Great Cormorants were censused on a pan-European level in January 2013. Cormorants were found in an enormous winter range, spanning from the Baltic Sea and Atlantic Ocean in the north to the Mediterranean and coasts of North Africa. This large-scale exercise in which more than 5000 volunteers took part resulted in a total of 641,650 Cormorants counted (630,000–672,000 estimated). Based on the breeding census data of 2012 we estimated a total number of birds in January 2013 of slightly over 1 million, including the birds from the Ukrainian and Russian parts of Black Sea, Sea of Azov and north-western Caspian. Using the summer counts to produce a corrected estimate for the area that was actually covered during the winter count gave an estimated 695,000 individuals for January 2013. Total coverage was considered good and comparison to the previously conducted winter count of 2003 revealed, corrected for coverage, a 15% increase. As both counts were carried out during a period of cold weather, it is unlikely that birds were missed due to movements to the east and north-east of the range, from or into areas that are difficult to assess. Some 33% of all Cormorants were found to occur in areas with a temperature of –5°C or lower, suggesting that many birds can survive under conditions that may be marginal, i.e. at a high risk and/or cost. Compared to the previous count a relatively lower number of Cormorants were found under low temperature conditions, –5 °C down to –10°C, coinciding with the moment of active ice formation of shallow and stagnant freshwaters. It may well be that the actual ice cover in 2013 was such that birds had to leave these areas. In contrast to the opinion that wintering under low temperatures is marginal, it is possible that these northerly wintering grounds are more rewarding in terms of food profitability. This is because cold-blooded fishes congregate at certain spots and are less mobile at low temperatures and thus relatively easy to catch. Given the current and previous work carried out we recommend a long-term monitoring of these processes, which operate at a huge geographic scale. A repeated pan-European count with intervals of about ten years could detect the major patterns, while also providing a useful method of monitoring changes due to the expected further warming of winter conditions.

This study is based on recoveries of c. 86,850 Great Cormorants ringed in the Russian Federation during 1939–2014. Data were obtained from the Bird Ringing Centre of Russia. During 1939 to 2014 a total of 1667 Cormorant recoveries (1.9%) were added to the ringing database. The majority of the recoveries were made in the Caspian Sea, Azov Sea, Black Sea, Baltic Sea and in Kazakhstan inland waterbodies, including the Aral Sea (seven recoveries), Lake Alakol (46), Lake Balkhash (28), Kapchagay Reservoir (10) and Lake Zaysan (5). Non-breeding birds (assuming breeding at age > 3 years) comprised 89% of ring recoveries from 1939 to 1974, 95% from 1975 to 1990 and 100% from 1991 to 2014. This study is the first to describe the migration patterns for this species on a continental scale. From Kaliningrad in the west to Vladivostok in the east seven mega clusters of waterbodies and wetlands exist where Cormorants breed and were ringed. Although showing some overlap, birds belonging to a certain cluster were recovered in distinct wintering areas, often more than 2000 km from the breeding areas and separated by high mountain ranges. The general migration pattern is discussed for four geographic regions. The occurrence of east–west migration patterns is briefly discussed and deserves further study as well as the role of high mountain ridges possibly shaping the movements between summer and winter areas.

Two subspecies of the Great Cormorant breed in Norway, the continental Phalacrocorax carbo sinensis in the south, along the Skagerrak coast, and the marine P. c. carbo from central Norway and northwards. Here we review the information existing until 2017 on population status and trends, breeding performance and diet of these two subspecies in Norway. The most recent national population estimates are approximately 2500 (in 2012) and 19,000 (in 2012–2014) breeding pairs of sinensis and carbo, respectively. The sinensis population established itself in 1996 in Rogaland at the south-western tip of Norway, and in 1997 in Østfold close to the Swedish border; in both areas it increased for about ten years. Since then, the numbers have stabilised. For carbo, the population increased from 21,000 pairs in the early 1980s to 27,000 in 1995, and then decreased to the current number of 19,000 pairs. Significant annual variations in clutch size and reproductive output have been observed, but the drivers of these changes have not been identified. Unidentified gadoids and Atlantic Cod Gadus morhua were the most common prey of carbo, whereas inshore species such as Corkwing Wrasse Symphodus melops, Rockcook Centrolabrus exoletus, Goldsinny Wrasse Ctenolabrus rupestris and Black Goby Gobius niger were the most common prey in the eastern Skagerrak caught by sinensis. Carbo took very large numbers of 1–3-year-old gadoids during the year, and we cannot exclude the possibility this can have local effects on fish mortality rates.

Great Cormorants Phalacrocorax carbo in the eastern Baltic are expanding in numbers. Eight colonies in Estonia, Finland and the Russian Federation were investigated in 2007. Brood size and condition of large nestlings were determined and indicate that food provisioning was not a limiting factor. Food items consisted of both pelagic and benthic prey, with marked differences among the colonies. Eelpout Zoarces viviparus and Roach Rutilus rutilus were the most important prey according to fish mass. For pelagic prey this was Sprat Sprattus sprattus. Additional data in later years support the conclusion that the species experienced a rapid expansion at the time of investigation. Two factors are likely to have contributed to improved conditions for Cormorants in this part of the eastern Baltic. First, the low abundance of predatory fish such as Cod Gadus morhua and Pikeperch Sander lucioperca as a result of overfishing by commercial fisheries as well as climate change has, most likely, caused small benthic and pelagic prey fish to thrive. Second, increased eutrophication has probably fuelled the fish production. Based on fish species taken and the number of Cormorants present, the conclusion seems justified that any interaction between Cormorants and fisheries is unlikely at the scale of the Finnish Gulf, although locally measurable effects of Cormorant consumption on commercial yield may exist. As a visible, relatively new apex predator to the system, the Great Cormorant may well be an indicator of the ecological condition of the system. Monitoring of Cormorants (numbers, distribution, food, breeding success) may therefore provide useful data which can be used to assess the recovery of the benthic and pelagic parts of the food web.

