Cormorant references, A

Cormorant papers:

references, abstracts and comments. Where there is no abstract, an abstract has been written, where abstracts are too long they have been abridged. Abstracts in languages other than English have been translated into English. CRGB = Cormorant Research Group Bulletin. The comment is personal, it points out errors and possible follow-ups, it is begun: CP:

Note the following lists on the web:

Cormorant Food Habit and Potential on Sport and Commercial Fisheries: An Annotated Bibliography by John. L. Trapp & Shauna L. Hanisch, U.S. Fish and Wildlife Service, February 2000.
Literature list to Kormorane im niedersächsischen Binnenland - "Weiterführende Fachliteratur zum Kormoran" by Meike Kleinwächter a.o.

A, B, C, D, E, F, G, H, I, J, K, L, M

Bédard, J., Nadeau, A. & Lepage, M. 1995: Doublecrested Cormorant culling in the St. Lawrence River Estuary. Colonial Waterbirds 18: 78-85.

Bédard, J., Nadeau, A. & M. Lepage (2000): Double-Crested Cormorant Culling in the St. Lawrence River Estuary: Results of a 5-Year Program. Symposium on Double-Crested Cormorants, Technical Bulletin No. 1879, Dec. 1999.

Modeling indicated that lowering the double-crested cormorant population from 17,361 to 10,000 pairs could be attained only by a combination of techniques: culling breeding birds in arboreal colonies to lower breeding stock and egg spraying in accessible ground nests to lower recruitment. The 5-year program was launched in 1989; culling was halted 4 years later because the population had fallen below the threshold of 10,000 breeding pairs. A greater vulnerability of males to shooting (203:100) probably accounted for the faster-than-predicted drop in numbers. Egg spraying spanned the entire 5-year period, during which 25,095 nests were treated with inert mineral oil. As predicted by the model, spraying lowered recruitment, but only after a 2-year lag. Culling should be considered a last-resort form of intervention whenever softer techniques (egg spraying, mechanical nest destruction, and carefully planned disturbances to the nesting colonies to enhance predation and abandonment) are not sufficient or practical to produce population control. Population control should be based upon careful planning (included detailed censuses, population modeling, and prior communication with the public) and be conducted under close scientific supervision. Internet version of this report.

Belyea, G. Y., Maruca, S. L., Diana, J. S:, Schneeberger, P. J., Scott, S. J., Clark, Jr, R. D. Ludwig, J. P. & C. L. Summer (2000): Impact of Double-Crested Cormorant Predation on the Yellow Perch Population in the Les Cheneaux Islands of Michigan. Symposium on Double-Crested Cormorants, Technical Bulletin No. 1879, Dec. 1999.

The Michigan Department of Natural Resources, in conjunction with the University of Michigan and the U.S. Fish and Wildlife Service, initiated a research study to determine the impact of double-crested cormorants (Phalacrocorax auritus on the yellow perch (Perca flavescens population in the Les Cheneaux Islands area of northern Lake Huron. Aerial and nesting colony counts were conducted to monitor cormorant abundance. We collected 373 cormorants to study food habits via stomach-content analysis. We found that (1) cormorants fed heavily on yellow perch in early spring, but over the entire season only 10 percent of their diet was perch; (2) alewives (Alosa pseudoharengus) and sticklebacks (Culaea inconstans, Pungitius pungitius, Gasterosteus aculeatus) made up the major portion of the cormorants' diet; (3) cormorants removed only 2.3 percent of the available perch biomass (v. 1.8 percent by anglers over the same period); (4) most fish taken by cormorants were less than 150 mm long; (5) total annual perch mortality was about 45 percent, of which less than 9 percent was due to cormorants; and (6) cormorants accounted for only 0.8 percent of the mortality of legal-size perch (> 178 mm), whereas summer sport fishing accounted for 2.5 percent. Thus, although the impact of cormorants on the perch population may vary slightly from year to year, we conclude that cormorant predation has minimal impact on the local perch population. Internet version of this report.

Berglund, T. (1958): Om skarvarna Phalacrocorax carbo sinensis i Kalmarsund. Vår Fågelvärld 17: 44 - 49.

Bertalanffy, L. von (1938): A quantitative theory of organic growth (Inquiries on growth laws. II). Human Biol. 10: 181-213.

Blanco, G., Gómez, F. & J. Morato (1995): Diet composition and prey size of the Great Cormorant (Phalacrocorax carbo sinensis) wintering in rivers and gravel pits of Central Spain. Ardeola 42: 125 - 131.

Boudewijn, T. J. & S. Dirksen (1995): Impact of contaminants on the breeding success of the Cormorant Phalacrocorax carbo sinensis in The Netherlands. Ardea 83: 325 - 338.

In The Netherlands Cormorants reached a minimum number of breeding pairs in the mid 1960s. Since the end of the 1970s the population is increasing again, and many new and rapidly expanding colonies have been settled. However, in a colony in Dordtse Biesbosch, situated in the centre of the sedimentation area of the rivers Rhine and Meuse, breeding success was very poor during 1987-1989. In order to establish the factors responsible, a study of breeding biology was carried out in 7 colonies, representing a wide range of contamination levels in surrounding feeding areas. Because literature on breeding biology of the species is relatively scarce, this paper firstly presents data on the breeding biology from a colony of Cormorants in a relatively clean area, producing average numbers of fledglings. Secondly, the different stages of breeding are compared for all colonies studied, together with several other parameters. From these data, possible factors causing the large differences in reproductive success between the colonies are evaluated. The low breeding success in Dordtse Biesbosch (c. 0.5 fledged young/pair) and, to a lesser content, in some colonies upstreams along the river (c. 1.2-1.7 fledged young/pair) can be considered the cumulative effect of (1) a later onset of egg-laying and a related decrease in clutch-size, (2) a reduction of eggshell thickness, probably to a level low enough to cause breaking of part of the eggs, (3) a high embryo-mortality, resulting in reduced hatching success and (4) a high mortality of young, especially in their first weeks. It is concluded that organochlorine contaminants are the most likely cause: pp-DDE (through eggshell thinning) and PCBs (through direct toxic effects on the embryo).

Bregnballe, T. & J. Gregersen (1995): Development of the breeding population of Cormorant Phalacrocorax carbo sinensis in Denmark 1938-1994. DOFT 89: 119-134.

Bregnballe, T. (1996): Udviklingen i bestanden af Mellemskarv i Nord- og Mellemeuropa 1960-1995. DOFT 90: 15 - 20.

Bregnballe, T. & J. Gregersen (1997): Age-related reproductive success in cormorant Phalacrocorax carbo. Ekologia polska, 45: 127 - 136.

Bregnballe, T., Frederiksen, M. & J. Gregersen (1997): Seasonal distribution and timing of migration of Cormorants Phalacrocorax carbo sinensis breeding in Denmark. Bird Study 44: 257 - 276.

The recent expansion of the Cormorant Phalacrocorax carbo population in Europe has led to management conflict throughout Europe, increasing the relevance of describing the migration pattern of each country's breeding population. We use 2279 recoveries and 16 769 resightings of 4735 colour-ringed individuals to describe dispersal and timing of movements of Danish Cormorants. Most Cormorants dispersed from the colonies to coastal areas and freshwater lakes in Denmark, Sweden and northern Germany in July. Southward movements took place throughout July to December. Major departure from the post-breeding areas occurred from August to mid-October, with many birds staging at Dutch and Alpine lakes between September and October, and with arrival in the Mediterranean mainly from October to November. Spring migration was fast, occurring from mid-February to March. First-year birds migrated south faster and reached the wintering areas sooner than adults, but left these later and moved north more slowly than adults. Wintering occurred from Portugal in the west to Greece in the east, and from Denmark in the north to North Africa in the south. The majority of Cormorants spent the winter in Mediterranean France, Italy, Yugoslavia, Albania, Algeria and in particular in Tunisia. Adult males stayed closer to the breeding areas in winter than females, and adults tended to winter further north than first-year birds. Sex differences in body size and advantages of arriving early at the breeding sites may explain why males wintered further north than females.

Bregnballe, T. & T. Rasmussen (2000): Post-breeding dispersal of Great Cormorants Phalacrocorax carbo sinensis from Danish breeding colonies. DOFT 94: 175 - 187.

Bregnballe, T. & J. Eskildsen (2002): Menneskelige indgreb i danske skarvkolonier 1994-2001. Arbejdsrapport fra DMU nr 162 2002. Internetversion av denna rapport.

Bur, M. T. & S. L. Tinnirello (2000): Diet of Double-Crested Cormorant in Western Lake Erie. Symposium on Double-Crested Cormorants, Technical Bulletin No. 1879, Dec. 1999.

Sport and commercial interest fishing groups are concerned about potential impacts double-crested cormorants (Phalacrocorax auritus may have on fish species. Our objectives for this study were to determine the diet of the cormorant in western Lake Erie and the diet overlap and competition for resources with piscivorous fish, such as walleye (Stizostedion vitreum). The stomach contents of 302 double-crested cormorants collected in western Lake Erie consisted primarily of young-of-the-year gizzard shad (Dorosoma cepedianum), emerald shiner (Notropis atherinoides) and freshwater drum (Aplodinotus grunniens). In the spring, freshwater drum were the most frequently occurring food in the stomachs and constituted the greatest portion of the diet by weight. Young gizzard shad became the most abundant prey and made up the largest percentage of the diet by weight in the stomachs from the end of July through October. Emerald shiners were abundant in the diet during June, September, and October. The fish species that cormorants ate resembled, by proportion, the species mix found in trawl catches. The diets of cormorants and walleyes were similar from July to October with significant overlap. Results from this study suggest impacts of cormorants at current population levels in Lake Erie are not detrimental to sport and commercial fishing. Therefore, control for the purpose of reducing competition for prey fish with walleye is not warranted at this time. Internet version of this report.

Callaghan D. A., Kirby J. S., Bell M. C. & C. J. Spray (1998): Cormorant Phalacrocorax carbo occupancy and impact at stillwater game fisheries in England and Wales. Bird Study 45: 1 - 17.

This study provides an assessment of Cormorant occupancy and impact at stillwater game fisheries in England and Wales, during 1988/89 to 1992/93. A total of 167 waterbodies operated as 'put-and-take' trout fisheries was included within a questionnaire survey, and this provided the bulk of the data used in the analyses. The results show that Cormorants are widespread at inland stillwater game fisheries in England and Wales throughout the year, with higher densities present during October-March. Peak counts by fishery managers at 45 of the fisheries increased by about 20% per year, regardless of season. Site use by Cormorants is investigated and the results imply that large, low altitude sites that are close to rivers in southern England are more likely to be used by these birds. The survey revealed that Cormorants are widely perceived by fishery managers to be responsible for significant economic losses through consumption and/or injury of stock fish. While this may be justified locally, we found no overall relationship between Cormorant density and anglers' catches of Rainbow Trout, the principal stock fish.

Debout, G. (1988): (transl.) Reproductive biology of the Cormorant (Phalacrocorax carbo) in Normandy, France. Ois. Rev. Fr. Orn. 58: 1 - 17.

Debout, G., Røv, N. & R. M. Sellers (1995): Status and population development of Cormorant Phalacrocorax carbo carbo breeding on the Atlantic coast of Europe. Ardea 83: 47 - 59.

About 37,000 pairs of Cormorants currently breed on the North Atlantic coasts of western Europe (France, Britain, Ireland and Norway) representing about 83 % of the world population of the nominate form of the species. Colonies typically number 10-500 pairs, with a maximum of 1400 pairs and, with one or two exceptions, occur almost werever there is suitable habitat. The preferred breeding sites are cliffs, stacks and rocky offshore islands though some, notably in Ireland, breed in trees. Amongst the factors which determine colony size the extent of suitable feeding areas (water less than 10 m deep) within 30 km of the colony and the proximity of other colonies are shown to be particularly important. The general trend in population numbers in the past 10-20 years has been upward, probably as a result of increased protection both through legislation and by the establishment of safe havens such as nature reserves. Rates of increase vary somewhat from area to area but have typically averaged a few percent per annum. The only area where declines continue is northern Scotland. Two cases of sudden decreases in breeding numbers are described (in Norway in 1985-87 and in SW Wales in 1991). Both appear to have been due to periods of adverse weather at the pre-breeding or early breeding stages acting to keep fish stocks in deeper water where the Cormorants could not catch them, perhaps coupled in the former case by reductions in fish stocks.

De Nie, H. (1995): Changes in the inland fish populations in Europe in relation to the increase of the Cormorant Phalacrocorax carbo sinensis. Ardea 83: 115 - 122.

Fish populations in Europe react on increasing eutrophication in a general pattern. However this pattern is locally different and strongly depends on the 'natural', pre-eutrophicated situation. The general trend is toward unstable fish populations with dominance of small, short living, early maturing fish like Perch Perca fluviatilis, Ruffe Gymnocephalus cernuus, Smelt Osmerus eperlanus and/or carp-like fish (cyprinids) mainly Roach (Rutilus rutilus) and Bream Abramis brama. The high-bodied Bream is negatively selected for by Cormorants in favour of Roach, Perch, Ruffe and Smelt. Social fishing on shoaling young fish of species like Perch, Ruffe, Smelt and Roach in relatively turbid, eutrophic lakes with few submerged water vegetation can be considered as an adaptation of the inland breeding population of Cormorants to eutrophication caused by human activity. Where lakes and rivers are still clear and relatively unpolluted, the Cormorant returns to its habit as solitary hunter, predating on Eel Anguilla anguilla, Trout Salmo trutta or cyprinid species of running waters like Chub Leuciscus cephalus.

Dieperink, C. (1995): Depredation of commercial and recreational fisheries in a Danish fjord by cormorants, Phalacrocorax carbo sinensis, Shaw. Fish. Man. and Ecol. 3: 197-207.

Dirksen, S., Boudewijn, T. J., Noordhuis, R. & E. C. L. Marteijn (1995): Cormorants Phalacrocorax carbo sinensis in shallow eutrophic freshwater lakes: prey choice and fish consumption in the non-breeding period and effects of large-scale fish removal. Ardea 83: 167 - 184.

