II REVIEW OF REPLIES OF QUESTIONNAIRE - PART C: OPEN WATERS

L. Marion - Laboratoire d'Evolution des Systèmes Naturels et Modifiés, Université de Rennes (France)

1. GENERAL INFORMATION

1.1 Total Area of Open waters

Most countries do not indicate the area of each kind of open water. In consequence it is not possible to evaluate the total biomass of fish available to piscivorous birds based on hypothetic densities for the different types of open waters.

The surface area covered by open waters is very different from one country to another. Fish populations and piscivorous bird problems are then also very different.

Table 5

Surface area of inland waters in various countries of Europe

Catches by sport fishermen are low in the United Kingdom (0.71 kg/year), Belgium (1.67 kg/year), mean in the Netherlands (3 kg/year) and in Switzerland (5 kg/year), high in Norway (7.13 kg/year), very high in France (10 kg/year), Poland (13.33 kg/year) and in Czechoslovakia (15.44 kg/year).

In Europe, there are an estimated 15 million sport fishermen, catch about 6.4 kg/year, that is to say a total of about 100 000 t. This figure seems rather high in comparison with data on predation by birds (for example, the European heron population will consume the equivalent of about 10% of the fish caught by sport fishermen).

2. BIRD PROBLEMS AND PREDATION

Populations of piscivorous birds and characteristics of open waters differ between countries. Consequently, bird problems depend on local situations in each country. Unfortunately, some responses to the questionnaire only list piscivorous birds, and do not give information on problems.

Grey heron (Ardea cinerea): France, Belgium, Hungary, Austria, Federal Republic of Germany (in regulated brooks and small rivers), Switzerland (only on trout in winter in small rivers), experience problems with the species, but predation is not quantified. In France, the problem is largely subjective in that fishermen feel that herons cause considerable damage, but fish specialists of the National Institute of Agricultural Research (INRA) consider that damage is negligible. Ireland, United Kingdom, Norway, Denmark, Finland, Poland, the Netherlands and Cyprus do not mention this species as a problem.

Great cormorant (Phalacrocorax carbo): Problems are experienced with this species in Romania (Danube Delta), and also in fisheries on IJsselmeer and estuaries in the southwest part of the Netherlands where problems especially concern breeding populations in spring and summer which cause great damage. This population and a secondary one in Denmark, migrate to the Federal Republic of Germany, France, Switzerland and Italy, where they cause considerable problems in winter. Birds attack fish stocks in big rivers, lakes, estuaries, ponds which support fisheries. Problems are reported from lakes in Italy, big rivers and lakes in Switzerland, and open waters in the Federal Republic of Germany. This species is not mentioned as a problem in Belgium, Ireland, United Kingdom, Poland, Finland, Norway, Czechoslovakia, Austria, Cyprus, Israel.

Gulls: In Finland, Larus spp. have a period of active predation from the middle of July to September, when fish fingerlings have reached a length of 5–7 cm. This coincides with the main period of stocking in small rivers and lakes. In Ireland, the black-headed gull, Larus ridibundus, predates on elvers as they ascend into freshwater (March to June). Problems are also mentioned in Austria, but without further definition. This species is not mentioned as a problem in the Federal Republic of Germany, Denmark, Norway, Cyprus. In Czechoslovakia, it is supposed that this species mainly catches fish which are not in good health.

In Finland, herring gull (Larus argentatus), common gull (Larus canus), terns (Sterna paradisea, S. hirundo), act in the same way as L. ridibundus. They are not mentioned in other countries, except for the herring gull which causes problems in Hungary in October and in Italy throughout the year.

Red breasted merganser (Mergus serrator), merganser (M. merganser): Cited in Finland as having the same predation pattern as gulls. In Norway causes problems in salmon rivers. It is also present in the Netherlands and Ireland where the problem is not important.

