intercafe – cormorants and the european environment

13 SUMMARY AND SYNTHESIS

13.1 The -Cormorants- of extracted per hectare of Cormorants can therefore be Human triptych surface water annually. In waters considered gross indicators of the poor in nutrients (oligotrophic trophic state of a water systems This chapter summarises the facts, and mesotrophic systems), peak and, given the relatively small figures and implications derived standing stock of fish ranges from amount of fish taken by them, are from the discussions in the previous between 40–100 kg per hectare. not considered the prime factor that chapters and attempts to synthesise In eutrophic systems this range is governs fish populations. In most these into a coherent view of between 200–400 kg per hectare. natural and semi-natural systems, the ecological aspects within Thus, based on these rather crude Cormorant numbers (and associated the Fishes-Cormorants-Human estimates, in most cases Cormorants predation levels, incorporating the Fisheries triptych. The aim here is take roughly not more than10% length of time the birds are present to summarise the ecological factors (and possibly up to a maximum of at a site) are dependent upon the and mechanisms that play a role in 20%) of the peak standing stock ‘available’ fish biomass and not the abundance and movements of of fish. As such they are not much vice versa. birds and fish, and to relate these different from other avian fish to the management practices of predators (see synthesis in van In accordance with this people in waters that are visited Eerden (1997) for Great Crested relationship, at a European scale by Cormorants. For the three Grebes (Podiceps cristatus), most Cormorants are found in players in this triptych, conclusive Goosanders (Mergus merganser) eutrophic, large-scale and rather factors are discussed before the and Smew (Mergus albellus). shallow water systems. The bird relationships between them are population is thus likely to be sketched as a working model. At In water systems which have determined by the available habitat, a later stage we plan to use this been exploited for a long time either in summer or in winter. logical framework to quantify the by both Cormorants and human Estuaries, large lake and shallow relationships in order to forecast predators (i.e. fishermen), the coastal systems are among the future developments. greatest proportion of fish mass most frequently used habitats, there consists of young individuals, followed by large rivers in winter. 13.1.1 Cormorant either of the current year or At the moment it is not possible the previous one (the so-called to easily distinguish between any As measured over the large sample 0+ and 1+ age-classes). Fish clear summer or winter foraging of waters in INTERCAFE’s biomass generally correlates with bottle-neck, which would be database, the density of Cormorants Cormorant use of a foraging site, more important to set carrying expressed as average number per areas with higher fish abundance capacity and thus determine overall unit surface area is fairly constant. generally attracting more birds and/ numbers in the species. However, So, regardless of the size of surface or birds remain there for a longer the extreme geographical spread waters, Cormorants spend, on period. Most often this general in the Cormorant’s winter range average, about 10 bird-days per relationship is associated with the compared to that of the breeding hectare per year at a site but can trophic level of the water. In other range in summer suggests that, spend up to a maximum of 100 words, the amount of nutrients in ultimately, the availability of bird days per hectare per year. This the water determines the level of ice-free foraging grounds (and use by Cormorants corresponds to fish production and this in turn associated fish stocks) in the winter roughly 4.5 kg (but up to 45 kg) governs Cormorant numbers. period is likely to be limiting.