During the period October 1989 - April 1992 a study on Cormorant feeding ecology was carried out in two shallow lakes in The Netherlands: lake Veluwemeer and lake Wolderwijd (3240 and 2600 ha respectively). Increasing numbers of Cormorants use these lakes for feeding in the non-breeding season; the maximum number, usually reached in October or November, amounted to 1314 in 1991. The effect of Cormorant predation on fish stock was studied for two reasons: assessing possible damage to commercial fishery and assessing the role of the birds in a large-scale biological management programme that has been carried out in lake Wolderwijd during the period of study. As part of this programme, fish stock in this lake was reduced from203 to 46 kg/ha. Most important prey species was Ruffe (60 % of fish-mass in 1991/92), which has no value to either commercial fishermen or anglers. Perch, Pikeperch, Roach and Smelt were found in most samples as well, while Eel, the only species commercially caught in the lakes, was hardly found at all. Mean daily intakes ranged from 146 to 699 g with highest values in October and March. In 1989/90 total consumption in the two lakes was estimated at 3.7 kg/ha. In September 1991 - March 1992 however, after fish removal, 80 % went to lake Wolderwijd, where smaller fish were taken. Nevertheless, total consumption amounted to 12.5kg/ha, against 2.1 kg/ha in lake Veluwemeer. As most of the fish consumed belongs to species potentially hazardous to water quality (transparency) the Cormorants seem to support biological management. It is discussed whether the larger proportion of small, shoaling fish after removal of large fish could have enhanced possibilities for mass flock fishing, attracting the birds to lake Wolderwijd. On the other hand, achieving the long-term goals of biological management is likely to reduce the fishing possibilities for Cormorants.

Dirksen, S., Boudewijn, T. J., Slager, L. K., Mes, R. G., van Schaik, M. J. M. & P. de Vogt (1995): Reduced breeding success of Cormorants (Phalacrocorax carbo sinensis) in relation to persistent organochlorine pollution of aquatic habitats in The Netherlands. Env. Poll. 88: 119 - 132.

Cormorants (Phalacrocorax carbo sinensis) breeding in the heavily contaminated sedimentation area of the rivers Rhine and Meuse have a severely reduced breeding success as compared to several other Dutch colonies. A detailed analysis of reproductive performance in combination with chemical analysis of eggs and food from colonies in differently contaminated aquatic habitats is presented. The differences in breeding success between colonies are caused mainly in the egg-stage of breeding. Eggshell thinning and increased embryonic mortality cause the differences in hatching success. The observed effects seem to be related to chlorinated hydrocarbons. Significant correlations are found for concentrations of DDE with eggshell thinning and for concentrations of PCBs with hatching and breeding success. The correlations between concentrations of chlorinated hydrocarbons in eggs and biological effects measured in the field are established both on colony and individual clutch level.

Engström, H. (1998): Conflicts between Cormorants Phalacrocorax carbo L. and fishery in Sweden. Nord. J. of Freshw. Res. 74: 1148 - 155.

Engström, H. 1998: Mellanskarvens ekologi och effekter på fisk och fiske. Fiskeriverket Rapport 1998:1, 5-29.

Engström, H. (2001): The occurrence of the Great Cormorant Phalacrocorax carbo in Sweden, with special emphasis on the recent population growth. Ornis Svecica 11: 155 - 170.

The population of Great Cormorant Phalacrocorax carbo sinensis in Sweden has increased considerably in size during recent decades and currently Sweden holds about a quarter of the total Northwest European population. In 1999, the population contained an estimate of 25,600 pairs, distributed over about 154 colonies. The increase was particularly strong between 1986 and 1994 (mean annual increase 31 %), and the population grew from 1800 to 15,500 pairs. After the mid-1990s, the population increase within most of the core area appears to have leveled of, while now fluctuating in size. However, in some northern breeding areas (including the coasts of Södermanland, Uppland, Gotland and several lakes) the population continued to grow at a high rate. With growing cormorant numbers, conflicts with human interests, mainly fishery, have increased. Hunting and egg pricking have frequently been used as methods to reduce cormorant densities locally and to solve fishery related problems. It seems, however, as if these measures, in most areas, only have had limited effects in terms of stabilising or reducing population size of cormorants.

Engström, H. (2001): Effects of Great Cormorant predation on Fish Populations and Fishery. Dissertation. Uppsala 2001.

The strong increase in number of Great cormorants Phalacrocorax carbo
in Sweden in recent years has led to conflicts - particularly with fishery. This thesis focuses on the possibkle effects of cormorant predation on fish populations. In total, data from 15 lakes in South Sweden were included in this study while most studies were carried out in Lake Ymsen. The results suggest that the impact of cormorant predation on natural fish populations was small, and I observed no decline in fish mass after cormorants established. Cormorant predation on eel was difficult to evaluate because of several confounding factors.

Ruffe, roach and perch (Sw.: gärs, mört och abborre) were the most important prey species to the cormorants and most fish taken were small. Cormorants do not seem to catch species and sizes in proportion to their occurrence in the fish community.

Total fish removal by cormorants varied considerably among lakes (0.2 - 15.0 kg/ha) and cormorant population sizes at the different lakes were significantly positively correlated with fishery catches, which in turn was significantly positively correlated with total phosphorous levels. Thus, cormorant densities in lakes, and perhaps elsewhere, seem to be governed chiefly by fish densities. The fact that cormorant predation appears not to reduce fish densities suggest cormorants to be regulated by other means than prey depletion. The mechanism behind population regulation could be a behavioural response of fish, making fish more difficult to catch for the cormorants.

In recent years, cormorant populations have been subjected to intensive legal and illegal actions with the aim to reduce cormorant numbers. However, the actions currently carried (out) are well below the efforts needed to limit population sizes. To conclude, cormorants appear to compete little with fishery, with regards to free-living fish. The main problem is that cormorants sometimes damage and take away fish in fishing gears.This paper on the web

Engström, H. & L. Jonsson (2003): Great Cormorant Phalacrocorax carbo diet in relation to fish community composition in a freshwater lake. Die Vogelwelt 124, suppl.: 187 - 196.

Eskildsen, J. (2002): Skarver 2002. Naturovervågning. - Danmarks Miljøundersøgelser. - Arbejdsrapport fra DMU, nr. 172. Internet version of this report

The number of occupied nests of great cormorants Phalacrocorax carbo was counted once in all breeding colonies in Denmark in 2002. The total number of nests in Denmark amounted to 40,540 in 2002, which was an increase of three percent compared with 2001.

Eskildsen, J. (2003): Skarver 2003. Naturovervågning.- Danmarks Miljøundersøgelser. Arbejdsrapport fra DMU, nr. 190. Internet version of this report

In 2003 the number of cormorant nests was estimated at 37,313, which was a decrease of eight percent in relation to 2002. The decrease was especially recorded in colonies along the coasts of Kattegat, which all showed an increase in 2002. In 2003 the number in most of these colonies is back on the same level as 2001. Furthermore, numbers had also decreased in some of the old and large colonies as Vorsø, Ormø, Brændegård Lake and Tyreholm. In spite of the decrease in nests in 2003, the number of breeding cormorants in Denmark has been stable during the last 10 years averaging 39.000 nests. The management plan for cormorants from 2002 made it possible to regulate nests, especially in West- and North Jutland. Due to this plan a "management-number" was defined to describe the number of cormorant nests excluding the number of regulated nests. The "management-number" was 36,968 nests in 2002 and 32,571 nests in 2003. On the basis of "management number", of cormorant nests the decrease from 2002 to 2003 is 12 % . Some of the non-breeding cormorants are able to initiate a breedingattempt if the right possibilities are present. Therefore the regulation of cormorant nests may not have an essential effect on the total number of breeding-attempts. Cormorants have been removed from certain localities, e.g. Vinterleje Pold and Høje Sande in Ringkøbing Fjord and Agger Tange north of Thyborøn. On other localities such as Rugård Lake, eastern Djursland, it has been difficult. In 2003 the cormorants made attempts to establish new colonies especially in eastern Jutland and the county of Storstrøm. Altogether it means that the number of colonies is still close to 50, the number of which has been stable for the last 3-4 years. Danish cormorants are regulated by shooting during migration and in some wintering areas. Whether this affects the breeding number of cormorants in Denmark is not known. However, it is likely that food availability is the dominant factor that determines the number of cormorant nests in Denmark. Local factors too affects the breeding numbers such as spring-floods in ground-breeding colonies, disturbance of various characters, deliberate or not.