Table 6

Total catch and number of professional and sport fishermen

^a Part-time
^b Two-thirds salmonids
^c Professional
^d 16 t eel, 21 t pike, 24 t pike-perch, 28 t carp, 11 t tench, 37 t salmonids, 69 t fingerlings
^e Sport
^f 2 000 t eel, 700 t perch, 300 t pike-perch
^g 3 848 t brown trout, 1 004 t char, 307 t grayling, 391 t gwyniad, 586 tpike, 783 t perch, 65 t eel, 145 t others
^h 75% cyprinds, 15% sheat fish, pike, pike-perch, chub, perch, 10% sturgeon and Danube herring
ⁱ salmon

In Romania, the bird predations are different. There are problems in the Danube Delta with pelicans, cormorants, grey heron, purple heron, night heron, gulls, terns, spoon bill, glossy ibis, great white heron, but their action is not precisely evaluated. Damage occurs essentially between March and September, especially in dry years when the level of the water is low and the fish are concentrated in small areas. There is no problem in natural open waters in Isreael and Czechoslovakia.

3. FISH DISEASES

See Part D: Fish Diseases. The assessment of this problem in open waters is very difficult. Distinction has to be made between the theoretic possibility that birds act as vectors of diseases, and their real action in the field, in comparison with other vectors or global conditions favouring the development of diseases. This second aspect was not dealt with in the responses to the questionnaire. Nevertheless, in most countries birds do not cause serious disease problems in natural fish populations of the open waters. They are mentioned as definitive hosts of parasites of the larval stages of fish (Ligula intestinalis, Diplostomum volvens, Neacus cuticola, Tetracotyle ovata), and as possible vectors of pathogenic bacteria and viruses. Thus they are assumed to transmit diseases from one water body to another, by droppings or injury to fish. But direct contamination and comparing with other vectors is not given, and for French specialists the problem is of minor importance as compared to the health conditions of fish, their food, the characteristics of water, the general conditions of management of fisheries, etc. For parasitic diseases, birds transmit eggs of parasites not to the fish but to an intermediate host (crustacea, mollusc), and densities of these, which directly transmit parasites to the fish, seems to be the most important factor. For other diseases, the action of birds is probably not very important (the body temperature of birds is not suitable for fish viruses), as compared to the probability of contamination from water or other fishes.

In the responses to the questionnaire, data are scarce on parasitic diseases. In Hungary, about 4.5% of 3–6 year old Abramis brama and Rutilus rutilus populations in Lake Balaton are infested by Ligula and Digramma, and 10% of the cyprinids in natural waters are infested by displostomosis. Some ponds or lakes bearly infested by diplotosma are only cited in this country and in Norway. Gulls are the essential definitive hosts of most parasitic species on fish. Inversely, birds may have a positive effect on fish diseases by taking sick or dead fishes as mentioned by the Federal Republic of Germany, the Netherlands, France for Ardea cinerea and Larus ridibundus (also a non-piscivorous bird: Milvus migrans), and for Phalacrocorax carbo in the Netherlands. In Romania and Italy, birds eat also sick, dying or recently dead fish.

4. BIRD PREDATOR POPULATIONS (LEGAL POSITION)

The status of piscivorous birds differs according to country, species, period and area. It may even differ with province in the same country as in Austria, where the grey heron is protected only in Salzburg and upper Austria. Information is lacking for some countries and it is not possible to describe precisely the general situation. Answers to the questionnaire generally concern only birds cited as causing problems; legal status of other species was rarely mentioned.

Table 7

Legal protection for piscivorous birds in EIFAC countries

P = yes
N = no

In open waters there are in general no indications that fish populations have changed due to bird predation. However, in the Netherlands, the feeding behaviour of the cormorant at the heavily fished feeding areas near the big colonies has changed, probably due to the increased number of cormorants. Formerly, cormorants fished mostly individually and preferred larger fish (eel, pike-perch). Now on the fishing areas of big cormorant colonies on Lake IJsselmeer “social fishing” by some hundreds and even thousands of cormorants together is a normal phenomenon. In this way cormorants are fishing jointly on shoals of small fishes, which were formerly not preyed upon. Today, 50% of the number of fish caught are smelt (Osmerus eperlanus), one may ask if this changed behaviour points to a modification of the fish population due to heavy fishing at a restricted area within flight distance of the colony.