We describe long-term changes in the distribution of 2249 freshly dead winter recoveries of 94,352 Great Cormorant chicks ringed between 1940 and 2018 in Denmark. The entire wintering range was divided into four major compartments to assess changes in (1) migratory distance and (2) the spatial distribution of recoveries. In the south-eastern wintering compartment, the mean distance to winter recovery sites declined from the winters 1946/47–2000/01 to those of 2001/02–2018/19 by 528 km (corresponding to a reduction of 36%). In the southern-central wintering compartment the change was gradual from before the mid-1980s to the winters 2006/07–2018/19 with a reduction of c. 700 km (corresponding to 41%). There were no temporal changes in migration distance for Cormorants wintering in the south-west. From 1991 onwards, recoveries were recorded in increasing proportions in the south-western compartment (from 21% in 1946/47–1990/91 to 60% in 2001/02–2018/19). The proportion recovered in the southern-central compartment varied between 34 and 45% up to the mid-1990s and then fell to 4–6% during the winters 2006/07–2018/19. The proportions recovered in the south-eastern compartment ranged from 9 to 18% until 1990/91 but fell subsequently to 0.6 to 2%. Long-term changes in the geographical origin of Cormorants recovered in Croatia further confirm that declines in numbers of recoveries of Danish-ringed Cormorants in the south-eastern wintering area reflect a true westward shift in winter distribution. The composition of recoveries in Croatia revealed that the south-eastern wintering areas were increasingly dominated by Cormorants from breeding colonies in the central and eastern Baltic region. We conclude that Danish Cormorants shifted their winter distribution westward from the 1990s onwards and shortened their migration by wintering further north. We hypothesise that this westward shift represents a response to increased competition with birds from breeding colonies located further east in the Baltic Sea, where populations increased markedly from the 1990s onwards.

After a period of continuous increase and range expansion, the Baltic Great Cormorant population has stabilised in large parts of its range in recent years. Ringing recoveries reveal that considerable proportions of the population winter in areas that can be affected by prolonged frost periods. There is evidence that winter severity is an important density-dependent regulation factor: if the population is large, ice cover of coastal and inland water surfaces during harsh winters affects the population by reducing the availability of food resources. As long as the population remained small, however, it was not affected even by very cold winters, since the remaining accessible food resources were presumably still sufficient. The analysis presented here uses the average winter temperature in Germany as a proxy for winter severity in the frost-affected parts of the wintering areas of Baltic Cormorants. The Baltic Cormorant population in 1980–2016 is estimated from annual counts in Denmark, Schleswig-Holstein, Mecklenburg-Western Pomerania, Estonia, Finland and Gotland, which account for about 50% of the total population. The interplay between winter severity and density dependence is analysed using a linear and a non-linear regression model approach. The non-linear model gives a better description of the relationship between the size of the Baltic breeding population during the year (n), the winter temperature T_n, and the population size during the previous year (n–1). According to the model, a population of less than 41,400 breeding pairs would not suffer declines during even the coldest winters recorded since 1882. In 1989, the Baltic Cormorant population exceeded for the first time the threshold value for density-dependent regulation caused by severe winters. The winter 1995/96 was then the first one cold enough to cause a population decline. According to the model, during the years 2002/2003, 2005/06, 2008/09, 2009/10 and 2010/11 the winters have been cold enough to reduce population numbers. Furthermore, the model shows that the regulative winter effect is restricted to the low temperature range.

The number of wintering Great Cormorants Phalacrocorax carbo sinensis in France has strongly increased since the 1970s, mainly due to the protection of the ‘continental’ sinensis subspecies in countries north of France. This increase has led to the establishment of a pioneering inland breeding colony in western France, while previously only the largely marine ‘Atlantic’ P. c. carbo subspecies occurred on the Channel coast. The marine subspecies was attracted and bred in this new inland settlement of sinensis, which rapidly became the largest colony in France. This paper analyses the migration pattern of birds from this colony by analysis of the dispersal of colour-ringed birds between 1989 and 2008. Interestingly, besides a classic south-west migration pattern, birds from this colony also displayed a pattern heading north-east, up to countries such as The Netherlands, from where the founders (sinensis) of this colony probably originated. Sightings and recoveries revealed that about 25% of the adults and 19% of the first-year birds headed north-east. Due to this north-east migration direction, the overall annual dispersal point was located only 50 km south-west of the colony, although in December and January this midpoint was located about 320 km south-west of the colony. The birds largely avoided Brittany, presumably to avoid competition with individuals of the carbo subspecies, and the main wintering areas of sinensis from other colonies, both in France (east, centre and south) and in Spain. Over the years 1989–2008, in the breeding period, the mean dispersal distance was shorter for adults than for young birds (54 km vs. 144 km, respectively) but in winter adult birds migrated further than young ones (305 km vs. 221 km, respectively). The mean annual dispersal distance in winter varied from 106 km to 527 km (all age-classes taken together). Migratory distance was not related to mean winter temperature. For adults, dispersal distance correlated with the annual number of breeding pairs in the Grand-Lieu colony between 1990 and 2003, but not between 2004–2008. Emigration (breeding in another colony) was recorded up to 2011 to 11 inland colonies and one coastal colony (founded more recently than Grand-Lieu), nine of them in France, two in Spain and one in The Netherlands. Annual emigration rate was negatively related to colony size in Grand-Lieu. The study points to the existence of density-dependent effects on distribution patterns of Cormorants outside the breeding season but also suggests connectivity and interaction among colonies that are hundreds of kilometres apart.