Eskildsen, J. (2004): Skarver 2004. Naturovervågning. Danmarks Miljøundersøgelser. - Arbejdsrapport fra DMU, nr. 199. http//arbejdsrapporter.dmu.dk. Internet version of this report

In 2004 the number of counted cormorant nests in Denmark was estimated at 39.631 nests in 59 colonies. This was an increase of 6% compared to 2003; the colonies near the coast of Kattegat holding the majority of the increase. The number of colonies was the highest number ever and 14% more than in 2003. The number of nests in 2004 is very close to the average of 39,000 nests during 1994-2003, varying between 36.700 and 42.800 nests. Regulation of cormorant nests was intensified in 2004 compared to years before. A minimum of 6,700 nests was regulated, which corresponds to 17% of the total number of counted cormorant nests in Denmark in 2004. Regulation of nests increased by 43% compared to 2003. In 2004, 83% was regulated by oiling, i.e. spraying the eggs with a fluid that closes the pores of the eggs and consequently kills the embryo. Regulations took place in 23 colonies (39%). Despite the intensified regulations, the number of colonies increased in 2004 compared to 2003. The availability of food resources seems to be the main factor of the fluctuations during the last 10 years.

Frederiksen, M. & T. Bregnballe (2000): Diagnosing a decline in return rate of one-year-old cormorants: mortality, emigration or delayed return? J. An. Ecology, 69, 753-761.

Frederiksen, M. & T. Bregnballe, T. (2000): Evidence for density-dependent survival of adult cormorants from a combined analysis of recoveries and resightings. Journal of Animal Ecology, 69, 737-752.

Frederiksen, M. & T. Bregnballe, T. (2001): Conspecific reproductive success affects age of recruitment in a great cormorant, Phalacrocorax carbo sinensis, colony. Proc. Biol. Sci. 268: 1519 - 1526.

Few studies have addressed the proximate factors affecting the age at which individuals of long-lived bird species are recruited into the breeding population. We use capture-recapture analysis of resightings of 16 birth cohorts of colour-ringed great cormorants, Phalacrocorax carbo sinensis, in a Danish colony to assess the evidence for two hypotheses: conspecific attraction (earlier recruitment when the colony is large) and conspecific reproductive success (earlier recruitment following years of high breeding success). For both males and females, conspecific reproductive success was the most important covariate explaining the interannual variation in age of recruitment; colony size was also important for females. These covariates explained nearly 60% of the year-to-year variation for both sexes. The age of recruitment increased for cohorts born after 1990, and this increase was correlated with a decline in breeding success in the colony; we interpret this as an indirect and delayed density-dependent effect. Females were recruited earlier than males (mean age of recruitment for cohorts born before 1990: 2.98 years versus 3.53 years); the most plausible reason for this is a skewed sex ratio in favour of males in the adult population. Recruitment of males may thus, to some extent, be constrained by the availability of females. This study provides the first evidence that conspecific reproductive success can affect the age at which individual birds start to breed.

Frederiksen, M., Lebreton, J.-D. & T. Bregnballe (2001): The interplay between culling and density-dependence in the great cormorant: a modelling approach. J. Appl. Ec., 38: 617 - 627.

Frederiksen, M., Bregnballe, T., van Eerden, M.R., van Rijn, S. & J.-D. Lebreton (2002): Site fidelity of wintering cormorants Phalacrocorax carbo sinensis in Europe. Wildlife Biology, 8, 241-250.

Frederiksen, M., Lebreton, J.-D. & T. Bregnballe (2003): Modelling the effect of winter culls on Great Cormorant Phalacrocorax carbo sinensis population size in Europe: the importance of spatial variability in culling intensity. Vogelwelt, 124 (suppl.), 325-330.

Frere, E. & P. A. Gandini (2001): Aspects of the breeding biology of the red-legged Cormorant Phalacrocorax gaimardi on the Atlantic coast of South America. Mar. Orn. 29: 67 - 70.

Red-legged Cormorants Phalacrocorax gaimardi breed in Argentina, Chile and peru. In Argentina their breeding range is restricted to a short section of coastline in southern Patagonia. We studied two colonies located on high rocky cliffs, 2 to 4 m above the high tide line. At one colony, nests were protected from prevailing winds whereas at the other colony most of the nests were exposed. Of active nests, 15 % had two eggs, 66 % had three eggs, and 19 % had four eggs, to give a mean cluch size of 3.04 0.47. Egg dimensions were 60.3 2.4 x 37.1 1.4 mm. The incubation period ranged from 34 days to 38 days with chicks hatching from mid-November to the first week of December. Red-legged Cormorants lay more and smaller eggs than do those of two sympatric cormorant species, the Rock Cormorant P. magellanicus and the Imperial Cormorant P. atriceps, probably as a result of differences in foraging ranges. Avian predation on eggs seems to be an important mortality factor for this species and wind has also an important effect on breeding success, possibly exacerbating avian predation.

Förlin, L. (1999): Undersökning av tånglake i Göta älvs mynning, Stenungsund, Brofjorden och Fjällbacka. Bohuskustens Vattenvårdsförbund.

Glaser, L. C., Barker, I. K., Weseloh, D. V. C., Ludwig, J., Windingstad, R. M., Key, Douglas, W. & T. K. Bollinger (1999): The 1992 epizootic of Newcastle disease in double-crested cormorants in North America. Journal of Wildlife Diseases 35: 319 - 330.

In the summer of 1992, morbidity and mortality in juvenile double-crested cormorants (Phalacrocorax auritus; DCC) attributable to Newcastle disease virus (NDV) was observed for the first time in seven northern USA states and one Canadian province, and recurred in three western Canadian provinces. Based on clinical signs and laboratory diagnostic findings, DCC mortality from NDV occurred in 59 of the 63 nesting colonies and two of three non-colony sites investigated. An estimate of in excess of 20,000 DCC died, with mortality rates ranging from < 1 to 37% in Great Lakes colonies to 20 to 92% in Minnesota (USA) and North and South Dakota (USA) colonies. Sick juvenile white pelicans (Pelecanus erythrorhynchos) exhibiting signs similar to sick cormorants, and dead pelicans were observed in Minnesota and North Dakota. Mortality rates in pelican colonies were as high as in the adjacent cormorant colonies, but no cause for the mortality of an estimated 5,000 pelicans was determined. No evidence of NDV was found in other species nesting in proximity to affected cormorants. Although the source of the NDV infection is unknown in cormorants, the simultaneous onset of the epizootics in juvenile birds over a wide geographic area implies that the virus was acquired by adults prior to migration and was carried back to nest sites, exposing susceptible nestlings. The possible transmission of this virus from free-ranging wild birds to domestic poultry is a concern. Based on repeated epizootics in cormorants since 1990, NDV seems to be established in DCC.