The relation between increasing populations of piscivorous birds and legal protection is mentioned by France, Belgium, the Netherlands, Federal Republic of Germany and Denmark for great cormorant and grey heron. In the Netherlands, due to legal protection, the population of the great crested grebe has increased from 300 to 7 500 breeding pairs, the great cormorant from 1 200 to 11 000 breeding pairs and the grey heron to more than 10 000 breeding pairs. Otherwise in the Netherlands, under the influence of modification of habitat and pollution, some uncommon fish-eating birds which are not important to fisheries, have become scarce (kingfisher) or are decreasing in numbers (purple heron, little bittern). Conversely, in Austria grey heron and black-headed gull are not protected but are increasing in abundance, even “exploding” in some places, perhaps because they are more hunted in Yugoslavia, birds migrate to Austria. Nevertheless, factors affecting the population dynamics of birds are very complex and cannot be attributed solely to human disturbance.

5. MEASURES

Preventive measures to control damage by birds at large open waters are probably neither practicable nor effective, except with drastic measures on populations of birds in breeding sites, which are not used in western Europe (EEC particularly) because birds are now considered to represent a patrimonial value. In Romania, authorities reduce the egg number in the nests of the piscivorous birds, and also adult birds by shooting, with a certain success because these measures are repeated each year all over the delta of the Danube. Even in this country the problem of damage persists, and in Poland the destruction of nesting places is practised without apparent effect. To stop totally any damage, it would be necessary to suppress the entire population of birds, which is hardly possible. The only measure for protecting small areas from visits of birds could be by wiring which may be successful.

6. RESEARCH

There are two governmental working parties on the problem of piscivorous birds in France and Switzerland. A scientific working group also exists in Bavaria. The governmental working parties have published their final reports.

Previous to the report in France (Marion, 1983) there was a local study on a fish farm (trout culture) in western France (herons) and on fisheries (cyprinids) in Camargue. In 1985 a small study was also made on a fish farm in Alsace (herons). The French Government actually finances five important studies on herons, great cormorant and black-headed gull in several provinces (ponds in Brittany, between the Loire and Gironde, Arcachon, Camargue and ponds in Forez). The grey heron population dynamics is also being studied in a long-term programme, to determine the effect of protection of this species (several publications).

In Switzerland, six publications treat “heron problems” and an investigation concerning cormorant is being made. In the Federal Republic of Germany, the predation by herons was studied in Bavaria on carp ponds. Another study on measures to protect fish farms and ponds from herons was made in Nordrhein Westfalen. In the United Kingdom, a study was made on predation by herons in fish farms.

In Ireland, only the food of cormorants was studied in open waters. In the Netherlands, research has been done in 1939 and 1952 on the kind and extent of the cormorant problem on commercial fisheries on the IJsselmeer, and recently at a fish farm (Lelystad). In Belgium, studies on herons and kingfishers were made on predation and protection measures.

In Denmark, veterinary research was carried out on parasitic diseases transmitted by birds. Another study is being made on the possible damage to fisheries by great cormorant, based on the stomach contents collected by fishermen shooting these birds (coastal waters) and from birds drowned in nets (lakes). In Romania, systematic research is being done in the Danube Delta concerning the dynamics of the piscivorous bird populations as well as on their influence both in the open waters and in fish ponds. In Poland, predation and relation with diseases were not studied, but a study was made on the percentage of the food given to fish in farms which was consumed by ducks and bald coot (2–7%). In other countries (Austria, Czechoslovakia), no investigations have been made on these subjects.

Except for great cormorant in certian favourable conditions (the Netherlands, Federal Republic of Germany, France in Camargue, Romania), these studies have shown that predation on fishes in open waters by piscivorous birds is not very important, in comparison with losses through bad management, water problems, diseases and predatory fish species.

7. ECONOMICAL IMPACT

As can be seen from the replies of the questionnaire, bird damage in open waters is local (cormorants) and mostly not of great economical importance, except in some areas: