various fish species from fresh, brackish, or marine waters, depending on seasonal changes in food availability in each habitat [18].

Important foraging associations with other species
None significant.

Diet
At sea, Great Cormorants feed principally on bottom-living fish over bare or vegetated substrates, which they may deliberately disturb from the bottom before seizing at short range. However, cormorants may also take schooling fish, such as sandeels Ammodytes sp. or spawning capelin Mallotus villosus, and individuals shift flexibly between benthic and pelagic foraging [13]. Great Cormorants feed mostly on fish <20 cm in length but occasionally to 75 cm or 1.5 kg. Small numbers of decapod crustaceans may also be consumed. The prey that predominate in most studies are flatfish, blennies (Blenniidae), sea-scorpions Taurulus bubalis, sculpins (Cottidae) and gadoids, with sandeels, salmonids, labrids and eels Anguilla anguilla also being important at some colonies (Table 1).
In Shetland, U.K. in 1989, 13 species of fish (and a single crab, Cancer pagurus) were found in 120 chick regurgitates: Saithe Pollachius virens (33% of regurgitates), sandeels (19.8%), and sea scorpion (18.6%) being most frequent, although there were significant differences between the four colonies studied [27]. Mills (1969) found flatfish were the commonest prey taken at Ord, Scotland in June-July 1965. Sandeels were the commonest prey item (occurring in 41% of regurgitates) at another colony in Caithness [27]. At the Lamb, Scotland, cormorants fed on Common Dab Limanda limanda, plaice Pleuronectes platessa, Viviparous Blenny Zoarces viviparus, sea scorpion, butterfish Pholis gunnelus, cod Gadus morhua, Saithe, pollock Pollachius pollachius and whiting Merlangius merlangus [24]. At the Farne Islands, U.K. in 1961-63, cormorants also consumed a wide variety of fish, with flatfish the most important (31% by weight), followed by gadoids (19%), eels (13%), sea scorpions (10%), salmonids (10%) plus various other species, but excluding clupeids [29]. Consequently, the Great Cormorant is considered to be an opportunistic feeder whose diet varies by locality and season and generally reflects the species most readily available within the foraging range (a result of differences in bottom substrate causing different availabilities of prey).
Birds breeding on the Chausey Islands, France, in 1995 fed both in mid-water and on the bottom on at least 22 different fish species (benthic species comprised 67% of otoliths, pelagic species 29% [12]. In Norway, Great Cormorants are recorded to eat small gadoids and sand eels throughout their range, but also eat other fish species when available [1]. In Dutch coastal waters, Great Cormorants (P. c. sinensis) follow beam trawlers and dive in the wake, apparently feeding on disturbed fish rather than discards taken by typical ship-following seabirds [5]. However, this feeding method has not been reported from other areas. Otoliths from pellets showed that birds from the area of Diskofjord, West-Greenland feed on at least 13 different species of fish [15]. Sculpins (Gymnocanthus tricuspis and Myoxocephalus scorpioides) were largely predominant both by number and by mass in all years, followed by capelin and gadoids, although the proportion of these two prey types varied among years. Research in Cornwall and in the Wadden Sea has shown that more than 40% of prey items consisted of juvenile flatfish [21]. Studies of wintering birds in Greece show a variety of fish are eaten but one or two dominate depending on the site, particularly mullet (Mugilidae) [23].


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Table 1: Summary data from principal studies of diet of Great Cormorant in Great Britain and Ireland Source: Kirby et al, 1996