Górski, W., Pajkert, Z. & I. Gorba (1990): [Competition and commensalism - two types of interaction between cormorant, Phalacrocorax carbo sinensis, and herring gulls, Larus argentatus]. Prz. zool. 34: 527 - 532.

Górski, W. & Z. Pajkert (1996): Interactions between Great Cormorant Phalacrocorax carbo sinensis and Herring Gull Larus argentatus argentatus in their common breeding sites. CRGB 2: 2 - 5.

Gorski, W. & Z. Pajkert (1997): Interactions between cormorants Phalacrocorax carbo and herring gulls Larus argentatus in their common breeding sites. Ekologia polska 45(1): 161 - 164.

Govedic, M. & F. Janzekovic (2003): The diet of Great Cormorant Phalacrocorax carbo on the Drava river in the winter of 1995/96 (Slovenia). Acrocephalus 24: 11 - 19.

Diet of the Great Cormorant Phalacrocorax carbo was studied by means of regurgitated pellets collected in March 1996 at night roost along the Drava river near Miklav`na Dravskem polju. Altogether, remains of 741 fish were found. Total weight of these fish was estimated at 115 kg. The diet consisted of 14 fish species (Chub Leuciscus cephalus, Nase Chondrostoma nasus, Barbel Barbus barbus, Grass Carp Ctenopharyngodon idella, Gold Fish or Prussian Carp Carassius auratus, Bream Abramis brama, Common Carp Cyprinus carpio, Danube Roach Rutilus pigus virgo, Roach Rutilus rutilus, Perch Perca fluviatilis, Ruffe Gymnocephalus cernuus, Striped Ruffe Gymnocephalus schraetzer, Zingel Zingel zingel and Pike Esox lucius). The diet was dominated by Perch (52.5% by number, 53.1% by mass) and Nase (14.0% by number, 22.3% by mass). Most of the fishes consumed by Cormorants belonged to the 18-22 cm (32.1%) size class. Average length of consumed Perch was 21.9 cm (median 21.5 cm, Q1-Q3: 18.9-25.2 cm) and 26.7 cm of Nase (median 25.3 cm, Q1-Q3: 22.3-31.9 cm). Average length of all prey in the diet of Great Cormorant was 21.3 cm (median 20.9 cm, Q1-Q3: 18.1-25.2 cm, min-max: 6.1-46.3 cm). Specimens of the first quartile constituted 6.4% mass of all prey, of the second and third quartiles 42.2%, and of the last quartile 51.3% mass of all prey. Length frequency distribution of the Perch, especially low proportion of small Perch in the Cormorants' diet, depended on standing waters' ice cover. Small Perches are abundant in standing waters, as they feed on plankton, which is most abundant there. In the winter of 1995/96 all standing waters in the Drava region were covered with ice and fishes in these waters were inaccessible to Cormorants. But as Ruffe and bigger Perches are not restricted to plankton diet, they also frequented flowing nonfrozen waters and were thus accessible to Cormorants. The proportion of Perch in Cormorants' diet was probably higher than in feeding habitat, while the proportion of Nase, Barbel and Chub was probably lower than in feeding habitat.

Grémillet, D. (1997): Catch per unit effort, foraging efficiency, and parental investment in breeding great cormorants (Phalacrocorax carbo carbo). ICES J. Mar. Sc. 54: 635 - 644.

The feeding ecology of breeding great cormorants (Phalacrocorax carbo carbo) was studied in 8 males and 6 females using automatic nest-balances and a radio-tracking system. Birds were shown to be extremely efficient predators with median catch per unit effort of 15.2 g of fish taken per minute spent underwater (n=89; gross foraging efficiency 3.25) in males and 9.0 g min-1in females (n=91; gross foraging efficiency 3.46). Mean daily food intake was also high (828±166 g, N=7 in males and 829±271 g, N=6 in females) and was positively related to brood biomass. Catch per unit effort was not related to foraging range, showing that birds do not tend to visit distant foraging areas to compensate for prey stock depletion around the breeding site. Furthermore, breeding adults responded to increasing food requirements of the brood by increasing the number of foraging trips per day, but maintained constant foraging effort, load size and catch per unit effort. Parents thus increased their overall foraging effort and food intake but kept their body mass constant.

Grémillet, D. & R. P. Wilson (1999): A life in the fast lane: energetics and foraging strategies of the great cormorant. Behavioral Ecology, 10: 516 - 524.

Body insulation is critically important for diving marine endotherms. However, cormorants have a wettable plumage, which leads to poor insulation. Despite this, these birds are apparently highly successful predators in most aquatic ecosystems. We studied the theoretical influence of water temperature, dive depth, foraging techniques, and prey availability on the energetic costs of diving, prey search time, daily food intake, and survival in foraging, non breeding great cormorants (Phalacrocorax carbo). Our model was based on field measurements and on data taken from the literature. Water temperature and dive depth influenced diving costs drastically, with predicted increases of up to 250% and 258% in males and females, respectively. Changes in water temperature and depth conditions may lead to an increase of daily food intake of 500-800 g in males and 440-780 g in females. However, the model predicts that cormorant foraging parameters are most strongly influenced by prey availability, so that even limited reduction in prey density makes birds unable to balance energy needs and may thus limit their influence on prey stocks. We discuss the ramifications of these results with regard to foraging strategies, dispersal, population dynamics, and intraspecific competition in this avian predator and point out the importance of this model species for our understanding of foraging energetics in diving endotherms

Grémillet, D., Storch, S. & G. Peters (2000): Determining food requirements in marine top predators: a comparison of three independent techniques in Great Cormorants, Phalacrocorax carbo carbo. Can. J. Zool. 78: 1567 - 1579.

Assessment of food requirements is a key feature in the evaluation of the ecological status of the marine megafauna. However, this remains technically difficult because prey intake by marine top predators occurs mainly under water, out of sight. In this paper, we compare three independent methods currently available for use in quantitative dietary studies: (1) time-energy budget; (2) stomach-temperature measurements; and (3) automatic weighing. To this end, concurrent measurements were performed on Great Cormorants (Phalacrocorax carbo carbo) breeding in Normandy. According to the time-energy budget method, breeding males required 690 g of fish while incubating, 1050 g when rearing small chicks, and 1350 g when rearing large chicks; corresponding values for breeding females were 500, 760, and 970 g. These measurements are similar to estimates derived from automatic weighing data, which gave a mean food intake of 540 and 390 g for incubating males and females, 1150 and 830 g for those tending small chicks, and 1410 and 1010 g for those tending large ones, respectively. Stomach-temperature measurements, which can only be performed for birds raising small chicks, were lower (640 g fish in males and 450 g in females) than those obtained using the other two methods. We compare these results with former estimates obtained at the same study site and for other Great Cormorant subspecies and discuss the relative accuracies of the three techniques. Finally, we stress that better assessment of the ecological status of marine top predators requires further technical improvements and additional investigations outside of the reproductive phase.

