INDOOR AND POND REARING TECHNOLOGY FOR FRY AND FINGERLINGS OF WELS, SILURIS GLANIS L.

L. Horváth

Warm Water Fish Hatchery, Százhalombatta, Hungary

ABSTRACT

The culture of wels, Siluris glanis, is based on the fact that it is a good quality food fish and that it can be used as a predator in warm water polyculture. Propagation and rearing require special techniques and health control during these phases is critical.

After reproduction and rearing of the yolk sac fry in the hatchery, the fry are grown further in either basins, small protected ponds or even larger ponds. With the former two methods the fingerlings of 3–4 cm are harvested after one month and also put into bigger ponds. The food that is provided consists of cut tubifex and in the ponds zooplankton and chironomids and especially fry and fingerlings of cyprinids (carp, tench, silver carp).

RESUME

Le silure glane, Siluris glanis, est élevé en pisciculture à cause de la qualité de sa chair et du fait qu'il peut servir de prédateur en polyculture. La reproduction et l'alevinage sont assurés à l'aide de techniques spéciales.

Après résorption de la réserve vitelline, les alevins sont élevés en bacs ou en petits étangs. Après un mois, les alevins de 3 – 4 cm sont transférés dans de grands étangs, où ils sont élevés en polyculture avec des cyprinides (carpe commune, tanche, carpe argentée etc.). Parfois les larves sont directement transférées dans de grands étangs. Dans les différentes phases d'élevage, les alevins sont nourris de tubifex (en bacs) et de zooplancton, chironomides et larves de cyprinides (en étangs).

1. INTRODUCTION

Wels is a good predator in intensive carp ponds (Szalay, 1963). Its oxygen demand is considerably lower than that of other carnivorous species in Middle-Europe (Marthüsev, 1973). Its feeding habits are also advantageous, as feeds are not made up completely by life fish, but by tadpoles, insects, worms and cadavers (Jászfalusi, 1954). Its meat has a high palatibility without bones.

These advantageous characteristics justify large scale propagation. Production in fish farms started in the twenties (Vásárhelyi, 1968). For a long time its natural spawning environment (willowroot nests) were imitated. Recently an induced spawning method was elaborated (Grosev, 1968; Fijan, 1975; Horváth, 1977).

The essence of this method can be described as follows:

-   breeders are treated with carp pituitary

-   ripe sexual products are stripped

-   fertilization is carried out by the dry method

-   eggs are hatched in vertical incubators (Zug-glasses)

-   non-feeding larvae are kept in mesh-cloth cages.

Large quantities of fry can be produced by the induced reproduction which can serve as basis for large scale rearing, both in basins and ponds (Huisman, 1975; Tamás and Horváth, 1976).

2. METHODS OF REARING IN THE FIRST SUMMER

The rearing of wels can be carried out by more than one method depending on size and age. Methods are shown in fig. 1.

2.1 Fry rearing

Fry rearing is practised in protected environments, this being the most delicate stage of fish. This can be carried out in the hatchery, in basins, or outside in small nursery ponds or sometimes in bigger ones.

2.1.1 Rearing in basins

After first feeding the fry are transferred from cages to troughs in the hatchery. In table 1 we can see the data of the reproduction result of recent years and in table 2 some characteristics of the rearing environment in troughs are given.

Essential requirements of rearing in basins can be enumerated as follows:

-   Suitable feed in appropriate size (cut tubifex)

-   Feeding of fry and cleaning of the troughs several times per day

-   Prevention of spread of diseases

-   Parallel to the growth of the fry the stocking density has to be decreased

-   Good quality of water sypply.

Rearing of fry in basins lasts only 2–4 weeks, in this time fish reach the size of 2.5 – 4 cm. In this stage they can be transferred to ponds for further rearing.

Rearing in basins requires special technical facilities and its man power demand is high, consequently it can be economic only in case of mass production.

2.1.2 Rearing in small ponds

The rearing in small ponds comprises the grow out of feeding fry kept in basins for 2–4 weeks. During the rearing in ponds, the following important conditions have to be secured:

-   No fish of bigger size should be in the ponds

-   Possibility of easy harvesting

-   Temperature above 20°C

-   Plankton stock of adequate size without species that would endanger the young wels

-   Possibility of additional water supply.

Duration of pond rearing is 3–4 weeks

Data on pond rearing are shown in table 3.

