II REVIEW OF REPLIES OF QUESTIONNAIRE - PART A. POND FISH CULTURE

C.M. Bungenberg de Jong - Organization for Improvement of Inland Fisheries (Nieuwegein, the Netherlands)

1. GENERAL INFORMATION

1.1 Total area of Fishponds

Estimate of the total area of fishponds (hectares) and the total production (tons) for most countries is given in Table 1. The information obtained from Italy relates to a few locations only as data on the total area of fishponds are lacking. In the case of the United Kingdom, the area of fishponds as estimated from the total production is probably limited to between 100 and 200 ha.

1.2 Production (fish species)

The most important fish species reared in Europe is the common carp (Cyprinus carpio), but the following cyprinid species are also raised: silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), grass carp (Ctenopharyngodon idella), tench (Tinca tinca), roach (Rutilus rutilis) and some other species from lesser importance.

The east Asian herbivorous fish species are mainly raised in countries with favourable climatic conditions (Hungary, Israel, Romania) whilst in west European countries tench and roach are produced often in polyculture with carp. The bulk of the production consists of cyprinid species and at the most 10% consists of the following predatory species: pike-perch (Stizostedion lucioperca), pike (Esox lucius), wels (Silurus glanis), perch (Perca fluviatilis) and some other species. In Israel, Tilapia spp. and mullets are produced, as well as common carp, silver carp and grass carp.

In most European countries the production level is not high (200–500 kg/ha), but in Hungary (1 400 kg/ha) and in Israel (4 000 kg/ha) higher yields are obtained as a consequence of intensive feeding. Here birds find much higher densities of fish in open waters.

1.3 Number of Pond Farms

Accurate assessment of the number of pond farms is difficult due to the great number of small ponds which are maintained as a side line to some other activity. Austria mentions 30 pond farms of more than 20 ha (a number of them of more than 100 ha) and a great number of small farms with an area of 0.6–10 ha.

Czechoslovakia states that, from the total pond area of 52 000 ha, there are 42 000 ha in the hands of the State Fishery Concern: grouped into 16 State Fish Farms each with 1 500–1 700 ha of ponds. France mentions the total number of ponds to be 5 000.

2. BIRD PROBLEMS

The following answers were received to the question of whether or not there was a local waterbird problem serious enough to hamper fish production. Austria, Belgium, Federal Republic of Germany, Hungary, Israel, Italy and the Netherlands report serious waterbird problems hampering fish production. Czechoslovakia, France, Poland and Romania experience such problems on a local scale. United Kingdom reports some problems with predation by birds. The following problems are encountered in order of importance (see Table 1):

1. predation - not only causes losses due to direct predation by piscivorous birds, but also as consequence of severe injury;

2. interference with artificial feeding - not only direct by consumption of food, but also indirect by the disturbance of the feeding process of fish due to concentrations of aquatic and predatory birds on the feeding places;

3. spread of fish diseases - transport of infectious fish diseases to ponds, pond farms, which were free of fish diseases before. Furthermore, a number of aquatic birds function as host in the life cycle of some parasites, and

4. disturbance during wintering - when the ponds do not freeze in winter, as in most of western Europe, severe problems can be caused by aquatic birds during wintering.

Table 1

Characteristics of pond farms in Europe and the degree to which they experience bird-related problems

3. PREDATION

A complete list of all piscivorous birds has been received from a number of countries. In the following we will restrict ourselves to those species which are (locally) common and play an important quantitative role in bird predation. Consequently only a restricted number of bird species is listed for the different countries of the EIFAC region (Table 2).

Some species listed as major predators in some countries are absent or very rare in others. Most instructive in this respect is the situation in Romania, especially in the Danube Delta where all the categories of piscivorous birds mentioned are present. In Romania the order of importance of the different predator categories and separate bird species is as follows:

Table 2

Predation by fish-eating birds, numbers refer to order of importance in each country

Other countries with experience of pelicans and cormorants are in complete agreement with the Romanian conception that these two categories are the most feared predators in pond fish farming.

In the EIFAC region generally pelicans are of only local importance as they are only known from Romania and Israel. These birds are also considered as the major fish-eating bird in Israel where they are present from October to March. Directly after pelicans, cormorants are undoubtly the most harmful predators in pond fish farming. In both Hungary and the Netherlands the great cormorant is ranked as first among the fish-eating birds in pond farming.