Grémillet, D., Wright, G., Lauder, A., Carss D. N. & S. Wanless (2003): Modelling the daily food requirements of wintering great cormorants: a bioenergetics tool for wildlife management. J.Appl. Ec. 40: 266-277.

- Great cormorants are large piscivorous birds which occur in Asia, Australia, Africa, Europe and North America. Their European breeding population has increased by at least 15% per annum over the last 15 years, reaching a total of 200,000 pairs in the late 1990s. There are concerns that this increase is adversely affecting freshwater fish populations throughout Europe, but real assessment\par requires a detailed knowledge of cormorant food requirements. The daily food intake (DFI) of great cormorants has been measured during the breeding season, but little is known about DFI in winter when these poorly insulated birds experience consistently low temperatures. DFI is likely to vary widely according to abiotic and biotic conditions, making predictions about impact particularly difficult. We modelled DFI for great cormorants wintering at Loch Leven, Scotland, using behavioural data recorded via radio-tracking of free-ranging individuals, metabolic measurements obtained from captive birds, and published data. DFI was estimated to be 672 g/day (predicted maximum range 441-1095 g/day, values similar to DFI of great cormorants breeding under temperate conditions and of other aquatic bird species.During winter great cormorants at Loch Leven decreased their average dive time and increased dive efficiency (higher proportion of time spent underwater). They nonetheless spent 130 min/day in the water and allocated more than a third of their daily energy budget to diving. In view of the need for the sound management of cormorant\par populations, we present a general bioenergetics model, based on simple behavioural and dietary inputs, that computes an estimate of DFI outside the breeding season for a range environmental conditions and habitats. An interactive computer programme for this model is available (http://www.cepe.c-strasbourg.fr) to help scientists and managers estimate local values for average, minimum and maximum DFI.

Hald-Mortensen, P. (1994): Danske skarvers fødevalg 1992 - 1994. Miljø- og Energiministeriet, Skov- og Naturstyrelsen. København.

Hansen, K. (1984): The distribution and numbers of the southern Cormorant Phalacrocorax carbo sinensis in Europe. DOFT 78: 29 - 40.

(transl.)By means of inquiries information about the European poopulation of Southern Cormorants was collected. (...) The Southern Cormorant still breeds in most countries within its traditional distribution. It has become extinct in Lithuania, Belgium and Austria, and only recently recolonised France (1981), the mainland of Italy (1982) and Czechoslovakia (1982). Thera never were records of breeding from Luxemburg or Switzerland. The total population is estimated at 21,299 pairs in 1982. Out of these 67 % breed in Northern Europe, and almost 40 % of them in The Netherlands and denmark combined. Romania is the only country in S. Europe with a population of importance (19 % of the total population).

Harris, M. P. & S. Wanless (1993): The diet of Shags Phalacrocorax aristotelis during the chick-rearing period assessed by three methods. Bird Study 40: 135 - 139.

Harritz, P. H. (1982): Skarv Phalacrocorax carbo sinensis ynglende på us&aeliG;dvanlig biotop i Danmark, 1982. DOFT 76: 80.

ICES (International Council for the Exploration of the Sea) (1984): Report of the working group on seabird ecology. Wilhelmshaven, Germany, 20 - 23 March, 2000.

[CP]: This is very competent work, and it is much quoted, but it is strangely anonymous, and the authors seem to be appealing for a continuation of their work and for the fact that the state of seabird populations be considered as an indicator of the state of fish population, lakes and seas, marine ecosystems. I would be interested to hear something about the working conditions of this expert group, whenever a member feels like talking. Internet version of this report.

Johansen R., Barrett R. T. & T. Pedersen (2001): Foraging strategies of Great Cormorants Phalacrocorax carbo carbo wintering north of the Arctic Circle. Bird Study 48: 59 - 67.

This study describes how 30 Great Cormorants Phalacrocorax carbo carbo managed to catch sufficient food for their daily energetic needs under conditions of reduced daylight and cold while wintering north of the Arctic Circle. Activity observations showed that the Great Cormorants' daily foraging pattern was generally bimodal, with morning and evening feeding peaks. They compensated for shorter daylengths in midwinter by starting to forage later and ending progressively earlier at lower light intensities. Fishing constituted only a minor part of their time-activity budget, and was one of the most efficient reported in marine birds. The Great Cormorants spent less than 60 minutes a day fishing in midwinter. Although subzero ambient temperatures and blizzards contributed to increased heat loss in midwinter, this potential energy loss did not seem to be compensated for by an increase in fish intake. Instead the Great Cormorants seemed to economize energy expenditure by halving the time spent at sea, and halving the number but doubling the mass of each fish taken.

Jonsson, B. (1979): Skarvarna och yrkesfisket i södra Kalmarsund. Calidris 8: 171 - 220.

Kato, A., Watanuk, Y. & Y. Naito (2001): Foraging and breeding performance of Japanese cormorants in relation to prey type. Ecol. Research 16: 745 - 758. Seabirds are high trophic predators in marine ecosystems and are sensitive to change in food supply and thus seabirds can be used as monitors of the marine environment. In order to study the foraging responses of Japanese cormorants Phalacrocorax filamentosus breeding at Teuri Island, Hokkaido to changes in fish availability, the diet was assessed from the regurgitations of parents and chicks, and diving behavior was measured by using time-depth recorders. Breeding performance (brood size, chick growth, breeding success) was monitored using conventional methods to study their breeding responses. Japanese cormorants changed the diet and foraging behavior over four summers. The birds fed mainly on epipelagic schooling fish when they were available and on demersal fish when pelagic fish availability was low. They tended to dive deeper and longer in a year when they fed mainly on demersal fish than the other years, reflecting the change in the depth distribution of prey fish. Chick growth rate did not differ among years, but fledging success was lower in the years of demersal fish as their meal delivery rate was low. When epipelagic schooling fish were considered scarce, parents maintained chick growth by reducing brood size. High variability and unpredictability in pelagic fish abundance are key factors affecting the foraging and breeding performance of Japanese cormorants, which could potentially be used to monitor fish resources.

Keller, T. (1995): Food of Cormorants Phalacrocorax carbo sinensis wintering in Bavaria, southern Germany. Ardea 83: 185 - 192.

Following the spectacular population increase in continental Cormorants, numbers of wintering birds on lakes and rivers in Bavaria, southern Germany, have risen sharply. Because of a growing concern among commercial fishermen and anglers, a diet study has been carried out at two major roosts: one at a lake (lake Chiemsee) and one at a dammed river (lower Inn river). Main prey species were cyprinids (Rudd, Roach, Chub and Bream), but at lake Chiemsee the commercially most important Whitefish also appeared in the diet during spawning in December/January. Mean daily intake per bird increased in the course of winter, because of larger specimens taken. Possible causes and effects of this phenomenon are discussed. Assuming an average daily uptake of 400 g fresh mass, it was estimated that at lake Chiemsee 3.3 % of the total annual fish production was taken by Cormorants (vs. 28 % by commercial fishermen). The total catch of Whitefish by Cormorants amounted to a mere 3.2 % of the total commercial catch of this species. Larger proportions were scored for Eel (22.3 %) and Pike (6.2 %). At the lower Inn river it was estimated that Cormorants took 21 % of the total annual fish production. Both in view of species composition in the Cormorants' diet and of the consumption estimates specified above, it is considered unlikely that the birds impose a serious threat to commercial fisheries. Interference with recreational angling (e.g. for Grayling) may, however, occur.