We analysed trends in the breeding population of Great Cormorants in Greece during the period 1988–2014 when at least 20 colonies were recorded. The number of colonies doubled over the study period reaching 14 in 2014 from seven in 1988. There was also a continuous increase in the number of nests: from 952 in 1988 to 9256 in 2014, presenting an annual change of +26.5% ± 0.05 (SD). Most of the colonies were situated in freshwater lakes, three in deltas and one on the seacoast. Nine colonies were mixed with other colonially nesting species (mostly from the Ardeidae family). Of the 14 colonies 11 were found in trees. Two colonies were situated on the ground, four in bushes, two on reed rhizome islets and one on cliff ledges. Five out of the 14 active colonies in 2014, comprising 87% of the total recorded nests, were in wetlands of low altitude (0–45 m a.s.l.). The rest were situated in mountainous wetlands at altitudes ranging from 235 to 853 m a.s.l. One colony, at Lake Kerkini, contained the majority of nests in Greece (6650, being 72% in 2014). The second largest colony, at Lake Volvi, had 900 nests, followed by the Greek part of Prespa with 625 nests (in three colonies) in 2014. The percentage of nests in newly established colonies increased after 2003, reaching its highest value (14%) in 2009. The reason for the growth in both colonies and nests over the years is attributed to the increasing availability of fish, the protection status of wetlands and the absence of disturbance. Particularly after 2002 the species increased in numbers and established colonies in new areas, which is probably related to the expanding breeding populations of the species in north-eastern European countries and a corresponding increase in wintering numbers in Greece. This needs further attention by monitoring and research.

Historical sources from European countries show that the persecution of Great Cormorants started centuries ago and was widespread. As an exception, in The Netherlands protective measures for Cormorants were declared from 1500 onwards. In this paper an explanation is given for this striking difference. In The Netherlands, Cormorants were game species and a food source. To make this use sustainable, Cormorants were protected. In other countries, Cormorants were neither considered to be a game species nor a source of food. On the contrary, in most countries Cormorants were seen as pest birds. They were persecuted to protect the pond culture of fish. In The Netherlands, however, this kind of pond culture was uncommon. Because of these differences with the rest of Europe, in The Netherlands the general opinion was more in favour of protection than of persecution. In consequence, the Dutch population of Cormorants was much larger than elsewhere in Europe. The survival of this Dutch population amidst the depleted populations in other countries was an important prerequisite for the increase of the European Cormorant population in the 20^th century.

Though the Japanese population of Great Cormorants Phalacrocorax carbo has increased in recent decades, information on its spread to the northern island of Hokkaido has not been reported outside of Japan. The purpose of this paper is to update the scientific community about the distribution and breeding ecology of Great Cormorants and to provide comparative information on the abundant and phenotypically similar resident Japanese Cormorant P. capillatus. Several ornithological groups and researchers were contacted in order to gather information about the current distribution and breeding activity of the Great Cormorant in the region. This paper consolidates the findings of Japanese research groups, translated publications and direct observations.

The foraging of Great Cormorants Phalacrocorax carbo in Alpine rivers is a relatively recent phenomenon. Wintering populations in Slovenia increased in the 1980s. In the 1990s, the increase affected the upper sections of rivers where the habitats of Grayling Thymallus thymallus and trout Salmo and Oncorhynchus spp. are located. The upper section of the Soča River system is known as one of the biogeographic hotspots for (sub)alpine rivers, where several endemic or rare fish species are found. The aim of this study is to evaluate the diet of the Cormorants in this area and to compare it to some other threats to the fish fauna of the river. A total of 75 Cormorant stomachs were examined. Of these, 68 (90.7%) contained fish or recognisable fish remains. The birds' diet consisted of eight species of fish: Marble Trout, Rainbow Trout, Common Nase, Vairone, Common Chub, Italian Chub, Italian Barbel and Bullhead. The increase in numbers and the diet of Cormorants in the river basin of Soča is discussed in the context of the increase of other threats to this ecosystem, such as fish stocking, gravel extraction and tourism, especially angling and water sports. A more comprehensive environmental assessment of all these factors is needed, as their increase runs parallel with the arrival of the Cormorants in the upper section of the river. There is therefore reason to believe that Cormorants are only a small part of a larger problem in the conservation of the river ecosystem.