Foraging range
Several studies have reported cormorants foraging up to 20–25 km from their wintering roosts [3] or breeding colonies [9, 10, 22, 28, 32, 35], although some have been recorded up to 35 km away [26].
In one study, 89 foraging trips of eight pairs of cormorants at the Chausey Islands were recorded using radio transmitters [11]. The study found that Great Cormorants foraged mainly in an area situated between the Islands and the western coast of the Cotentin, c.17 km to the east [11]. This is a shallow (<25 m depth), extended (c. 600 km²) coastal area in the very productive North Sea/Channel shelf ecosystem. The majority (c.65%) of the trips were within 5 km of the colony, and a further 20% were within 10 km; the maximum feeding range recorded was 35 km from the colony, but these represented less than 5% of the total. Short foraging trips were usually to habitats with granitic boulder, the favoured habitat of labrids, the commonest prey in the diet. A similar study focussed on breeding cormorants fitted with radio transmitters in the Diskofjord, West-Greenland, which is an area consisting of deep (>200 m) fjords with very steep shores [15]. All feeding locations were within 15 km of the breeding colony, and the average foraging range was 6.7 ± 2.5 km, with the foraging zone restricted to a very narrow band (max. 300 m) along these shores.
In winter 1999/2000, 1,800 wintering Great Cormorants were observed along the coasts of eastern Jutland and Læsø, Denmark, almost all of which were recorded within 10 km of the coast, with few individuals actively foraging further offshore [30, 31]. Up to 90% of the observed birds occurred in water shallower than 10 m. Most birds were recorded on islets close to land, and 75% of the birds were seen within 3 km of the coast. Offshore cormorants peaked in numbers in the depth interval between 10 and 12 m. The average flock size recorded within 3 km of the coast was 26 individuals, with a maximum of 890; whereas average flock size at a distance of more than 3 km from the coast was only 3 individuals, with a maximum flock size of 220. A study of Great Cormorants wintering at Sørfjord, northern Norway found that a total of 72% of the foraging sites were within 4 km of the night roost and 96% within 8 km (n=181), and mean of 3.2 km ±2.0 SD for the whole study period (October-March) [16].
A major foraging review noted that in the North Sea Great Cormorants were never observed further than 5 km from land, and only in waters <10 m deep [2].
Another study, which focussed on two breeding pairs of cormorants (P.c.sinensis) in 1999 in a riparian wood along the Ebro river, north east Spain, found that the average distance between the main feeding areas (Ebro and Aragon rivers) and the nest sites was 3.8 km [20].
Diving and depth association
Great Cormorants will dive to depths of up to 35 m, although they usually forage in less than 10 m [14]. One study recorded cormorants diving (apparently to the bottom according to dietary composition) in waters 4.6-19.8 m deep, with a mean depth of c. 11 m [33]. Nearly all of the Great Cormorants recorded in the Baltic Sea were observed in waters less than 10 m in depth [8].




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Table 2: Showing average value (km) of foraging range in different country/sea areas.