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Preferred Cormorant foraging obtaining higher prey yields than rivers, Dace, Nase and habitat consists of semi-turbid described above (often less than are commonly taken besides the (Secchi depth 60–90 cm), 10% of standing crop), even if the bottom-dwelling gobies. In all relatively shallow (2–7 m deep) Cormorants have adopted social habitats Cormorants also take prey water with a high abundance of hunting in these circumstances (van that are highly valued by Man. In small fish (10–20 cm), within Eerden & Voslamber 1995). standing freshwater systems these reasonable distance (<15 km) of are Pikeperch, Eel and Whitefish a fixed place — either a colony Cormorants are opportunistic (Coregonus) species, in coastal where birds breed or a night roost. foragers and do not select particular waters (Clupea harengus), Cormorants are so called ‘central species of prey fish, their energy- Whitefish and Sole, and in riverine place foragers’ making foraging expensive foraging behaviour systems Grayling, Trout and Charr trips from a communal site. Roosts meaning that they generally almost species, and Atlantic . and colonies tend to be spread out always eat the most abundant prey regularly in the landscape in order species that they can find. On a Although the major proportion to optimise the balance between European scale, more than 100 of Cormorant diet consists of the energy expended in flying to fish species are regularly taken economically unimportant species, (and between) foraging sites and by Cormorants, with mid-water some birds do feed on the less the energy gained through foraging. living Cyprinids such as Roach common, commercially important The Cormorants’ ’harvest’ of fish and Bream the most important species. Whether this predation in natural waters is determined group across Europe. These are leads to any effect on the prey fish (besides by the availability of fish) followed by the Percids Perch is impossible to determine just by the foraging distance to and and Ruffe and, in coastal waters, from the percentage occurrence in from the ‘central place’ (i.e the Eelpout, Viviparous Blenny, Gobies the diet. However, it is important to individual’s current colony and/ and Sculpins as the commonest consider two important facts. First, or roost site) and the presence prey. In smaller, fast-flowing the Cormorant is not necessarily and location of other roosts or colonies. Cormorants spend a lot of energy during diving, and so their energetic return needs to exceed this and thus food intake rate can not drop below a certain threshold. This threshold is obviously higher if energy expenditure increases (e.g. flight distance and/or diving depth increases and/or ambient temperature decreases). The same is true for breeding adults rearing young compared to birds having no nestlings to care for. The important conclusion here is that habitat conditions and physiological state ultimately govern Cormorant numbers through energetic rules. These energetic constraints limit Cormorants with respect to the potential extraction of prey fish. In typical Cormorant habitat Productivity in Cormorant colonies varies according to the availability of food. the murky water conditions (by Most clutches contain 3–4 eggs (range 2–6) but the number of young that either suspended inorganic matter fledge varies greatly from 0.5–2.7 per nest on average (range 0–4). or algae) prevent the birds from Photo courtesy of Florian Möllers.

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selecting any particular species of water levels and high nutrient loads eutrophic (i.e. nutrient-enriched) fish while foraging in a particular reedbeds are known to retreat as shallow waters. water body. This implies that, on a result of the erosive mechanical a large geographical scale, rare power attacking the stems at the Due to predicted global climate and protected fish species are same point (e.g. Ostendorp et change, the climate window unlikely be threatened by this al. 1995). The man-made lakes that Cormorants operate within predator. Second, when relatively used for water storage, for energy will likely tend to move farther large numbers of Cormorants are supply or recreation, often have northwards. The current shifts in on migration (or making shorter artificial (hard stone or concrete) Cormorant distribution associated movements in search for ice-free or very steep shores without much with this will thus lead ultimately foraging sites) and visiting small vegetation which would serve as to more available space in summer running waters (e.g. in the pre- shelter. In all these cases, fish tend to breed and, perhaps more Alpine region), the fish there may to be more susceptible to predation important, to winter as well. The well be temporally vulnerable to by Cormorants although here, too, present expansion of breeding predation. The amount of available it is unlikely that their populations Cormorants in the Baltic region is shelter will determine the fishes’ are negatively affected by predation certainly in accordance with this chance of escaping predation in the longer term. scenario. Whether this also holds and could limit any effect at the for southern areas in the Russian population level. Connectivity to The trend to increase the number Federation is not known. Besides other parts of the catchment area of hydropower reservoirs over earlier access to, and the availability is also important with respect the past decades has undoubtedly of, a larger breeding area there is to recovery from predation contributed to the wintering site also likely to be an effect of global losses. Generally, fish survival is possibilities of Cormorants. This is change in winter. Milder winters considered to be higher in more especially true for Spain, Portugal will eventually lead to more birds natural water systems but the effect and Italy, but also for Greece and beginning to winter in the same of stocking with naïve stock (both elsewhere in the Mediterranean region as they breed, as suggested particular breeds or individual where these water bodies are strongly by current increased fish), in combination with modified now important foraging waters numbers in the northernmost range habitats and water flows due to for Cormorants. Furthermore, the of the current ‘traditional’ wintering river management will usually lead resulting patches of drowned forest area. Parts of UK, The Netherlands to greater predation effects than in often serve as roosting areas for the and Belgium, northern Germany natural systems. birds in these waters. and deeper water areas in Denmark, Sweden and Poland will probably The role of artificial water bodies By contrast to the situation in thus see increasing Cormorant in relation to the predatory effects winter, when numbers are dispersed numbers in winter. This might also by Cormorants is similar. As fish over large areas, the area of be true for the northerly regions in here are often no longer able to available foraging waters in the the Balkans, Bulgaria and Romania. perform their annual movements, North Sea-Baltic region and the The future use of the Mediterranean (e.g. from rivers to the deeper parts NW parts of the Black Sea should, by Cormorants is also very of lakes, or from lakes to coastal at a European level, be considered interesting in this respect. North areas) they tend to concentrate at core areas for Cormorants. Both Africa (the traditional southern very specific places. This is often coastal, lake and larger river fringe of wintering sinensis birds) the case at weirs, sluices, locks, deltas and lagoon systems are is supposedly not used so much by near dams and other obstacles, present at a density that has no wintering birds anymore. Whether often in the deeper water sections. parallel elsewhere in Europe. It the overall increase in the Nordic The regulation of water levels is in these two ‘mega-regions’ populations of Cormorants will that accompanies the hydraulic that Cormorants breed (and part mean an increase in wintering restructuring of water bodies often of the population also winters) in numbers in this area again is means a degradation of densely large numbers, fully in accordance difficult to predict, although not vegetated natural shores. At fixed with their apparent preference for unlikely.