Furthermore, the great expansion of the cormorant populations in Denmark and the Netherlands, where they are totally protected, migrating cormorants cause more and more problems in Austria, France (fish farms Camargue), Germany (fish farms Schleswig-Holstein) and Switzerland.

Cormorants cause considerable incidental damage to fish by pursuing the shoals under water causing a panic reaction. Fish endangered in this way will hide in the vegetation on the banks or cluster in harvesting sumps for a long time. When disturbance by cormorants is continuous, the stressed fish will not leave their hiding places, do not eat and as a consequence will finally die of starvation, or disease. It is a remarkable fact that fishponds used for raising two-year old carp are frequently practically without fish when fished in the autumn due to comorant attacks. It apears that the fish population is totally fished out by cormornts, but in fact it is the stress effect which has the major impact on the fish population. Because this cormorant predation often means total bankrupt, especially in fish farms raising stocking material.

Herons are mentioned as the most important predator category in pond fish farming in Austria, Belgium, Czechoslovakia, France, Germany, Israel, Italy, Poland, Romania and the United Kingdom. Gulls are considered more harmful in Hungarian fish farms than herons, but less so than cormorants, whereas grebes cause more damage than herons, but less than cormorants in the Netherlands.

Gulls (terns) are generally ranked after herons in Belgium, Czechoslovakia, France, Germany, Israel, Italy, Romania. In Austria and Poland the great crested grebe causes more damage in pond fish farming than do gulls (terns). The damage caused by fish-eating birds differs according to species.

Herons

Herons operating along the banks are restricted to the shoreline or to wading in shallow water. They have a limited radius of action and eat fish of up to 250 g. Pond farms mainly raising 300–1 200 g carp for consumption in ponds with more than 100 cm water depth will have restriced losses due to herons. Otherwise, herons can cause considerable damage in ponds with an insufficient water level or ponds loosing water towards the end of the growing season, especially in those fish farms raising stocking material.

Gulls (terns)

The black-headed gull (Larus ridibundus) is the most common gull species on fish farms. They are observed particularly during the growing season on shallow fingerling ponds. Black-headed gulls, which occur often in very great flocks, can involve high losses of fingerlings especially during the autumn harvesting. In Hungary, herring gulls (Larus argentatus) visit the harvesting sumps in October in very high numbers. During a short period of time they can cause higher losses by direct predation and injury than black-headed gulls during the whole season.

Piscivorous birds pursuing fish under water

The most efficient fish-eating birds belong to this group: grebes, mergansers, cormorants and pelicans have an unlimited radius of action covering the whole fish ponds. The limiting factor is mainly the size of prey: grebes take fish of up to 120 g, cormorants up to 600 g and pelicans catch fishes up to 1 kg. Unlike herons and gulls, these typical aquatic birds rarely cause problems at the time of harvesting when the ponds are emptied.

Besides such typical fish-eating ducks as mergansers some diving ducks also belong to this group. Common species, such as the tufted duck (Aythya fuligula) and the pochard (Aythya ferina) have proved dangerous to smaller fish in wintering ponds. In summertime fish will normally swim rapidly enough to escape these non-specific fish predators. But in winter, when the fishes move only slowly, diving ducks can seriously disturb one-year old carps in the wintering ponds. When the fishes are not small enough to be eaten by the ducks, they injure up to 80% of the fish population, which die later.

4. INTERFERENCE WITH ARTIFICIAL FEEDING

Three main problems arise where artificial feeding is practised. Firstly, there is direct competition where the feeds intended for fish are eaten by birds. Secondly, fish may be disturbed by birds and receive less food than normal and finally there is the concentration of predatory birds on the feeding places, resulting in a higher predation and an increase in the number of injuries. This problem is not restricted to conventional methods of feeding, but may be even worse when automatic feeders are used.

In order of importance coots (Fulica atra), ducks and local swans were mentioned. Of the ducks, the mallard (Anas plathyrhynchus) and the diving ducks (Aythya ferina, Aythia fuligula) are considered of importance in this respect. Grain, as well as pellets are eaten by these birds. Birds quickly learn to operate automatic feeders, with the consequence that pellets fall into the water and are lost by disintegration. The financial losses caused are substantial and in Hungarian pond farms are comparable with losses caused by predation.