Since 2006, Great Cormorants Phalocrocorax carbo sinensis have wintered in the area of the river Geul, a right-bank tributary of the river Meuse, in the province of Limburg in the south of The Netherlands. Although the number of birds there is relatively small (approximately 30 birds), the local sports fishery sector is concerned about the possible impact on wild Brown Trout Salmo trutta, particularly on the young year-classes, through predation by wintering Cormorants. The number of birds, as well as their estimated fish consumption, was studied in the winter of 2012. Analysis of 70 diet samples (pellets) taken from the roost local to the area, showed that predation was primarily on young year classes of cyprinids, like Roach Rutilus rutilus. These cyprinids, and probably also the few trout consumed, were thought to have been mainly taken from farmed fish ponds in the direct neighbourhood of the river Geul. Besides predation of larger cyprinids, the Cormorants also took abundant small riverine fish species (2–10 cm) like Rhine Sculpin Cottus rhenanus and smaller cyprinid species like Minnow Phoxinus phoxinus and Gudgeon Gobio gobio. These riverine fishes have increased recently due to ameliorated water quality. The estimated fish consumption by Cormorants in the present study suggests limited or no impact on Brown Trout during winter.

Aquatic habitats are subject to multifactorial changes including global warming, invasive species colonisation, modification of organic and micro-pollutant discharge and, for large rivers in Europe, drastic physical modification (e.g. channelisation, impoundments). The Meuse River in Belgium is one of these multi-stressed environments, in which recent decreases of fish populations were observed, with the loss of 90% of Roach Rutilus rutilus biomass in only a few years. In the light of this fish stock collapse, diet modification and local population evolution of a key avian predator, the Great Cormorant Phalacrocorax carbo, were examined. The diet composition and daily consumption rates of the Great Cormorants feeding in the river were largely similar to that seen before the fish population collapsed. Numbers of wintering Great Cormorants decreased by 90%, as did Roach numbers, and thus the predation pressure was adjusted to the decreased fish availability. The number of night-roosts and locations remained unchanged and no redistribution to adjacent habitats was observed at the regional scale. We suggest a bottom-up chain of responses where a fish collapse forced a reduction in Cormorant numbers, being the main piscivorous avian predator, rather than a modification of Cormorant prey composition and/or a local redistribution to adjacent wetlands (top-down). The factors that govern the establishment of a small and productive breeding population remain to be explained, but we hypothesise that the start of breeding could well have been alleviated by the large decrease in number of wintering birds.

The recent increase in Cormorant numbers in the UK and the increasing use of inland feeding sites by the birds have created conflicts at many freshwater fisheries. Information on the prey composition of Cormorants foraging at such sites is important in understanding these conflicts and in providing a sound basis for management. Two subspecies of Great Cormorant, the continental subspecies Phalacrocorax carbo sinensis and the Atlantic subspecies Phalacrocorax carbo carbo occur in the UK and forage sympatrically. This study presents information on the winter diet of over 1400 Cormorants shot at inland fishery sites over an eleven-year period. A wide range of prey species were identified, but five fish species compromised almost 80% of the prey items recorded by number, and most prey items were small (≤10 cm). Dietary differences, in terms of the species, sizes and number of fish consumed, were apparent between birds of different sex and subspecies, consistent with the differing sizes of the birds. The largest birds (male P. c. carbo) tended to consume fewer, larger fish and the smallest birds (female P. c. sinensis) more, smaller items. Bird size also appears to influence foraging site selection, with male P. c. carbo more likely to forage on fisheries, such as put-and-take trout fisheries, where larger prey items were available.

Monthly aerial bird counts showed a strong increase in the number of wintering Great Cormorants Phalacrocorax carbo sinensis since the late 1990s at Lake IJsselmeer but not at Lake Markermeer-IJmeer. Compared to the 1980–1990s, breeding numbers also increased in this part of the system. The resulting increased exploitation of fish stocks was thought to have been possible because of a long-term increase in the stock of Ruffe Gymnocephalus cernuus, despite a clear overall decline of total estimated fish biomass in the lake during the same period. The most likely cause of these shifts was thought to be the intensive commercial fishing regime, removing the large predatory fish first, followed by a strong reduction of stocks of large Bream Abramis brama, in turn paving the way for increases in the stocks of Ruffe. Increased predation by Cormorants on the enhanced stocks of small fishes was possible because of ameliorated underwater visibility in Lake IJsselmeer. Starting in 2000, there was a strong shift in both temporal habitat use and associated fish consumption by Cormorants towards the winter period. The local breeding birds, exploiting the same age- and size-structured community of fishes in the spring, thus face an already-depleted food resource. Compared to the 1980–1990s, fish consumption by Cormorants in winter increased by a factor of ten, whereas that by breeders did so by a factor of 1.6. Our calculations showed that the actual harvest of available fish stock by wintering and breeding Cormorants together was c. 5% in 1985–2000 and c. 15% in 2001–2015. The disproportionate division of the overall consumption (‘harvest’) of the fish stock towards the wintering birds is a strong argument for direct competition with their conspecifics breeding locally. In conclusion, we calculate that because of the increased winter exploitation initiated by the activities of an intensive commercial fishery, the fish consumption in summer and early autumn by breeding Cormorants and their offspring was suppressed by a factor of six.