2.1.3 Rearing in bigger ponds

This method combines fry rearing with rearing of fingerlings, because it is impossible to harvest bigger ponds after 1 month. Wels are reared in polyculture with fry of cyprinids. Stocking is carried out simultaneously. Ponds are treated before stocking to secure dense Rotatoria stocks. Ponds should have:

-   a safe water supply

-   a good water quality

-   cover in some places by water plants

-   a depth of 1.20 – 2 m.

Wels in such ponds is an additional fish, stocking rates 10 000 – 20 000 per hectare, survival rates vary widely; often they are very poor. Wels are not fed; feed requirements are covered by natural feed organisms: larvae of insects, frogs and fish etc. Production data are shown in table 4.

2.2 Fingerling rearing

This phase is simular to the previous one in several aspects, e.g. pond requirements. There is a difference in preparation of pond, as planktonic organisms of bigger size are required. Wels fingerlings of 2–3 cm are stocked together with cyprinids of the same size. With this method higher survival rates can be obtained (Table 5).

3. FEEDING

Wels is a fast growing predatory fish. During the period under consideration its appetite is very high, depending on the water temperature. In the beginning, it feeds on Rotatoria, small crustaceans, worms, insect-larvae, etc. When the amount of feed is not sufficient, cannibalism may appear at a length of 3–4 cm. With feeds amply available, canibalism will not occur. Starving wels fry loose weight and are more susceptible to diseases.

It is the task of the fish breeder to influence the biocenosis of ponds in such a way, that feed organisms are always present in high quantities.

3.1 Feeding of fry

Most appropriate feed for wels fry in basins is tubifex cut into 1–2 mm size pieces. Starter feeds in ponds are Rotatoria and small, inferior Crustacea. Although fry can take Crustacea of bigger size, the ponds for rearing are managed to develop a Rotatoria stock. Reason for this is, that copepods of bigger size may damage wels fry. Rotatoria in ponds can provide sufficient feed for fry during the first few days. After this, bigger planktonic organisms have to be available. This is done by inoculation of the ponds with mezo-zooplankton. Mezo-zooplankton provides sufficient feed during the first months of rearing. Better growth and survival rates can be achieved by stocking fry of cyprinids, e.g. tench, in the rearing ponds.

When wels are not harvested after one month, but left in the same ponds to the end of the season, a sufficient amount of forage fish has to be present in the rearing ponds. At present, common carp is the cheapest for this purpose. The best method is to stock carp fry together with wels fry. There exists however, the danger that carp fry may grow too fast and become a feed competitor, while at the same time they will be too large to be eaten by the wels. Although it is more expensive, stocking of tench or silver carp fry is therefore more effective. Tench grow slowly and consequently it will serve as forage fish trough the whole season. This is confirmed by the fact that tench stock is usually depleted by the end of the season, while that of carp may actually show good survival rates.

In biculture of wels and silver carp, there is no feed competition. Silver carp keeps the water clean, thereby diminishing the danger of diseases. As silver carp is a quickly moving fish, the wels will only feed on it when there is no other nourishment to be found. In such a biculture, survival and production of silver carp will be high also.

3.2 Feeding of fingerlings

In the basins wels fingerlings are fed with tubifex. Because of the great appetite of such wels, this method is uneconomic. Several experiments were carried out for rearing with pellets, but up to now only initial results were obtained

When fingerlings of 1 month old are reared in ponds, chironomids and crustaceans are already not sufficient, consequently forage fish have to be provided. If wels of 1 month are stocked in ponds, where cyprinid larvae are present, the latter will be erradicated. Therefore, when bigger wels are to be stocked they should be stocked together with cyprinids of approximately the same size. The principles for choosing appropriate forage fish species are the same as said above.

Since it is not economic to produce only wels in rearing ponds, the most common method of wels rearing is in polyculture, as an additional fish. In such polycultures, it is advisable to stock 10–20 per cent more cyprinids than usual. It is also common practice to stock the ponds again later, with younger cyprinids, as forage fish. Supplementary feeding of wels reared in polyculture with high animal protein feed is not economic, because these feeds are taken by the carps.

Experiments are running for use of butchery offals and offals of the canning industry.

4. VETERINARY ASPECTS OF WELS REARING

Fry and fingerlings of wels are endangered by many diseases, which sometimes kill the greatest part of the stock. It is not our aim to go into details concerning diseases. However, since they play a decisive role in fry rearing of wels, we have to mention some major problems.