5. DISEASES

See Part D of Review of Replies to Questionnaire.

6. MEASURES

From the replies to the questionnaire it is apparent that all possible attempts have been made to prevent bird damage in pond farms. Unfortunately, however, most methods have proved to be ineffective (Table 3).

Table 3

Non-lethal protection methods

+ = effective;
t = temporary effects;
N = no results

Two non-lethal methods are commonly used to prevent bird damage:

1. complete or partial exclusion using physical barriers, and
2. various frightening devices (acoustic and visual deterents).

(a) Physical barriers

1. Complete enclosure. In general enclosing ponds to exclude all fish-eating birds require 3–5 cm mesh netting secured to frames or supported by overhead wires. This method is successfully employed in the Netherlands to prevent winter damage to carp fingerlings (± 25 g) by diving ducks. In contrast with trout farms this effective method can only be used on a small scale in pond farms because of the high costs.

2. Lines or overhead wires. Ponds can be covered with overhead wires or monofilament lines suspended horizontally in parallel dug in one axis or a crossing on two axes. Spacing between the wires or lines should be based on the type of birds causing damage.

   In Israel, against pelicans, strings are stretched across ponds at 30–40 m intervals and around the circumference of pond embankments (0.5 m high). If the strings are properly installed they are very cheap and effective. Experiments have been made in the Netherlands on a large scale with overhead lines crossing in two axes of 20 × 20 m on larger ponds (10 ha) to prevent damage by cormorants. In the first year this method proved to be successful, but in subsequent years the cormorants changed their behaviour and learned to pass through patterns as small as 5 × 5 m. In the USA it was found that 120 cm spacing was effective in deterring gulls. To exclude mergansers 60 cm spacing is necessary and a minimum of 30 cm is required for herons and terns.

3. Perimeter fencing or wire. Perimeter fencing or wire around ponds provided some protection and is most effective for herons. But after some time also herons learn to avoid these obstacles.

(b) Frightening devices

Generally the use of frightening devices (automatic exploders, bird control shells, alarm or distress calls and lights) is not effective, because most birds rapidly become accustomed to them.

The replies to the questionnaire indicate that automatic exploders can be used under certain circumstances. For example, this method is often used with success some days before emptying a fishpond to prevent damage by herons, and in the Netherlands it was found that it is possible to deter mergansers on 10 ha-ponds by automatic exploders. Acoustic methods can be more effective if reinforced by shooting. In the replies, the use of distress calls is not mentioned and all respondents agree that the use of visual deterents has no value.

In several countries, interference with artificial feeding can be prevented. In Czechoslovakia, for instance, protection against birds is carried out by installation of wire nets in the water column (25–30 cm under water level) above the feeding places. Feed drops through the wire net and can be taken by fish but not by birds.

7. LEGAL POSITION

See Part B Legal Position and Part C Bird Predator Populations (Legal Position).

8. INDEMNIFICATION

No provision for indemnification for damage caused by protected birds exists in countries responding to the questionnaire. Indemnifications have been granted in the case of severe damage by cormorants only in Schleswig-Holstein (Federal Republic of Germany).

9. ECONOMICAL IMPACT

The replies to the questionnaire gave an evaluation of the economic losses caused by bird produced damage by country as follows:

Austria

Losses by birds due to predation have increased considerably in the last few years and in places have reached an unacceptable level. In several ponds containing one-year old coregonids damage caused by herons and grebes was estimated at 20–30% of the yearly production. Damage to one-year old carps was estimated to be 20% and to two-year old carps more than 10%.

Belgium

Damage caused by piscivorous birds differs from fish farm to fish farm. Predation by herons, the main predator, caused a loss of production of between 10% and 60%.

Czechoslovakia

Birds, especially herons, hamper fish production locally. Rough estimates indicate that birds decrease the production of carp by 10% in these localities. Carp fry to yearlings are primarily affected by birds.

France

The specialists of the National Institute of Agricultural Research and the Working Party on Piscivorous Birds differ from fishfarmers in considering bird predation of slight importance. The only exception is the great cormorant which can cause local problems in the Camargue and in Brenne.