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13.1.2 Fishes Large numbers of Cormorants across Europe are known to be associated with eutrophic water bodies. After protective measures were taken in most European countries from the 1970s onwards, enhanced fish production, combined with less clear waters, have provided better feeding conditions for Cormorants. Eutrophication is considered a major driver behind the Cormorant’s recovery. The phenomenon of nutrient enrichment is still apparent in large parts of eastern and southern Europe, whereas in western and northern regions there is a reversed pattern of lower nutrients levels due to a strong reduction policy in the 1980s and 1990s. The Cormorants’ range expansion into more easterly and southerly regions of Europe Figure 13.1 Example of algal blooms in the Baltic Sea as depicted by satellite images has thus been facilitated by two taken during summer 2006. More blooms occur in southern and shallow water effects: those of a milder climate areas. Notice also the apparent bloom in the southern part of Lake Peipsi, and that and of greater fish abundance and the southernmost (and most polluted) part (Lake Pskov) has not been included in this availability due to eutrophication. picture (Source SYKE, Helsinki). As an example of the large-scale effect of eutrophication in the the Baltic Sea. The productivity amount of productivity. It is Baltic Sea, Figure 13.1 shows of the Baltic has increased at theoretically able to produce even the extent of algal blooms as least two-fold in the last 100 more, and the ‘unused’ primary visible by satellite, with peak years. This is ultimately apparent production may detrimentally affect values appearing in a wide belt in less-clear water, but has had the ecosystem. The timing and in the southern Baltic ranging drastic effects on other parts of species composition of seasonal from Öland all the way east to the ecosystem too. Although algal blooms is resulting in lower Latvia. The blooms in the shallow large cyanobacteria blooms are consumption levels of primary waters in western Estonia and the a natural phenomenon, their production in the food web (see pp. easternmost of the Finnish Gulf present intensity most certainly 147–54 of Wahlström et al, 1996, near St. Petersburg are obvious. In is not. Regardless of the reasons SYKE 2006). In combination with coastal areas these correspond to for increasing productivity in the higher water temperatures, the those regions where Cormorants Baltic Sea, it is impossible to assess increased organic matter content spend the summer months, how this additional production is in the water reduces oxygen levels either breeding or in post-fledge transferred through the food web. on the seabed, and this promotes aggregations. There are no proper estimates of blooms of toxic algae. As a the total biomass of top predators consequence, the water becomes Human influence is most likely (fish, birds and seals) over the more turbid (less clear) and benthic the main reason behind such past century, but the Baltic Sea is organisms receive less light and large-scale ecological changes in certainly able to sustain a large suffer from anoxia (that is periods

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when oxygen content in the water due to consumption by algae during nighttime is near zero), whilst the reduction of benthic filter feeders further increases the production of algal biomass. As a consequence, there is less seaweed and submersed macrophytes and fish populations become simplified. For example, pelagic plankton-feeding fish such as Herring and fish that prey by sight start to disappear from the system. This species shift causes more tolerant prey fish species like Gobies, Butterfish, Eelpout and Cyprinids to increase and, together with turbid water, offers very favourable foraging conditions for Cormorants on a very wide scale.

