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Interactions between cormorants, fish and fisheries

Cormorant species are native to every continent in the world, other than Antarctica, and in Europe they are one of the most widely studied of all wild birds. Across Europe, there have been large increases in the numbers of Great Cormorants Phalacrocorax carbo over the past 30 to 40 years. Cormorants are now thought to be more numerous in Europe than ever before; populations have returned to some areas after a long absence and have also moved into other previously unoccupied areas. This increase in numbers and distribution has brought the birds into conflict with man.

Fishery & Cormorants

Cormorants are ‘generalist’ fish predators

Cormorants feed almost exclusively on fish. They are opportunistic predators – rather than specialists – that utilise a wide variety of aquatic habitats. Here they consume a wide variety of fish species, although their diet commonly reflects the seasonal availability of prey at a particular site. They consume a wide size-range of fish, from as little as 3 cm in length (which they very often swallow underwater) to as much as 50 cm (and longer for some Eels Anguilla anguilla). Though such big fish are relatively rare in their diet, cormorants eating big fish are highly conspicuous as they manipulate and try to swallow their prey on the water’s surface. However, despite their ability to take fish of a wide range of sizes, small- to medium-sized fish (10-25 cm) typically predominate in cormorant diet. The birds feed individually or in flocks, sometimes working together to increase their foraging efficiency. Cormorants eat only what they need to survive, including meeting the energetic costs of breeding and migration, or to feed their chicks while they are in the nest. On average, a cormorant requires around 500g of food each day, although the weight of fish eaten can vary both from day to day and seasonally. Cormorants can also damage and scar fish, especially larger specimens which they catch but are unable to swallow. This damage increases the risk of disease, mortality and stress in affected fish.

Cormorants can cause problems for people

The large rise in cormorant numbers across Europe has resulted in widespread problems or ‘conflicts’, mainly because many of the water bodies where cormorants choose to feed are also sites that are of direct interest to people – for example, as commercial or recreational fisheries or for fish rearing purposes (aquaculture). However, conflicts can also arise where fish populations or species of special conservation interest are seen to be threatened by cormorant predation.

Such problems are often complicated

Cormorant-fish-fishery conflicts are complex and dynamic and a diverse range of fishery interests is affected by cormorant predation across Europe. The birds naturally feed in freshwater, brackish and marine habitats, and so can affect commercial fisheries, fish farms (intensive and extensive), and recreational angling in natural, semi-natural, or artificial habitats. The timing and extent of conflicts also varies enormously, since there are large variations in the numbers and distribution of cormorants across Europe, with birds undertaking broad-scale seasonal movements between breeding and wintering areas. Hence, conflicts occur at different times of the year in different places and can involve breeding, wintering and migrating birds. Moreover, in addition to the complex population dynamics of the cormorants, conflicts will also be influenced by the equally complex fish population dynamics, and seasonal and annual variations in external factors, such as weather conditions. This complexity means that assessing conflicts often requires that a diverse range of factors is considered. Obviously these complicated and ever-changing scenarios associated with many cormorant problems have implications for any resulting management actions.

Who views and assesses these problems?

As well as affecting a range of fish/fishery/aquaculture interests across a variety of aquatic habitats, cormorant-fish-fishery conflicts are also viewed in different ways by different stakeholders. This, in part, probably reflects the fact that the term ‘fishery’ is used to mean a number of different things. Thus, to fishermen a ‘fishery’ is the place where they fish; to fishery managers or fish farmers it means the enterprise managed; to angling clubs it means the water where they lease or own the fishing rights; to ecologists it covers the ecosystem, including its fish; and to policy makers the term describes the sphere of activity covered by the relevant law.

Conflicts can thus be viewed and assessed in various ways. For example, at the ecosystem level, the focus should be on biodiversity in general. This might mean the importance of a particular affected species (e.g. a rare or threatened fish species), or the overall status of the fish fauna since, for river systems, this is important to their achieving ‘good ecological status’ under the Water Framework Directive.

At the ‘enterprise’ level, the focus is likely to be on the economic effect of cormorants on a business (e.g. loss of production at a fish farm or reduced catches in a fishery). Further, at the individual level, perceptions that a cormorant conflict exists can also influence the amenity value of a fishery. For instance, anglers may be unwilling to visit a site that is known to be visited by cormorants, irrespective of whether this is influencing things like fish catches there or not. Such perceptions could thus affect the financial income of that fishery. Moreover, for many stakeholders, ecological and economic concerns may well overlap.

How are these problems evaluated?

Although cormorant biology and ecology has been the subject of considerable scientific research, understanding the interactions between cormorants and fish and fisheries often remains problematic. Losses may be quantifiable at fish farm sites, but reliable assessments are typically much more difficult for more extensive fisheries and other water bodies. Some studies have demonstrated that cormorants can have significant negative impacts on fish stocks and fisheries - resulting in reductions in fish abundance and biomass. However, other investigations have also indicated that the mere presence of cormorants at a site does not necessarily mean that a problem is occurring. While the birds can seriously deplete stocks at some sites, their impact elsewhere can be relatively minor. Thus, this usually means that conflicts require some level of evaluation on a case-by-case basis.

Case studies on platform

It is planned to illustrate the complexity of cormorant-fish-fishery conflicts by means of a range of case studies covering different fishery sectors and aquatic habitats. These case studies will be added to the platform as they become available; it is anticipated that the first case studies will be available in the first half of 2012.

Evidence of damage

Demonstrating unequivocal ‘cause-and-effect’ relationships in relation to environmental issues is often not easy, and this is typically a particular challenge when considering the interaction between cormorants and damage to fish stocks/fisheries. This, in large part, reflects the inherent problems and uncertainties of obtaining reliable quantitative information on fish stocks, particularly in large bodies of water. This can lead to assumptions that the absence of clear evidence means the absence of any problem. However, such arguments place an unreasonable burden of proof on fishery stakeholders, and it should be recognised that the absence of clear, unequivocal evidence of damage does not mean that it is not occurring.

Solving, or reducing, problems requires balanced consideration

In the wider context, it is important to remember that predation by cormorants is just one of a wide range of factors that can affect fish species and fish populations. Fish in natural aquatic habitats live within communities of plants and animals and, other than in fish farms, it is normal for some (often very many) of them to be eaten at various stages in their life-cycle. Predation by cormorants is therefore a normal part of the natural interaction between species. Nevertheless, cormorants can have significant negative impacts on fish species and fish stocks at particular sites and this will commonly occur at those places that are most valued by some stakeholders. Under such circumstances, management action may be needed. This needs to balance safeguarding fish fauna, stocks and fisheries with the conservation of the birds, although striking such a balance may not be easy. It is hoped that the information available on this website will both inform stakeholders and help such a balance to be attained.