References
1. Barrett, R.T., Røv, N. Loen, J., and Montevecchi, W.A. (1990). Diets of shags Phalacrocorax aristotelis and cormorants P. carbo in Norway and possible implications for gadoid stock recruitment. Mar. Ecol. Prog. Ser. 66:205-218.
2. BirdLife International (2000). The Development of Boundary Selection Criteria for the Extension of Breeding Seabird Special Protection Areas into the Marine Environment. OSPAR Convention for the Protection of the Marine Environment of the North-East Atlantic. Meeting of the Biodiversity Committee (BDC) Vlissingen (Flushing): 20 – 24 November 2000.
3. Boldreghini, P., Santolini, R., Volponi, S., Casini, L., Montanari, F.L. and Tinarelli, R. (1997) Variations in the use of foraging areas by a Cormorant Phalacrocorax carbo wintering population: a case study in the Po delta (Northern Italy). Ekol. Polska 45:197–200.
4. Brown, L. H., Urban, E. K. and Newman, K. (1982) The birds of Africa vol I. London: Academic Press.
5. Camphuysen, C.J. (1999) New feeding technique of Great Cormorants Phalacrocorax carbo sinensis at beam trawlers. Atlantic Seabirds 1:85–90.
6. Cramp S. and Simmons K.E.L (1977) Handbook of the Birds of the Western Palearctic Volume 1: Ostrich to ducks. Oxford University Press.
7. del Hoyo, J., Elliot, A. and Sargatal, J. (1992) Handbook of the birds of the world, vol 1: Ostrich to Ducks. Barcelona, Spain: Lynx Edicions.
8. Durinck J., Skov, H., Jensen, F.P. and Pihl, S. (1994) Important Marine Areas for Wintering Birds in the Baltic Sea. Ornis Consult report to the European Commission. 110 pp.
9. Engström, H. (2001). Long term effects of cormorant predation on fish communities and fishery in a freshwater lake. Ecography 24:127–138.
10. Goc, M., Iliszko, L. and Chelkowxka, N. (1997) Daily foraging rhythm at a Cormorant Phalacrocorax carbo colony during the breeding season. Suppl. Ric. Biol. Selvaggina 26:445–451.
11. Grémillet D. (1997). Catch per unit effort, foraging efficiency, and parental investment in breeding Great Cormorants (Phalacrocorax carbo carbo). ICES Journal of Marine Science 54:635–644.
12. Grémillet, D. and Argentin, G. (1998) Cormorans et pecheries autour des Îles Chausey. Le Cormoran 10 (47):196–202.
13. Grémillet, D., Remillet, G., Argentin, G., Schultze, B., and Culik, B.M. (1998). Flexible foraging techniques in breeding Cormorants Phalacrocorax carbo and Shags Phalacrocorax aristotelis: benthic or pelagic feeding? Ibis 140:113-119.
14. Grémillet, D., Wilson, R.P., Gary, Y. and Storch, S. (1999) Three-dimensional space utilization by a marine predator. Marine Ecology Progress Series 183:263–273.
15. Grémillet D., Kuntz G., Delbart F., Mellet M., Kato, A., Robin, J-P., Chaillon, P-E., Gendner, J-P., Lorentsen S-H. and Le Maho, Y. (2004). Linking the foraging performance of a marine predator to local prey abundance. Functional Ecology18:793–801.
16. Johansen, R., Barrett R.T. and Pedersen T. (2001). Foraging strategies of Great Cormorants Phalacrocorax carbo carbo wintering north of the Arctic Circle Bird Study 48(1):59-67.
17. Johnsgard, P.A. (1993) Cormorants, darters, and pelicans of the world. Washington: Smithsonian Institution Press.
18. Kameda, K., Matsubara, T., Mizutani, H., Yamada, Y. (2002). Diet and foraging site selection of the Great Cormorant in Japan. Japanese Journal of Ornithology 51:12-28.
19. Kirby, J.S., Callaghan, D.A., Hughes, B. and Underhill, M.C. (1996) Piscivorous birds in Britain and Ireland: an overview of current knowledge of conflicts with fisheries U.K. Nature Conservation 15:1-6.
20. Lekuona, J.M. (2002). Foraging ecology of cormorants Phalacrocorax carbo sinensis during breeding in a recently colonized area (Ebro Valley, northern Spain). Ardeola 49:241-247.
21. Leopold M.F., van Damme C.J.G., van der Veer H.W. (1998). Diet of cormorants and the impact of cormorant predation on juvenile flatfish in the Dutch Wadden Sea Journal of Sea Research 40: 93–107.
22. Linn, I.J. and Campbell, K.L.I. (1992). Interactions between white-breasted cormorants Phalacrocorax carbo (Aves: Phalacrocoracidae) and the fisheries of Lake Malawi. J. Appl. Ecol. 29: 619–634.
23. Liordos, V., and Goutner, V. (2007). Spatial patterns of winter diet in coastal wetlands of Greece. Waterbirds 30:103-111.
24. Mills, D.H. (1969) The food of the cormorant at two breeding colonies on the east coast of Scotland. Scottish Birds 5,:268-276.
25. Nelson, J. B. (2005) Pelicans, cormorants and their relatives. Pelecanidae, Sulidae, Phalacrocoracidae, Anhingidae, Fregatidae, Phaethontidae. Oxford, U.K.: Oxford University Press (Bird Families of the World 17).
26. Nelson B.J. (2006). Bird Families of the World. Pelicans, Cormorants, and Their Relatives. The Pelecaniformes. Oxford University Press.
27. Okill, J.D., Fowler, J.A., Ellis, P.M. and Petrie, G.W. (1992) The diet of cormorant Phalacrocorax carbo chicks in Shetland in 1989. Seabird 14:21-26.
28. Paillisson J-M., Carpentier, A., Le Gentil, J. and Marion, L. (2004). Space utilization by a cormorant (Phalacrocorax carbo L.) colony in a multi-wetland complex in relation to feeding strategies. C. R. Biologies 327: 493–500.
29. Pearson, T.H. (1968) The feeding ecology of sea-bird species breeding on the Farne Islands, Northumberland. Journal of Animal Ecology 37:521-552.
30. Petersen I.K., Fox A.D. and Clausager I. (2003) Distribution and numbers of birds in Kattegat in relation to the proposed offshore wind farm south of Læsø –Ornithological impact assessment. Department of Wildlife Ecology and Biodiversity. National Environmental Research Institute, Ministry of Environment, Denmark.
31. Pihl, S., Petersen, I.K., Hounisen, J.P. and Laubek, B. (2001) Landsdæk- kende optælling af vandfugle december 1999/marts 2000. Danmarks Miljøundersøgelser. 46 p. - Faglig rapport fra DMU, nr. 356.
32. Platteeuw, M., and van Eerden, M.R. (1995). Time and energy constraints of fishing behaviour in breeding Cormorants Phalacrocorax carbo sinensis at Lake IJsselmeer, The Netherlands. Ardea 83:223–234.
33. Ross, R.K. (1974) A comparison of the feeding and nesting requirements of the Great Cormorant (Phalacrocorax carbo L.) and double-crested cormorant (P. auritus Lesson) in Nova Scotia. Proceedings of the Nova Scotia Institute of Science 27:114-132.
34. Snow, D. W. and Perrins, C. M. (1998) The Birds of the Western Palearctic: Concise Edition. Oxford: Oxford University Press.
35. Veldkamp, R. (1997). Early breeding by Cormorant Phalacrocorax carbo sinensis at Wanneperveen, The Netherlands: profiting by spawning Roach Rutilus rutilus. Suppl. Ric. Biol. Selvaggina 26:99–109.
36. White, C.R., Day, N., Butler, P. J., and Martin, G.R. (2007) Vision and foraging in Cormorants: more like Herons than Hawks? PLoSOne i2(7): e639.doi:10.1371/journal.pone.0000639.


Compiled by: Ben Lascelles, Nigel Varty, Kate Tanner, Rory McCann