13.1.3 Human fisheries

Man as commercial fisherman Besides his effect through eutrophication, Man as a commercial fisherman plays Figure 13.2 long-term changes in catches and abundance of different key organisms an overriding role in many in the Baltic (after Österblom et al. 2006) and trophic relationships between , ecosystems. Through large-scale and plankton as well as the alternative predators pike and perch (right). The over- of Cormorant’s appearance since the mid 1990s runs parallel to that of the reappearance cascading effects result at many of the Grey Seal. lower levels in the food chain. This is a pattern that has been of over-fishing, is highly attractive versus 4.7 years) and are generally observed in many marine, river to Cormorants. smaller than their ancestors (49 cm and lake ecosystems across the compared to 56 cm). The Baltic globe, ranging from China, Canada, The next examples are again was not very rich in fish during the West Africa and also Europe (see taken from the Baltic Sea, further first part of the 20th century and, Scheffer et al. 2005). Ultimately demonstrating this area to be an compared to the present situation, this process leads to ever smaller increasingly important one for there were many seals (Phoca prey sizes, a phenomenon known Cormorants. By comparing Cod vitulina, P. hispida and Halichoerus as ‘fishing down the foodweb’ (Gadus morhua) from the Neolithic grypus) and harbour porpoises (Pauly et al. 1998). As well as the period (5300–3900 BC) with (Phocoena phocoena), regarded dominance of smaller-sized fishes, Cod from contemporary times, as the natural top predators in high pressure also results in researchers have discovered that the the system. Because of perceived a reduction in the reproductive age, species has evolved over a relatively competition with human fisheries, in other words fish stay smaller short period as a result of human these mammals were heavily and tend to reproduce at a younger overexploitation. According to a persecuted during the first half age as a result of the intensive recent scientific paper (Limburg of the 20th century (see Figures fishing. As described elsewhere, an et al. 2008), contemporary Cod 13.2 and 13.3). This and the effect abundance of small fish as a result attain adulthood earlier (3.7 years of eutrophication allowed higher

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productivity levels, which (after numbers of Cormorants seems Cod is the most valuable species World War II when fishing effort logical in this set of cascading for fisheries and the size of the was relatively low) resulted in large events. As a predator of both the stock has a large impact on stocks of both Herring and Cod, the near-benthic fish community and the economy for commercial cod now being the new top predator of the more pelagic schooling fish fishermen. Despite the advice in the system. species it may likewise have profited from ICES, that a substantially from the disappearance of Cod from decreased Cod fisheries could lead Catches of Herring increased the Baltic system. to improved long-term potential sharply to peak levels in the for catching more fish, politicians 1970s, followed by large catches This type of example is not have not yet had the courage of Cod during the 1980s. Both restricted to the Baltic. In many to take the necessary decisions species decreased as a result of fished ecosystems, that is to say (see Figure 13.4). Preliminary this heavy fishing pressure, and in nearly all large-scale waters, calculations indicate that a few then another , the there are indications that the large years of dramatically reduced sprat (Sprattus sprattus), began to predatory fish species have been fishing could lead to a rapid increase increase in the 1990s. This species over-fished, very often resulting of the Cod stock in this area (Hjerne is a food competitor of the Herring in conditions that are highly & Hansson 2001). From our results and, although of less economic favourable to Cormorants. and synthesis outlined above we importance than it, became heavily predict that the return of Cod would exploited too as a result of the Over-fishing during the 1980s thus mean that Cormorants are faced declining stocks of Herring and contributed to a decrease in the Cod with a less superfluous (small) fish Cod. Although not monitored stock, which has led to a number supply and that this might result in a specifically, the disappearance of the of effects on other components drastic decline in their population. Cod is likely to have had an effect of the Baltic Sea food web — it on the near-benthic fish community is becoming increasingly clear Man as fish farmer now released from predation by that Cod play an important part in Fish farms are artificially managed Cod. The appearance of large the dynamics of this ecosystem. water systems which tend to act

Very probable relationships When the Cod stocks decreases the stock of Sprat grows stronger and bigger and this leads to a decrease of zooplankton.

Probable relationships The decrease of Cod can also lead to a decrease of Perch and Pike because they have to compete with the stock of Sprat that has grown stronger and consumes a lot of food (zooplankton).

Interesting hypotheses When the Cod stocks decrease, the stock of Sprat grows stronger and bigger and this leads to a decrease in zooplankton. This could lead to an increase of phytoplankton which leads to muddy and hypoxic water.

Figure 13.3 Relationships between Cod and other trophic levels in the Baltic Sea. (From Österblom 2009). The abundance of small fish (here Sprat) in all routes is providing favourable food conditions for Cormorants.

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of over 10,000 kg per hectare of occur in some fishponds in Israel. The more fish in the pond, the more susceptible the system is to foraging predators such as Cormorants. The actual predation of fish is not the only problem; also the mere presence of foraging birds and the associated disturbance can cause additional mortality to fish because of the stress it induces.

Man as sports fisherman Historically, angling was purely the exploitation of wild fish stocks, but  for decades anglers have intensively Figure 13.4 over-fishing of Cod in the Baltic in recent times. After the strong decline managed their stocks to enhance in catches the 1980s, ICES advice on catches (green bars) has been consistently lower their sport. As water quality than the politically achieved decision on allowable catches (blue bars), whilst actual has declined and hydrological catches have been higher still (red bars). ‘improvements’ to sections of river have become common practice, the demand for active management as ‘honey pots’ to Cormorants by over time. For example, though has become stronger. The stocking offering high densities of relatively less intensive than during GDR of fish from other water bodies, small fish (very often Carp) kept times, Carp production in Saxony the introduction of non-native in shallow water. Fish density is still intensive and there are species, the release of naïve is almost always higher in these local problems with water quality, ‘fingerlings’ (i.e. ) to systems than it is in nearby natural including algae, the disappearance restore stocks are all commonly waters (up to a factor of 10 and of macrophytes, low oxygen applied techniques to increase even up to 100 times greater) and content and increased turbidity. catches and to make waters more water depth is usually less than Similar intensive production attractive for recreational angling. 1.5 m in most cases. Fish farms conditions occur in the larger pond Like fish farms, stocked angling can be used by Cormorants if a complexes in France, the Czech ponds and river sections become breeding colony is nearby but Republic and Poland. When Carp more vulnerable to predation because most fish farms tend to are grown for the consumption as densities of fish, relative to occur in regions without much market, fish farmers generally put adjacent habitats, increase. The use other open water, the number the one-year and two-year-old age of hand-reared fish stock further of breeding Cormorants in the classes to grow in larger ponds at increases the risk of considerable neighbourhood of fish farms tends higher densities. The 0+ (young- loss to predation. This is due to to be relatively low. More often, of-the-year) fish are generally kept the fact that fish reared in captivity Cormorants visit fish farms in late over winter in tanks or small ponds show little or no fear of predators. summer and autumn when the covered with nets. In less intensive As in fish farms, most Cormorant birds are on migration. In many situations like those in Brenne in predation at angling waters occurs places, Cormorants also frequently France, fish are grown over several during migration periods and also visit fish farm areas during the years. Here, the ponds usually in winter. During these periods, winter, especially if the fish farm have more aquatic vegetation birds very frequently switch area consists of many ponds and/ than do other systems and fish feeding location and look for or if larger lake or river systems densities are generally lower as alternative foraging sites and this are nearby. The way farm pond there is no artificial feeding. On is when these systems are most systems are used has developed the other hand, extreme densities commonly visited. Thus in pre-