Keller, T. (1996): Maßnahmen zur Abwehr von Kormoranen - Eine Übersicht. Ornitologischer Anzeiger 35: 13 - 23. Kennedy, G. J. A. & J. E. Greer (1988): Predation by Cormorants Phalacrocorax carbo on the salmonid populations of an Irish River. Aq. Fish. Man. 19: 159 - 170.

Kirby, J. S., Gilburn, A. S. & R. M. Sellers (1995): Status, distribution and habitat use by Cormorants Phalacrocorax carbo wintering in Britain. Ardea 83: 93 - 102.

This paper describes a study of Cormorants wintering in Britain based on information collected as part of the National Waterfowl Counts scheme for the four winter seasons from 1987/88 to 1990/91. Birds were found to be widely distributed throughout Britain, both inland and on the coast, with the greatest concentration in south-east England, north-west England and south-west Scotland. Evidence is presented which suggests that Cormorants switch from coastal to inland habitats through the winter so that by February only 50-54 % of birds were found on coasts. The total winter population is estimated to have reached c. 19,000 in 1990/91, and has increased by 74 % overall. Possible reasons for these changes are discussed.

Kirby, J. S., Holmes, J. S. & R. M. Sellers (1996): Cormorants Phalacrocorax carbo as fish predators: An appraisal of their conservation and management in Great Britain. Biol. Cons. 75: 191 - 199.

The breeding population of cormorants Phalacrocorax carbo has expanded from 6 400 in 1969-1970 to 7 200 pairs in 1985-1987 and has probably benefited from protection under the Wildlife and Countryside Act 1981. The available studies of cormorant diet show that they exploit a range of fish species according to locality and season but often concentrate on locally dominant species. No study in Britain (or Ireland) has accurately quantified cormorant losses to fisheries, and the significance of depredation by them therefore remains unclear. A wide variety of methods to protect individual sites against cormorants is available. Most remain untested in British situations, and many may be impractical at some large waters where cormorant predation is seen to be a problem. Shooting to reinforce scaring is regarded as a favourable option by the UK Government and is exercised mainly on Scottish river fisheries. Unlicensed control is probably significant in some localities.

Kiss, J. B. & J. Rékási (2001): Data concerning diet and nesting of Pygmy Cormorant (Phalacrocorax pygmeus) in the Danube delta, Romania. Sc. Ann. Danube Delta Inst. 2001: 130 - 136. Internet version.

(...) Our work was limited to nest and egg size of the Pygmy Cormorant, along with its range of food items (dietary selection). This field work involved the measurement and analysis of 132 nests, 165 eggs, and the stomach contents of 71 birds in the Romanian delta of the Danube. Egg size ranged from 40.05 to 50.17 mm lengths, and 20.28 to 30.9 mm widths, with an average value of 41.31 and 29.93. The egg index ranged from 1.17 to 1.92, averaging 1.49. The stomach contents of Pygmy Cormorants shot at fish ponds between 1973 and 1982 was measured quantitativel and qualitatively. This method is no longer used, or accepted. Food consumed comprised 34 items, consisting of 4 plant species, 10 invertebrates, 2 amphibians, and 18 species of fish. Amphibians were in the stomachs of 18.3 % of the birds. Among the fish 39.4% were identified as Cyprinus carpio and 19.7% as Carrasius auratus gibelio. The prey selection is more varied during warmer weather than during winter, when only fish are taken. From the point of view of economic damage , the status of the prey items is distributed in the following manner: Fish of high economic value: 48%, Fish of no economicvalue: 21% Fish of low economic value: 14%, Amphibians: 11%, Invertebrates: 6%. During the summer of 2001 a census was performed of the area's Pgymy Cormorant population by aircraft and canoe. The number of birds was estimated to be 9,000 nesting pairs. Considering that this species is on the UICN list of globally threatened birds, and that its Romanian population resides in the Danube Delta Biosphere Reserve, there is no conflict currently between the birds' preservation needs and local economic needs.

Koffijberg, K. & M. R. Van Eerden (1995): Sexual dimorphism in the Cormorant Phalacrocorax carbo sinensis: possible implications for differences in structural size. Ardea 83: 37 - 46.

A sample of Cormorants which accidentally had been drowned in fishermen's gear was used to study differences in morphological characters and diet between males and females. Males were significantly larger than females in all body dimensions, whereas size of juveniles and adults were the same. Discriminant analyses applied on length of body, wing and culmen, and bill depth, achieved highest classification rates by a function which makes use of body length, wing length and bill depth. Bill depth and wing length showed best segregation between both sexes in all functions.(...)

Konstantinou, I. K., Goutner, V: & T. A. Albanis (2000): The incidence of polychlorinated biphenyl and organochlorine pesticide residues in the eggs of the cormorant (Phalacrocorax carbo sinensis): an evaluation of the situation in four Greek wetlands of international importance. Sc. Tot. Env. 257: 61 - 79.

This study contributed to identifying the current levels of organochlorine pollutants in four Greek wetlands of international importance (the Evros and Axios Deltas, and Kerkini and Prespa Lakes), using the cormorant Phalacrocorax carbo sinensis as a suitable bioindicator in a region where such information is scarce. Residue levels of eight polychlorinated biphenyl (PCB) congeners and 13 organochlorine pesticide (OC) compounds were measured in cormorant eggs. Most PCBs and OCs (except dieldrin and endrin) were found in at least some of the study areas. Median concentrations of five PCBs (IUPAC 8, 20, 52, 138, 180) and of six OCs (alfa-BHC, beta-BHC, lindane, heptachlor, 4,4'-DDE and 4,4'-DDT) differed significantly among the areas. The median totals of the PCBs were highly significant among the areas, being unexpectedly highest in Prespa Lake (68.43 ppb), despite its remoteness, and lowest in Evros Delta samples (12.17 ppb). Aldrin that was found in samples from Evros, Axios and Prespa probably accumulated in wintering grounds. In all of the areas, the relative proportions of ?-BHC and 2,4?-DDD were the highest of all OCs. Fingerprint and cluster analyses illustrated overall differences in the PCB patterns, being greatest between the deltas than between the lakes, but, inversely, for OCs the differences were smaller in the deltas. Differences were attributed to large variations in the cormorants' diet between areas and different regimes of pollutant management in the two types of wetland. Correlations of pollutants varied considerably among areas and they were more diverse in OCs. The sum of OCs/sum of PCBs ratio indicates agrochemical pollution in all areas. An important finding was that levels of both pollutant groups were too low to have any biological implications on the cormorants and, additionally, suggest that they have a negligible impact on the environment of the wetlands studied.

Kortlandt, A. (1984): Patterns of pair-formation and nest-building in the European Cormorant Phalacrocorax carbo sinensis.. Ardea 83: 11 - 25.