In the Camargue, predation by cormorants is a “new and unexpected” problem, due to the explosive increase of the cormorant populations in the Netherlands and Denmark. Formerly cormorants were seldom seen in the Camargue, but today more than 6 000 cormorants are present from October to March. Confronted with this alarming situation the Government has ordered research to assess the damage caused by cormorants. Research at Tour du Valet has been carried out on five pond farms in order to estimate the damage caused by cormorants over the four years from September 1981 to March 1984. It was found that losses to these pond farms were 47 000 kg with an economic value of F 846 000 per year. As a consequence production costs have risen to an unprofitable level, several fishfarmers were forced to go out of business, and the total area of fishponds in the Camargue was reduced from 1 020 ha in 1981/82 to 500 ha in 1983/84. The cormorant problem is international and countries with rapidly increasing breeding populations such as Denmark and the Netherlands should take measures to control these populations. To do this the countries of the European Community should consult to fix the number of cormorant breeding pairs that are desirable within each country.

Germany, Federal Republic of

Bayern: Four fish farms with a total area of 265 ha of ponds have suffered damage of at least DM 125 000 per year. This is mainly caused by predation by herons, grebes and gulls, and interference with artificial feeding by coots, ducks and swans. One fish farm reported 30–50 herons on 30 ha of fishponds; another fishfarmer reported 120 herons on 75 ha of fishponds (1.6 heron/ha). Perhaps even more damage is caused by the great crested grebe, especially in fish farms raising stocking material. Most endangered are ponds with one-year old tench or one to two-year old carp which suffer losses of up to 80%. One fishfarmer was forced to stop raising small fishes and switched over to the less profitable production of carp for consumption. The feeding places of another farm with 70 ha of ponds were frequented by 500 ducks (mainy tufted duck), 400 coots and 15–20 swans, which disturb the fishes and cause high losses of food (grain, pellets).

The great cormorant has become established in Bavaria near the well-known fish farm “Birkenhof” (200 ha). Here a breeding colony of 70 pairs has been set up in the “Speichersee” at Ismaning near Munich. In a short time the visiting cormorants wrecked the farm: wintering ponds stocked with one and two-year old carp were practically totally fished out and the losses of two to three-year old carp of the order of 50–80%. As carps of up to 550 g are vulnerable to cormorant predation and larger carps up to about 700 gr can be severely wounded by cormorants failing to swallow them, the only alternative is to stock ponds with carps greater than this weight. Under these circumstances fish culture is no longer profitable.

Niedersachsen: In 1977 a questionnaire was distributed in the area of Hannover to gather more information on the economic impact of bird predation. Replies were received from 20 pond farms. Damage to these farms amounted to DM 225 000 (herons: DM 177 000, other birds DM 48 000).

Nordrhein-Westfalen: Three carp farms with a combined area of 200 ha sustain damage of at least DM 75 000/year, mainly caused by herons.

Schleswig-Holstein: Damage caused by herons is estimated on 10% of the yearly production (total area of carp farms 1 700 ha with a production of 340 t two and more year old carp plus 1.6 million one-year old carp. Local heavy losses of up to 80% have been reported due to cormorants which have caused financial losses amounting to DM 10 000 to DM 30 000 per fish farm.

Hungary

A special survey was carried out in four large fish farms with a combined area of 7 385 ha (34% of the total pond area in Hungary) in 1984 to gather more information on bird problems. Results of this survey of fish-eating bird species were as follows (in order of importance):

1. cormorant (Phalacrocorax carbo)
2. herring gull (Larus argentatus)
3. black-headed gull (Larus ridibundus)
4. grey heron (Ardea cinerea)
5. spoonbill (Platalea leucorodia)
6. purple heron (Ardea purpurea)
7. little egret (Egretta garzetta)
8. great white egret (Egretta alba)

Cormorants cause damage over the whole growing season of the fish. From March to July they visit ponds stocked with one or two-year old fish. From late summer they also appear on fingerling ponds. Losses due to predation by cormorants are about 200 t/year on the four large fish farms and approximately 300 t in the whole country (Ft 12 million). Damage due to black-headed gulls can also be observed during the whole breeding season in shallow fingerling ponds. These birds can involve great losses of fingerlings especially during the autumn harvesting.

Herring gulls and some other migrant gull species visit the harvesting sumps in October in a higher number. Within a short period of time they can cause higher losses both by direct predation and by injury than can black-headed gulls during the whole season.

On the four large farms losses due to all gull species are 100 t/year. It means 300 t at country level (approximately Ft 16.5 million in value).

The feeding habits of grey herons and purple herons in pond farms are rather similar to those of cormorants, but losses are much lower. In spring they visit ponds stocked with one-year old fish. In summer, as fish fry grow, they also can be found on fingerling ponds. In the fish farms investigated, the damage due to these bird species is about 30 t/year, so at a country level 90 t (approximately Ft 3.6 million).

Spoonbills are important only in a few areas. During summer they can eat a large number of valuable common carp and herbivorous fish fry. On the Hortobágy fish farm this means 1.5–2 million fry/year. At country level a value of Ft 1.5 million can be estimated.

The populations of little egret and great white egret are growing very quickly in some areas of the country due to protection. It seems possible that these species will cause serious losses on fingerling ponds in the future.

Only two species, mallard (Anas platyrchynchos) and coot (Fulica atra) are important in that they interfere with artificial feeding by eating both cereal grains and pelleted feeds as well.

At the largest Hungarian fish farm, Hortobágy (4 100 ha), feed losses due to mallards are estimated at 8 t/day, and due to coots at 1.5 t/day. On the Szeged fish farm (1 839 ha) 220 t/year are taken by mallards and 45 t/year by coots. At country level 4 000 t of feed intake by birds can be estimated (approximately Ft 24 million).

Summarizing the above data, the economical losses and extra costs to the Hungrian fishfarming sector can be roughly estimated as follows:

Israel

The main impacts of birds on aquaculture in Israel are through predation also interference with artificial feeding and disturbance during harvesting.

The following fish-eating birds are of special importance because of their economical impact:

Pelicans are present from October to March. Some individuals stay all winter, but most migrate to Africa during the months of January and February. They feed mainly on tilapia and carp winter stocks consisting of fish ready for marketing. They also raid fish ponds during harvesting. They take fish of up to 1 kg and effective predation takes place in fish ponds of 2 m depth (or less).

Storks and herons: (white, purple and grey), some herons remain all year round, whereas others migrate to Israel during October–December and again during March–May. They feed mainly on fish weighing less than 150 g. The main damage is in tilapia fry wintering ponds when the temperature drops to less than 12°C. Herons also predate on fish while they gather under the demand feeders or when they are sick.

Sea gulls are present all year round. They mainly disturb fish during the harvest by pecking on them. In addition to the wounds they cause, the disturbance of the fish considerably increases the turbidity of the water, which in turn, will eventually cause the death of many fish. Sea gulls also peck on sick fingerlings.

Interference with artificial feeding is mainly caused by herons and sea gulls while fish are gathered under automatic or demand feeders. The bird interference is independent of food quality. All together the estimated damage caused by birds on pond farms in Israel is approximately US$ 500 000 per year.

The Netherlands

Dutch pondfarming has problems with grebes, diving ducks and herons, which cause extra losses due to predation of 15–20% in one to two-year old carp and 5–10% in two to three-year old carp. These problems are, however, relatively slight as compared with those caused by cormorants. As an example of the effects of cormorant predations we cite the history of the fish farm (170 ha fishponds) near Lelystad in the eastern Flevoland polder, which have been successfully operated since 1963. After reclamation of the neighbouring southern Flevoland polder in 1978, a cormorant colony settled in the newly established State Nature Reserve “Oostvaardersplassen” (5 000 ha), approximately 13 km south-southwest of the fish farm. The development of this colony was explosive: from 1978 onwards estimated numbers of breeding pairs were 175, 790, 1 100, 2 000, 2 500, 3 900, 4 600 up to 5 100 in 1985. In the following years the colony was maintained at this level and a further increase in population took place in a daughter colony at a distance of 10 km south-southwest of the main colony.

Although plenty of food is available for the cormorants in Lake IJsselmeer with 185 000 ha of rich fishing grounds, cormorants visited the fish farm in increasing numbers from 1979 onwards. In 1979 regular groups of cormorants were seen fishing on the fishponds. By 1980 some ten cormorants visited the fish farm every day. In 1981 this number increased to several hundreds (300–600 yearly) with flocks of even more than 3 000 cormorants per day (April 1981).

Higher losses were estimated in carp culture in 1979, but in following years this enterprise was completely ruined due to cormorant predation, as can be seen from the following data:

In 1980 severe losses (35%) had already been registered in the production of three-year old carp, whilst the production of two-year old carp was a complete disaster: a greater weight of one-year old carps was stocked than was harvested as two-year old carps.

In view of this situation, the production of two-year old carp was abandoned and it was decided to grow only three-year old carp in 1981. Although the six ponds (area 63 ha) were stocked with carp of a mean weight of 450 g, the losses of two to three-year old carp increased in 1981 to 70% and production was zero.

There can be no doubt that these losses were inflicted during massive cormorant visits. Carp of the same origin were stocked at the same time in small experimental ponds (0.2 ha) as a control. As can be seen from the control column of the above table, losses in the experimental ponds which were never visited by cormorants were low and contradict the popular view that cormorants will only eat sick fishes.

During subsequent years a lot of trouble and money were investigated in the development of methods to prevent cormorant damage. Although the use of overhead lines and the application of ultrasonic sounds seemed promising at first, it was found that these methods were less successful after continued use, as the cormorants learned to avoid these obstacles. Near to bankruptcy, finally the greater part of the ponds was taken out of production and a flourishing fish farm was abandoned due to cormorant predation.

Poland

Opinions on whether bird problems do exist, vary from farmer to farmer. Predation by herons (Ardea cinerea), grebes (Podiceps cristatus), terns (Sterna sp.) and cormorants (Phalacrocorax carbo) (listed in order of importance) is considered serious with regard to fry and fingerlings. Ornithologists, however, claim that due to the rather small populations of piscivorous birds in Poland, predation on cultured fish will be in general negligible.

Interference with artificial feeding has been observed on several fish farms. A study of this problem in four fish farms, including the largest fish farm in Poland, the State Fishery Enterprise in Milicz, revealed that the dominant species feeding on fish food are: pochard (Aythya ferina), tufted duck (Aythya fuligula), mallard (Anas platyrhynchos) and baldcoot (Fulica atra). Population densities of these species estimated in four fish farms, totalling 4 000 ha of fishponds, varies until mid-July from 2.9 to 6.8 specimens/ha. After mid-July the populations increased from 5.3 to 15.2 specimens/ha. During the period when fish are feeding, these bird species consumed 2.0–7.4% of the food given to fish.

Romania

There are local serious problems from fish-eating birds. Most affected are: the Danube Delta and the floodplains of the Danube and the Prut, Siret, Mures and Olt rivers. Fish farms and especially rearing stations in these regions are strongly attached by fish-eating birds from March to October. Most vulnerable to predation are ponds of over 5 ha and an average water depth of 1 m. The turbidity of the eater is an element which contributes to the intensification of predation. There are cases where ponds have been totally fished out by predatory birds.

Interference by birds with artificial feeding is also a problem. Coots (Fulica atra), especially, interfere with artificial feeds distributed on special fixed places in the ponds. A great number of coots are attracted to the feeding places, plunging into the water immediately after the feeds are administered to fish. They eat pellets as well as a mixture of grits.

Predatory birds intensely stress pond fish during feeding, especially in shallow ponds. Gulls (Laridae) are very active now and the presence of uneaten feeds can be noted, because of attacks of predatory birds.

Fish-eating birds are also important vectors in the spread of fish diseases and parasites. As sick fish are highly vulnerable to predation, fish-eating birds may prevent the early discovery of diseases.

There are normally no problems with birds during the winter in fish farms. During mild winters however, when the water does not freeze, the great majority of the predatory birds does not leave the breeding and feeding areas. Piscivorous birds do not normally disturb fishes in wintering ponds, but during mild winters cormorants can cause heavy losses.

The damage caused by birds on pond fish farms in Romania is estimated at 20% of the fish production. It means a loss of production of 8 000 t and, in terms of monetary value, some US$ 6 400 000/year.

Other replies

Other replies did not contain estimates of economical impact. However, this does not mean that no bird problem exists. On the contrary practically all fishfarmers struggle in one way or another against birds, but losses are often difficult to distinguish from those arising from other sources.