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Alpine streams, Cormorants may would be treated as a kind of pest will also lead to higher use of local concentrate during periods of frost, species. Not withstanding the costs water bodies in the region of the as a consequence of the freezing of such an enterprise (which have North Sea and Baltic, if the current of their preferred lowland habitat. never been quantified), population pattern of milder winters continues. The effects of Cormorant predation modelling has shown that such a There is thus no reason to believe on fish stocks are generally found strategy would only have limited that there will be a shift towards to be lower in cases where (1) river effect. Given that the potential prey reduced Cormorant predation systems are connected (i.e. they are base for European Cormorants pressure on many of Europe’s more open systems allowing fish in natural waters remains ‘super fisheries in the near future at least. to move freely and to repopulate abundant’ (as is the case for the depleted areas), and (2) habitats majority of waters in Europe), Some stakeholder groups argue are relatively complex and offer the plasticity of the Cormorant strongly that Cormorants are the good cover/refuge for fish, often population is such that measures single most important cause of the deeper sections of rivers with to reduce it will very soon be the economic losses experienced natural shelter in the form of compensated for by increased by many fisheries, regardless of things like boulders and woody birth rates, higher survival and/or whether these be commercial debris or in ponds and small lakes immigration into the ‘population’ fisheries, fish ponds, or recreational with a strong natural growth of being managed. This section angling waters. However, as macrophytes and natural shore explores and synthesizes possible discussed elsewhere, the situation vegetation. solutions to the Cormorant conflict with respect to these different from an ecological perspective. fisheries activities is neither constant nor always sustainable 13.2 Epilogue: towards a Changing worlds but persistent from economic and/or social habits points’ perspectives (see Part Three solution of ‘the Cormorant European legislation and the local of Marzano & Carss, 2012). problem’ from an ecological protection of a species that has perspective? expanded widely in the eastern For example, the economic story part of its European breeding range of a decline in the prospects of the Concerted management activities has caused a significant increase Carp market became very evident to reduce Cormorant numbers in the number of Cormorants. during INTERCAFE’s work. overall have not yet been carried Landscape restoration activities, Customer demand for other fish out across Europe, but are as well as integrated protection species has redirected the market, continually demanded by some schemes such as those under and changing international trade fisheries sectors as the solution Natura 2000, have also greatly relationships have created a different to Europe’s Cormorant problem. improved the environment for perspective for the traditional Such coordinated management Cormorants. In the near future producers. From the ecological between countries and involving this situation will continue and perspective, Carp ponds can be seen the likely culling of many lead to larger populations of as ‘honey pots’ on the birds’ flyways thousands of Cormorants each Cormorants, related for example to and from the main foraging areas. year is certainly, on ecological to the poor environmental state Protection of these pond areas of grounds, considered inadequate of the Baltic. Along the coastal special interest is easier if wetlands to resolve the problem. This is areas of the Baltic States, in of sufficient surface area are because it does not recognise any Sweden, Poland and locally available to the birds as alternative of the causal relationships which in some states in Germany foraging sites when disturbance underly the present European (Mecklenburg-Vorpommern) for actions are undertaken. Cormorant situation. With any example, Cormorant numbers are large-scale culling or shooting likely to increase further. These Today, many commercial fisheries activities directed towards the higher numbers of birds will be face the prospect of over-fishing reduction of the total European increasingly often observed in the and, associated with this, ever Cormorant population, the birds Balkan countries in winter, but stronger regulation in the form of