With the aim of studying ecological specialisation between subspecies, we compared the components of breeding success in individuals of two recently sympatric subspecies, carbo (‘marine’) and sinensis (‘continental’), of the Great Cormorant in a continental colony. The subspecific origin of broods was determined using D-Loop mtDNA and microsatellites. Although there were no differences in clutch size and laying date between the subspecies, mean fledging success was lower for the marine subspecies (–30% according to mtDNA assignment, –38% according to microsatellite assignment) than for the continental subspecies, while mixed breeding pairs had an intermediate fledging success. These results showed that the marine subspecies is less well adapted than the continental one to inland water, which is considered to be the optimal habitat of the continental subspecies. According to these results and to the geographical expansion of the continental subspecies, we suggest that the proportion of marine subspecies in western European inland colonies could decrease when density-dependent competition increases due to saturation.

The necessity of balancing conservation issues with the protection of aquaculture activities drives research to quantify the impact of the Great Cormorant Phalacrocorax carbo and to identify the best mitigation strategies. This study aims to characterise the diet in relation to the possible impact of the Great Cormorant on the extensive aquaculture farms in Friuli Venezia Giulia region in the upper Adriatic Sea. The diet of the species was investigated by analysing 459 pellets, collected monthly during the winter 2006/2007 in the three most important roosts of the Friuli Venezia Giulia coastline. Cormorants proved to feed mainly on typical fish species of lagoon and coastal waters. By frequency, Sand Smelts Atherina boyeri (54%) and Flounders Platichthys flesus (26%) were the main prey. Flounders played an important role also in terms of biomass (35%), together with mullets (Mugilidae, 28%). Commercially-important aquaculture species (European Seabass Dicentrarchus labrax and Gilthead Bream Sparus aurata) represented only 2% and 14% by frequency and biomass. The diet analysis supports our previous studies, carried out by behavioural observations on Cormorants foraging at different depths, on interactions with the main prey and strategies of exploitation of the fish farms. Our data imply that the Great Cormorant does not have a large effect on aquaculture yield in the north Adriatic Sea.

Great Cormorants Phalacrocorax carbo sinensis are considered generalist predators feeding in a variety of marine and freshwater habitats on many different prey species. Recently however, there is increasing evidence that even seemingly generalist species can often be regarded as a diverse group of individual specialists sometimes induced by sex-specific foraging behaviour. To test whether this also holds true for Great Cormorants, the present study characterised the at-sea distribution and examined the time budgets of provisioning Great Cormorants by deploying GPS-loggers on 11 nesting males and females during the breeding season of 2012. Additionally, pellets were collected from underneath the nests of these tagged birds for detailed diet analysis, based on the otoliths and fish bones recorded within them. Diverse foraging patterns were recorded, ranging from exclusively freshwater feeding, to offshore foraging, with the furthest recorded distance from the colony being 28 km at sea. Time budgets during foraging trips revealed relatively long periods of resting ashore (55% of the trip ± 23%), followed by foraging (31 ± 18%), and time spent in flight (15 ± 11%). Flatfishes dominated in the diets of these Great Cormorants, indicating benthic-foraging strategies when foraging in saltwater (feeding on discards could be ruled out). The males in this study avoided freshwater foraging habitat and preferred offshore waters with a sandy bottom or nearshore waters alongside rocky breakwaters and within harbours. Females foraged in all habitat types, but proportionally more in inland freshwater bodies. Females spent more time on foraging than did males, particularly in marine habitats, whilst males rested more. It is proposed that these differences might have been caused by sex-specific foraging strategies, possibly resulting from competition at foraging sites.

Coastal breeding Great Cormorants Phalacrocorax carbo foraging in a shallow part of the Dutch North Sea preyed upon abundant marine demersal fish species. In 2010–2012 intensive fish surveys were performed in the Voordelta area and in 2009–2015 Cormorant pellets were sampled in the breeding colony of Breede Water, Voorne, four times per year between March and September. In total 48 fish species were detected in the diet, 38 being marine species. Mainly flatfish were consumed, and European Plaice, Common Dab and Common Sole were the most important prey according to fish mass. Experimental trawling revealed 65 species of fish of which gobies, Herring, Whiting, Sprat, European Plaice and Common Dab were the most abundant. Compared to the trawl data, Cormorants showed a preference for Common Dab and Common Sole and for other solitary bottom fish like sandeels and Shorthorn Sculpin. These species were all common in the area. With respect to uncommon and rare species, no preferential selection was recorded. Densities of flatfish were highest in foraging areas closest to the breeding colony and possible depletion effects were only recorded in Common Dab. This diurnal species was already being preyed upon early in the season. Nocturnal foraging habits in other flatfish species, in combination with burrowing behaviour and rounded body shape are effective anti-predator traits and this was reflected in lower frequencies of these species in the Cormorants' diet. Consumption of freshwater fish by Cormorants at the beginning of the breeding period enabled an early start to breeding, and the increasing availability of flatfish in late spring matched the peak demand of rearing nestlings. The almost exclusive predation on flatfish was probably caused by the near-bottom foraging behaviour of most Cormorants and this habit made the birds feed on other abundant demersal fish species as well, such as Whiting, sandeels, Shorthorn Sculpin, Lesser Weever and dragonet species. And although numerous in the system, this bottom-oriented feeding behaviour of Cormorants therefore resulted in a very limited predation on pelagic fish species. In total we estimate an annual extraction by Cormorants of some 100 tonnes of fish being c. 77,600 kg flatfish and c. 20,700 kg other marine fish. Although with a foraging range partly outside the coastal zone, the extraction of fish by Harbour Seals Phoca vitulina and Grey Seals Halichoerus grypus outnumbered that of Cormorants by a factor of 9. As seals are known flatfish consumers, this suggests that there is competition between mammalian and avian predators on demersal fish stocks in the coastal zone.