4.1 Rearing in basins

The danger of diseases is increased by the high stocking density. At the same time, a decay of water quality in the basins may occur. This further increases the possibility of infection.

4.1.1 Ectoparasites

During larval rearing in basins, usually Ciliate ectoparasites appear.

Chilodonellosis

Chilodonella sp. appear on gills and in the mouth.
Marks: fry loose weight, take air on the surface and are killed.
Treatment: a) prevention; formaldehide bathes: 1:5 000. The chemical is introduced into the troughs, and washed out by the water-flow. b) Curing; flush treatment with 5 per cent salt solution (2–5 minutes). Against Trichodina sp. and Costia sp. the same treatment can be applied.

Ichthyophthiriosis

Ichthyophthirius multifiliis appear on the skin.
Marks: white spot on the body.
Treatment: preventive; Malachite green bathes daily: 0.1 – 0.3 mg/liter.

4.1.2 Bacterial diseases

Bacterial diseases appear mainly in basins.

Mixobacteriosis

Flexibacter columnaris causes great losses in basins.
Marks: necrotic areas on skin, decay of fins.
Treatment: prevention and curing: Malachite green bath several times daily: 3 mg/liter.
Furazolidon bathes several times daily: 0.2 g/liter.

4.2. Rearing in ponds

The spread of diseases in ponds depends on the stocking density of wels, and on other species used for polyculture. When 25 fish are stocked per m² in polyculture with carp or tench, the probability of different infections is high.

4.2.1 Ectoparasites

Greatest losses under pond circumstances are caused by ectoparasites.

Trichodinosis

Trichodina and other common ectoparasites do no kill the wels fry, but they result in bad growth. Treatment of pond: with copper oxychloride: 4 mg/liter.

Ichthyophthiriosis

Ichthyophthirius multifiliis is the most dangerous parasite of wels fry. Frequently it causes total losses. Treatment of pond with Malachite green: 1 mg/liter. Treatment has to be repeated after 1 week.

Ancilodiscoidosis

Gill worms appear frequently in the second half of the season. No effective treatment known in pond. During harvest a flush treatment of common salt is applied: 2 – 5 per cent.

4.2.2. Bacterial diseases

In ponds with a good water quality bacterial diseases are rare. If such diseases occur, oxytetracycline treatment: 5 g/m³ followed by an increase in flow rate is recommended.

5. REFERENCES

Fijan, N., 1975 Induced spawning, larval rearing and nursery operations - Siluris glanis. EIFAC Technical Paper No. 25: 130 – 138.

Grozev Gr., 1968 Iskustveno razmnozavane i otglezdane na som. Ribno Stopanstvo (6–7): 28 – 29.

Horváth, L., 1977 Improvement of the method for propagation, larval and postlarval rearing of the Wels/Siluris Glanis L. Aquaculture 10: 161 – 167.

Huisman, E., 1975 Report of the first results concerning controlled reproduction and rearing of fry of Siluris glanis. EIFAC Technical Paper 139 – 141.

Jászfalusi, L., 1954 Poloumely chov sumcu. Cesk. Rybn. 9 (2): 26 – 28.

Marthüsev, F., 1973 Prodovoe rübovodsztvo. Vüss. Skola. Moszkva. 1 – 445.

Tamás, G. and L. Horváth, 1976 Intensive Welsbrutvorstreckung (Siluris glanis) in Kunststoffrinnen und Becken. Der Fischwirt. 26 (7): 41 – 42.

Szalay, M., 1963 Uj módszer a harcsaivadék mesterséges szaporitására. Halászat 9 (3): 95.

Vásárhelyi, I., 1968 A harcsa. Mg. Kiadó, Budapest 1 – 98.

6. TABLES AND FIGURE

Table 1. Results of wels propagation in 1974–78.

Table 2. Characteristics of the environment for wels rearing in basins

Table 3. Data of wels fingerling production in small (100–200 m²) experimental ponds. Rearing period: 1 month

Table 4. Production of wels in 1978. Ponds are stocked with 4–5 days old wels fry.

* no data

** eliminated

*** Warm water Fish Hatchery

Table 5. Production of wels in 1978. Ponds are stocked with 2–4 weeks old wels fingerlings.

Fig. 1. Methods for rearing of fry of wels