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catch quotas. The new EU Member The conflict in the ecological stocks of Pikeperch in IJsselmeer) States are still in a position of perspective of the triptych or Sprat and Eelpout Zoarces transition from the original State- The ecological conditions that viviparus as result of diminishing directed system towards a more Cormorants face today appear to Cod stocks in the Baltic). free-market one, with all its many be very favourable for the species’ social and economic problems. good population status. The ‘Fishing down the food chain’ leads Given this socio-economic banning of pesticides, a Europe- to the abundance of smaller fish transition, often associated with wide amelioration of water quality species and a shift in size classes, a lot of tension for the individual and protective status has all had from larger species towards a stakeholders, the confrontation their effect. However, the changes preponderance of smaller species with a relatively recent arriving in the hydrographical conditions and/or individuals. Finally, natural predator as the Cormorant of many rivers and lakes that have the introduction of non-native accellerates the tension (Carss et al. been undertaken over the same species for use in angling waters 2003, Carss & Marzano 2009, see time frame have led to conditions has contributed to unfavourable also Marzano & Carss 2012). where fish populations have limited conditions for many native migratory space compared to earlier species. All these changes lead Like angling, bird watching is conditions. This has had negative to a situation in which the large an outdoor activity which is an consequences for the spawning fish predators (or, still earlier, increasingly popular form of of fish and has ultimately lead mammalian predators like small recreation. Especially in the more to simplified ecosystems where whales, dolphins, and otters) are densely populated parts in Europe fewer fish species are present. As being ‘replaced’ in food webs by wetlands are visited by increasing described above, these less natural avian predators, in this case the numbers of people and Cormorant conditions also offer higher chances Cormorant. The greater ability of colonies or roosts are among the of predation by Cormorants. The this species to move over large sites offering guided excursions eutrophic status that persists locally distances, coupled with higher (e.g. in The Netherlands, Denmark) for many water bodies enforces this reproduction rates, allows these or the opportunities of more effect and, combined with a heavy birds to feed and breed over wide informal visits visiting bird hides pressure from fisheries activities, geographic areas. High Cormorant (e.g. in The Netherlands, France, has lead to a further shift in species numbers are thus an ecological Belgium, Germany, Denmark, composition towards the increase sign of the super-abundance of Sweden, Poland). These activities of commercially non-important small fish prey, a situation which are also becoming an increasingly species (e.g. Ruffe Gymnocephalus under natural conditions prevails important part of local economies cernuus as a result of over-fished in estuaries, lagoons or shallow through the associated transport, accommodation and specialized equipment sectors. Interestingly, this trend is more apparent in densely populated areas than it is in rural ones in Europe where traditional land-use patterns like fish-farming still prevail.

As the European Cormorant population links all these very different social and economic worlds, it is clear that no single Commercial fisheries have a great impact on fish composition in many solution to the conflict is ever likely European waters (species and size distribution shifting towards smaller to be successful, given the fact individuals). Moreover, discards of fish form an attractive prey for many fish- that the perception of the problem eating birds including Cormorants, Lake Markermeer, The Netherlands. differs across Member States. Photo courtesy of Florian Möllers.

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coastal zones and large river ponds, Cormorants operate at a both ends of the geographical scale, sections, but is also the result of ‘global’ scale across Europe and management of basic resources disturbed aquatic ecosystems. beyond. Their geographical range is thus considered vital for any encompasses a vast diversity solution of the current Cormorant The suggestion from all this is that of human social, cultural and problems. Cormorants may be good indicators economic systems. The social and for the environmental state of a economic backgrounds associated For the short term at least, two water systems, rather than the with people’s perceptions of types of conflict cases remain, both ultimate cause of the ecological what is important and acceptable associated with smaller-scale water disturbance to it. Ecological or not in relation to Cormorant- bodies. The first and geographically monitoring of the species fisheries interests is dealt with most widely distributed group (especially numbers, food, status extensively in Marzano & of cases is that of fish farms and distribution) may thus provide Carss (2012). Here we confine (see also Seiche et al. 2012), the valuable information and indicate the discussion to an ecological other related to small upland and any changes in water quality and perspective on the conflict where mountain rivers. Larger numbers effects of fisheries management it is clear that for large-scale open of Cormorants, often migrating programmes. Managing basic water bodies the problems with or moving to and from their core resources in an ecologically Cormorants are mainly related habitat (focused on large-scale sustainable way would seem a far to the improper management of water bodies) can cause problems more viable option than to adopt wild fish populations by Man. with local fisheries. From this large-scale management of the Devising sustainable fisheries investigation, the area of most predators themselves. for the predatory fish species intense reporting of conflicts seems seems the most effective way to coincide more or less with that A regional approach for solving to shift these systems into more part of Europe where large-scale local problems: the European balanced situations. This needs waters are scarce. At a continental Watershed Hypothesis as coordinated action and will, of level this may be seen as the example course, not be simple. The result, ‘European Watershed’, dividing the This work has shown clearly however, will have important coastal Baltic/North Sea/Atlantic the complexity of the ecological consequences on the European river catchments from the Black relationships associated with population of Cormorants and for Sea/Mediterranean. Since historical Cormorants feeding on fish. the systems themselves. As most times, people have tried to manage As ecological conditions and Cormorants in Europe rely on their local fish supply by creating management in different parts these larger-scaled water bodies, fish ponds and using small streams of the species’ range vary consistently applying the principle as a local source of water. On considerably, there is little chance of ‘wise use’ will initially slow their way to their wintering areas, of a single pan-European solution down the population increase and Cormorants pass these watershed that can cope with all these subsequently turn it back to lower areas with a relatively limited differences. Cormorants react to levels. This will be the ultimate area of foraging waters and may differences in food abundance and, task if one is to control Cormorant eventually be present there in large because of their ability to fly large numbers. As a result of lower numbers. distances; they are perhaps better overall Cormorant numbers, fewer able than other predators to detect cases of conflict will also arise Can we use this ‘European areas where fish are abundant, on those river sections, lakes and Watershed Risk Hypothesis’ for either naturally or as a result of small streams which do not have the alleviation of local problems? Man’s actions (i.e. management unnaturally high concentrations of It may well be possible if detailed of wild stocks or commercial/ fish. Natural management, aimed knowledge of timing of bird recreational enterprises). Spanning towards variable habitat structure migration is linked to a temporal the complete range of marine and natural water level fluctuations recording of the development of to freshwater and from large- will allow fish to migrate, weather patterns. Cold weather scale coastal waters to isolated and hide from predators. Clearly, at arriving from the north and tailwinds