Water transparency is an important factor affecting fish availability (underwater visibility) for diving birds. The diet of Great Cormorants Phalacrocorax carbo in relation to water transparency (range 1.4–4.0 m) was studied by pellet analyses at the submontane reservoir Dobczyce, Poland, from June to November. Although water transparency proved to be related to the birds' distribution, in the range of turbidities studied, no relationship was found with either fish species and fish size taken. Of 14 species in the diet, Roach Rutilus rutilus was dominant in all monthly samples (35–91% in 2002, 56–82% in 2004). Numbers of Great Cormorants and water transparency (range 0.4–1.4 m) were studied in the turbid lowland reservoir Goczałkowice during the migration period in autumn (August–November 2011 and 2012). Observations here suggest that the effect of water transparency on food uptake and habitat choice was only apparent below 0.6 m Secchi depth. The number of foraging Great Cormorants was not only affected by Secchi depth, but by a complex of factors (year, month, place, Secchi depth and water depth). We argue that, especially when the number of Great Cormorants is low, only highly detailed measurements of these factors (both spatially and as time series) can show the complex relationship between prey distribution, environmental conditions and the predator's behaviour.

The feeding ecology of the Great Cormorant Phalacrocorax carbo has been the subject of many studies in which the hard parts of fish, contained in dietary samples such as regurgitated pellets, were identified using morphological characteristics. However, morphological prey identification does not necessarily permit the reliable identification of all fish species due to digestion eroding diagnostic characters and/or morphologically indiscernible hard parts in a range of fish species. Molecular methods have the potential to overcome these obstacles by allowing the detection and identification of minute quantities of prey DNA present in pellets, faeces, and stomach samples. Moreover, DNA of the consumer (i.e. the Cormorant) is also present in dietary samples and can thus be employed for ecological studies too. Here, we present a methodological overview of two molecular approaches commonly used to study trophic interactions, namely diagnostic PCR and next generation sequencing, along with their main advantages and disadvantages. Regarding the use of consumer DNA contained in dietary samples, molecular sexing, i.e. the non-invasive sex determination of the sample-producing bird, is presented. We exemplify the potential of DNA-based methods for future research via a case study on pellets collected at Chiemsee (Germany), which were subjected to molecular and morphological prey identification as well as to molecular sexing. Compared to morphological prey identification, molecular analysis led to a 53% increase in prey species and genera, mainly caused by eight additionally detected cyprinid taxa. For 79% of the pellets, the sex of the pellet-producing Cormorant could be successfully determined via molecular sexing. Our findings highlight the exciting possibilities molecular methods offer for future studies on Cormorant feeding ecology, especially regarding evaluations of prey spectra and the non-invasive assessment of sex-specific differences.

Day-fresh Great Cormorant pellets were collected at a huge single winter roost at Kreupel island in Lake IJsselmeer, The Netherlands. The roost is situated on a sandy island, 3 m above the water level and is regularly used by 1000–8000+ Cormorants. By examining pellets and thus reconstructing the previous day's meal of Cormorants gathering at the roost, we explored spatio-temporal differences in the diet of individual birds. We found that structural differences in pellets (size, colour and structure) are related to their content (fish species and fresh mass of a daily meal). Structurally larger and lighter coloured pellets contained larger and coarser fish and represented a higher total fish mass. Pellets of different size and structure were non-randomly distributed at the roost. Birds positioned centrally in the roost produced on average larger pellets than birds at the edges. Cormorants having few or no food remains in their pellets were more frequently recorded on the edge of the roost than in the centre and in particular, they were more likely to be located to the rear of the roost. The same was true for pellets showing a bloody signature which may be linked to either injury by sharp fin rays or to a parasite induced effect. Pellet data were corroborated by observations of differential distribution of faecal splashes of Cormorants, showing that individuals in the centre of the roost produced most guano. Clear diet differences existed within the roost between birds in the centre and elsewhere in the roost, whereas smaller differences existed between the front, edge and rear of the roost. Our data suggest a social hierarchy based on higher foraging success of birds returning earlier to the roost and settling in the centre, hereby forcing later arriving and less successful Cormorants towards the edge, rear and front position. We conclude that position at the roost is a reflection of previous foraging success, possibly caused by differences in dominance between Cormorants during the period of communal foraging. From a methodological point of view, our study demonstrates that, because of structural differences in pellets and spatial differences in occurrence, it is important to collect pellets randomly if one is to describe a proper diet sample by using pellets at any given site. Especially in marshlands and under trees and bush growth with a lot of litter, the likely collection of larger, more conspicuous pellets will cause a bias towards the larger fish species and sizes. Also sampling only at edge locations or specifically at the centre of a roost may give biased results, as this may result in an under- or overestimation in reconstructed fish uptake (e.g. fish size, mass, species) from the waterbody under study.