Courtship signals (i.e. the crouching partner in the nest centre and the erect partner outside the centre) are performed by both sexes, except for the male wing-waving. These patterns are derived from the copulatory postures by degenderisation and partly de-aggressionisation. The role differences between the sexes in copulatory behaviour, territorial defense and individual mate preferences are expressed by aggressive vs. subdued vocalisations and by stiffish vs. supple motions which symbolise dominance vs. submission. The similarity of postures and the soft voices in the tender rubbing, billing and cooing which precede egg-laying, symbolise the equality of the forthcoming parental roles. Copulatory behaviour and pair-forming behaviour ("sex" and "love") represent etho-physiologically different systems: they are not closely synchronised, have different motivations, operate through different mechanisms and have different aims and feedbacks, even though they mix and interact. Nesting behaviour is organised by a hierarchical system of drives and feedbacks, but the organisation differs between males and females. In males the system acts independently, in females it is subservient to the parental system. Ontogeny and seasonal maturation of nesting behaviour proceed by the isolated development of (sub-)patterns which is followed by their ascending integration. The methodology of interpretation of the observed behaviour is illustrated by recounting a "true love story" from Cormorant life.

Leopold, M. F., Van Damme, C. J. G. & H. W. van der Veer (1998): Diet of cormorants and the impact of cormorant predation on juvenile flatfish in the Dutch Wadden Sea. J. of Sea Research 40: 93 - 107.

Lindell, L., Mellin, M., Musil, P., Przybysz, J. & H. Zimmerman (1995): Status and population development of breeding Cormorants Phalacrocorax carbo sinensis of the central European flyway. Ardea 83: 81 - 92.

This paper describes distribution and population development of the continental race of the Cormorant in its central and eastern European breeding range. After a period of severe human persecution until halfway this century, the population started to rise from about 1980 onwards, ranging from 14 % in Poland to 27 % in Sweden. Systematic disturbances of newly established settlements have locally lowered the overall growth rate, but may have been responsible for further eastward expansion of the breeding range. Considering the timing of the population growth and the level of the growth rate, the recent developments of this population do not seem caused by the increase of the western population. Contaminant levels, though at least locally high, are unlikely to have had any noticeable negative influence on reproduction and/or population development. The role of eutrophication is suspected to be of importance, but proofs will still have to be found.

Logerwell, E. A. & A. Christiansen (2004): Energy density of Steller sea lion prey in Western Alaska: species, regional and seasonal differences. PowerPoint presentation at Marine Science Symposium, Alaska Fisheries Science Center, January 2003.

The energy density of prey fish is a necessary component of foraging models that show how changes in prey abundance or distribution (natural or fishery-related) might impact the feeding success of Steller sea lions. Although values of fish energy density can be found in the literature, data are not available for many species that sea lions eat. This is particularly true for specific geographic regions or seasons. The goal of this Fishery Interaction Team project was to fill these gaps by collecting fish that are common in sea lion diets, but for which energy density data is unavailable during the seasons and in the regions that sea lions eat them. Internet version of this poster.

Lorentsen, S.-H., Grémillet, D. & G. H. Nymoen (2004): Annual Variation in Diet of Breeding Great Cormorants: Does it Reflect Varying Recruitment of Gadoids? Waterbirds 27: 161 - 169.

Great Cormorant (Phalacrocorax carbo) diet was studied during three years (2001-2003) in an area where Arctic Kelp (Laminaria hyperborea) is extensively distributed off the central Norwegian coast. A total of 608 diet samples, 378 (62.2%) chick regurgitations, 22 (3.6%) whole fish, and 208 (34.2%) pellets were collected from the colonies at regular intervals during the chick-rearing period. From these samples a total of 1,013 food items (after pairing the otoliths) were isolated, representing 18 fish species. Gadoids, mainly Cod (Gadus morhua) and Saithe (Pollachius virens) dominated the diet (75% numerically, 86% by biomass). During the first year of the study, Cod represented nearly 50% of the diet, but decreased to 13% in 2003. At the same time, the occurrence of Saithe in the diet increased from 23% to 65%. For Saithe age II-group fish dominated the diet in 2001, and I- and II- group dominated in 2002 and 2003. For Cod 0-group fish dominated the diet in 2001 and 0- and I-group fish dominated in 2002 and 2003. The decrease in Cod in the diet of the Great Cormorant most probably reflected the decrease in the Norwegian coastal Cod population, and that the increase in Saithe in the diet is related to the relative increase in the abundance of this fish prey as the abundance of Cod decreased.

Marion, L. (1995): Where two subspecies meet: origin, habitat choice and niche segregation of Cormorant Phalacrocorax c. carbo and Phalacrocorax c. sinensis in the common wintering area (France), in relation to breeding isolation in Europe. Ardea 83: 103 - 114.

The wintering population of Cormorants in France reached 66,000 birds in January 1992 against 14,000 in 1983. This increase largely concerned the inland areas (holding 55 % of the population instead of 28 % in 1983). The most important area is the Loire valley. Analysis of 704 new recoveries or sightings of individually marked birds since 1983 confirms the segregation of habitat between the two races, 84 % of sinensis birds being controlled or recovered in inland areas and 80 % of carbo birds at sea. The same difference is observed between the French birds from inland and coastal colonies (16 % and 97 % of ringing sightings or recoveries at sea). In spite of the colour-marking of 22,600 birds among all European breeding areas in the 1980s, only few proofs of exchange of breeders between countries occurred until 1993 (0.3 % of Dutch or Danish birds), with a total of 13 birds concerning Denmark, Germany, The Netherlands, France and Ireland, all of them within each race's breeding area. The new inland tree-nesting colonies in France and East Anglia (England) seem to concern sinensis incursions in vacant breeding carbo area. This juxtaposition of the two races, based on ecological segregation, is discussed.

Matteson, S. W., Rasmussen, P. W., Stromborg, K. L., Meier, T. I., Van Stappen J. & E. C. Nelson (2000): Changes in the Status, Distribution, and Management of Double-Crested Cormorants in Wisconsin. Symposium on Double-Crested Cormorants, Technical Bulletin No. 1879, Dec. 1999.

We reviewed and summarized historical data and conducted population surveys from 1973 through 1997 to determine the breeding status and distribution of double-crested cormorants (Phalacrocorax auritus) in Wisconsin. Breeding cormorants historically occupied large, isolated lakes and wetlands in northern Wisconsin, but there were no known nesting sites until 1919, when cormorants were reported nesting on Lake Wisconsin in south-central Wisconsin. From the 1920's to the 1950's, cormorants occupied 17 colony sites in 16 counties, though no more than 7 sites were occupied during any particular year. From the 1950's to the early 1970's, the number of cormorant nests and colony sites plummeted owing to bioaccumulation of DDT and its metabolites, human persecution at some colony sites, and habitat loss. The installation of 1,269 artificial nesting platforms at 13 locations in north-central, northeastern, northwestern, east-central, and southwestern Wisconsin, coupled with a decline in DDE levels in breeding birds, as well as protection as a State-endangered species, led to a marked recovery. Between 1973 and 1997, the State's breeding population grew at an annual rate of nearly 25 percent, from 66 nests at 3 colony sites to 10,546 nests at 23 colony sites. We estimated population trends for six geographic regions in the State determined by distinct distribution patterns of nesting birds. Cormorant populations for five of six regions increased during 1973 through 1997. Trends differed significantly among regions, with a greater estimated increase in Great Lakes' sites (P < 0.01). In 1997, 81 percent of the State's breeding population occurred on four islands in Green Bay on Lake Michigan. Increasing Lake Michigan cormorant populations have raised concerns among sport and commercial fisheries about impacts on yellow perch (Perca flavescens) although recent studies indicate that alewives (Alosa pseudoharengus) predominate in cormorant diets. Internet version of this paper.

Last addition (68 entries) 5.3.07.

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