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favouring bird migration are known needs. The work presented here has favouring the development of as important triggers for bird paved the way to such an approach, natural shores, allowing for migration (Alerstam 1981). As such, setting up the GIS framework naturally fluctuating water a GIS based warning system could and bringing together most of the levels wherever possible and be developed which may direct the biological data that are needed to promoting emergent and incidence of disturbance actions model Cormorant movements/ submerged vegetation used by to be taken against Cormorants. migration in relation to available fish to spawn, grow, and shelter Applying disturbance at a site before resources and such things as weather from predators. commonly used by the arriving birds and climate. ▪▪ Stimulating the communication during autumn and spring migration of information between is probably the most effective Recommendations in short different stakeholder groups, way to avoid settlement of larger From the ecological point of view, the exchange of common groups in a ‘new’ area. The same the main conclusions of this work practice to develop the probably holds for the pre-Alpine can be summarized and several sustainable use of complete lakes and streams at times of frost recommendations offered which are ecosystems which including when nearby still waters at lower considered crucial to the resolution Cormorants and other natural altitude freeze over. Based on GIS of European Cormorant-fisheries top-predators. information, the combination of problems: ▪▪ Developing an ‘early the water charts and the occurrence warning system’ of migratory of sensitive streams and fish-farm ▪▪ Reduction of the continuous Cormorants in risk-sensitive areas could be used to arrive at an over-fishing by man of stocks areas (the so-called EU integrated ‘early warning’ system. of large predatory fish species watershed area). Such an early warning system at sea and in large lakes, by ▪▪ Increasing the protection of system based on on-the-spot promoting the sustainable use of sensitive fish species and sites information has been shown to work fish resources. by netting, acoustic or other at a regional scale in Israel where it ▪▪ Reduction of extensive nutrient measures (see Russell et al. was successfully applied in case of loads that currently still affect 2012). managing conflicts in populations water systems, causing algal ▪▪ Adapting fish stock management of Crane Grus grus and White blooms and leading to simplified in the most vulnerable areas Pelican Pelecanus onocrotalus. This fish communities. by rearing larger-sized fish approach would combine detailed ▪▪ Removal of barriers in rivers and less prone to predation in geographical information and the lakes, thus restoring the aquatic recreational and commercial availability of resources, knowledge connections used by fish. fisheries. of bird migration habits and may ▪▪ Stimulating and restoring more ▪▪ On the spot disturbance or direct local managers to effectively natural conditions in smaller shooting of Cormorants at carry out protective measures. When still waters, avoiding intensive the most vulnerable sites carried out in such a sophisticated management with continuous when other measures fail, in way, the measures are likely to be stocking of non-autochthonous combination with the provision far more effective because they are fish species. of ‘buffer’ areas where the coordinated and timely reactions to ▪▪ Restoring habitat quality species is allowed to forage the birds’ movements and specific in aquatic systems by unmolested.

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