The recovery of the Baltic White-tailed Eagle Haliaeetus albicilla population since the 1980s is a conservation success story. However, the re-establishment of this predator in coastal and lake areas appears to affect Great Cormorants Phalacrocorax carbo during the breeding season. We collated published and unpublished records of White-tailed Eagle occurrence and interactions with Cormorant colonies as well as records of Cormorant anti-predator responses and effects on Cormorant breeding performance. We found evidence for immediate direct effects (mainly kleptoparasitism, predation of eggs and offspring) and indirect effects (facilitating nest predation by mainly gulls and corvids). Repeated disturbance from hunting White-tailed Eagles was also inferred to trigger changes in selection of nest sites, to cause complete abandonment of colonies and lead to regional redistributions of breeders. However, there are also observations of ‘unproblematic’ coexistence with White-tailed Eagles breeding inside Cormorant colonies. There is some evidence to suggest that nesting opportunities for Cormorants may become further restricted as White-tailed Eagle numbers continue to increase, and we argue that this – in some regions – will impose limitations on the distribution and size of breeding populations of Cormorants.

Breeding Great Cormorants Phalacrocorax carbo have been rapidly increasing in Italy since the 1990s, and have settled their colonies in sites already used by other colonial waterbirds. We investigated the patterns of interference between Grey Herons Ardea cinerea and Great Cormorants, the two species most prone to competition due to their similar preference for nest sites. The analysis of long-term population trends in colonies with both species compared to colonies without Cormorants, showed that the number of Grey Heron nests gradually decreased following the settlement of Great Cormorants. The two species exhibited a gradual spatial segregation in nest location, both horizontally and vertically, within the same colony. In some heronries we found clear examples of interaction, but we also report cases where the interaction was less clear. Thus, although Great Cormorants might be competitively dominant over Grey Herons for nest site occupancy in many of their shared colonies, the spatial competition dynamics might also be influenced by factors affecting population dynamics in the wider environment. We envisage specific observations and focus on areas with an apparent abundance of trees suitable for nesting by both species.

Density-dependent depletion of prey during the breeding season may affect breeding performance and colony development in seabirds and colonial waterbirds. Breeding Great Cormorants Phalacrocorax carbo sinensis are central place foragers, so parental flight distances to profitable foraging grounds and thus parental provisioning of offspring are likely affected by local food availability. Chick body condition in a given colony may therefore be influenced by the distance to feeding areas, changes in fish stocks and fish distribution as well as the number of Cormorants from neighbouring colonies using the same feeding areas. At three Danish Great Cormorant colonies with overlapping foraging ranges and variable access to shallow marine areas, Vorsø (VO), Mågeøerne (MA) and Stavns Fjord (SF), we investigated variation in mean chick body condition (n = 9697) in relation to colony location, year, colony size and estimated density of foraging conspecifics during 1993–2007. Foraging areas available to VO colony parents had the highest theoretical densities of conspecifics and the VO chicks were invariably in poorer condition than the chicks at the other two colonies. Chick body condition was negatively correlated with estimated foraging density, especially within a foraging range of 20 km. Body condition between all three colonies was weakly correlated over the years, suggesting that changes in macro-environmental conditions (e.g. fish density) only had moderate impacts on food availability, independently of Cormorant numbers. We conclude that both the geographical location of colonies, the size of the colony itself and the nearest neighbouring colonies and the associated variation in density of foraging Cormorants were major drivers of variation in chick body condition between colonies and years.

Ground-nesting Great Cormorants were monitored in three neighbouring colonies at Lake IJsselmeer, The Netherlands. Using aerial photographs taken during peak breeding time, nest density and nearest neighbour distance were determined for four sequential years. In addition, species and number of predators were determined. In total, five mammalian and nine avian predatory species were associated with the Cormorant breeding colonies. Spatial distribution of nests mostly showed dispersed and random patterns rather than a contagious pattern. The latter distribution, with less distance between nests than expected both from a random and equal distribution pattern, was found in the colony of De Ven in 2013 during the last year of its existence. The predator Red Fox Vulpes vulpes arrived at the colony in 2010. In all three colonies, nest density was highest and nearest neighbour distance shortest in colonies with the highest number of predators. At low to moderate predatory pressure, ground-nesting Cormorants left free space between nests that was used by adult birds during take-off and landing. During the last years of its existence the shrinking colony of De Ven showed an almost circular shape, with an extreme nest density and the lowest edge-to-surface area ratio. But with Foxes present, breeding at the fringe still caused greater losses due to direct predation. Breeding success fluctuated synchronously between colonies but was lower in colonies where the number of predators was higher. The arrival of Red Foxes in De Ven caused extreme losses of young and over the years resulted in a strong decline in number of breeders, eventually leading to complete abandoning of the site in 2014. Large gulls formed another important group of predators but did not cause the Cormorants to abandon the breeding site. In the Vooroever colony, bush and tree cover supplied shelter and allowed birds to breed in greater density without causing nearest neighbour density to decrease, as was the case when no cover was available. Greater nest density and reduced nearest neighbour distances are considered to be a pro-active response by individual birds to the presence of predators. When predator numbers increased, the within-colony open spaces that normally exist under circumstances of moderate density were filled up with nests, leaving little or no room for landing and departure. This leads to reduced edge effects and a circular shape of the colony, thereby potentially limiting predation risk. As a consequence of extreme high nest densities, breeding success was lower due to interference by other Cormorants. This study is the first to show that colony structure in waterbirds is affected by forces of attraction and repulsion between founding birds that are predator driven.

The Great Cormorant Phalacrocorax carbo is a widespread piscivorous waterbird. The competition for resources between recreational anglers and Cormorants has been causing serious conflicts between sport fisheries and environmentalists. This study aimed to compare fish catches by Cormorants and by recreational anglers in the upper Elbe River in Central Europe (Czech Republic). Cormorant diet was studied using regurgitated pellets, and catches of anglers were obtained from annual angling reports. Altogether 1478 Cormorant pellets were collected from which 6903 fish were measured and identified to species level. A total of 93,413 fish caught by anglers were identified to species level. Cormorant diet consisted of 24 fish species in six fish families. Cormorants caught smaller-sized fish (median mass 90 g) compared to fish caught by anglers (median mass 1700 g). The majority of fish caught by Cormorants were under the minimum legal catchable size for anglers. Species of moderate interest to anglers (mainly Roach Rutilus rutilus) dominated in Cormorant diets while Common Carp Cyprinus carpio dominated in catches of anglers. In conclusion, the direct competition for fish between anglers and Cormorants appeared low. However, as Cormorants consumed small fish that serve as prey for piscivorous fish species and that could potentially grow into legally sized fish for angling purposes, this still allows for indirect competition between Cormorants and sport fisheries.

A series of paired-pond trials were completed during the winters of 2005 to 2008 to examine the effect of introducing artificial fish refuges on Great Cormorant Phalacrocorax carbo foraging behaviour, prey capture rates and fish losses in trial ponds, and to help refine their potential use as a tool in addressing cormorant-fisheries conflicts. Investigations comparing refuge volumes within a year provided clear evidence that larger refuges offered fish better protection than smaller ones. When compared across years, there was no clear relationship between refuge volume and prey consumption rate by Cormorants. There was, however, a very large difference in the consumption rate when ponds with no refuge were compared with any of the refuge volumes tested, suggesting that, in the absence of any other cover, the addition of even very small fish refuges can markedly reduce the quantity of fish eaten by Cormorants. Further trials indicated that introducing refuges as a single larger structure was more effective at reducing fish losses than an equivalent volume of refuge arranged as a number of smaller, widely-spaced, discrete units (>50% reduction in prey consumed per Cormorant visit). Trials further confirmed that the beneficial effects of fish refuges were not compromised by the presence of marginal habitat; indeed, prey consumption rates were >30% lower in the pond with additional simulated reed beds. The impact of refuge presence on fish loss rates varied between species, with Roach Rutilus rutilus benefitting in particular. The results are discussed in the wider context of interactions between Cormorants and their prey and with regard to the potential use of fish refuges as a management tool.

In Israel, which is a bottleneck for some six hundred million migrating birds, the drainage of the Hula Lake in the north resulted in serious conflicts between fish-eating birds and the intensive fish farming and agriculture that became established on the reclaimed land. Implementation of various deterrence methods, including lethal control, failed to solve these conflicts. We followed the concept that only by understanding the biology of the organism in question could the tools (indications) for effective and sustainable management be devised that would solve such conflicts while helping the preservation of natural assets such as wetlands and their inhabitants. We demonstrated this concept in the solutions reached in the case studies of pelicans and cormorants and their interactions with intensive commercial fish ponds. We studied the physiological condition, food preference and energy demands, and ecological constraints of Great White Pelican Pelecanus onocrotalus, Great Cormorant Phalacrocorax carbo and Pygmy Cormorant Microcarbo pygmeus. As a result, we suggested different management solutions that are currently implemented with the full cooperation of the fishermen and the nature preservation authorities. Based on assessment of visual acuity to detect and catch fish under water Great Cormorants were effectively pushed towards Lake Kinneret where they could hunt more profitable fish than in the turbid fish ponds. The established roosts there were not controlled such as to lead the birds away from the fishponds. In case of the pelicans, part of the drained Hula valley was reflooded and extra fish was provided. Both measures effectively reduced the damage at the fish farms. We have shown that such solutions are economically helpful for fishermen and enable the preservation of the wetland habitat.

One of the most widespread and persistent environmental conflicts in Europe involves the Great Cormorant Phalacrocorax carbo. The ‘continental’ race P. c. sinensis comprises over 80% of the European breeding population and its numbers and geographical distribution have increased and expanded dramatically in recent decades. Consequently, Cormorants have increasingly come into conflict with fisheries interests across Europe, as many people believe that the birds are now so numerous that they cause declines in fish catches, with associated impacts on commercial and recreational fisheries. The Central European policy issue is thus how to deal with: (1) a large pan-European population of Cormorants, (2) very often breeding in some Member States but overwintering and preying upon fish in others, (3) where there is generally a lack of unequivocal scientific evidence for predation impact on fisheries and (4) where there are growing political calls for coordinated European management, whilst (5) many believe that the site-specific local/regional management advocated by some is ineffective. Using case examples and experiences from several pan-European studies and research networks, this paper describes the complexity of this issue and the diversity of associated opinions. Much of the controversy over Cormorants is fuelled by differences of opinion and, coupled with its persistence and entrenched nature, it has many of the characteristics of a so-called ‘intractable environmental conflict’. As such, this paper draws on a ‘reframing’ model proposed to deal with such situations and discusses the various ‘frames’ by which issues are viewed. It also proposes that future research might best focus on specific fisheries sectors that appear to be ‘hotspots’ for conflicts. Here, demonstration projects could involve a reframing exercise, coupled with new scientific research and practical experimentation within an adaptive management framework – one aim of which might be to increase the scope and geographical coverage of effective management activities.