The European catfish or wels (Silurus glanis) is one of the best predators for use in intensive carp ponds or ponds stocked with a polyculture of various cyprinid species. Its oxygen demand is considerably lower than that of other European carnivorous fish. Its feeding habits are also advantageous, as the diet is not restricted to live fish, but includes tadpoles, insects and their larvae, worms and dead animals. Wels meat is of high palatability, without bones and scales, its capacity for growth is amongst the highest of any fish.
Development of propagation methods
Propagation of wels has been developed into a successful large-scale industry on Hungarian fish farms. Production in fish farms started in the 1920s. The essence of the original method was to imitate the natural spawning environment of wels in the farms. Ignoring minor differences in practice, an outline of the method is as follows.
When the water temperature has been 20°C for a sufficiently long time, single pairs of male and female breeders (previously separated by sex and held in small wintering ponds) are introduced into small spawning ponds, each of which contains a nest made of willow roots. Under favourable weather conditions the couples spawn on the nests within two days.
The nest is sometimes left in the pond, where the eggs hatch in a few days. After a few weeks, fry and breeders are taken from the pond. When using this version of the method, heavy losses may occur due to the fry picking up various infections from the breeders.
In more developed variations of the technique, nests are transferred to another pond, or are taken apart and the egg-covered part is put into baskets or boxes covered with cheese-cloth, until hatching. After hatching, larvae are fed for a few days. More effective protection may be given by placing pieces of the nest in tanks or Zug devices for incubation. The material most often used for nests (washed willow roots) generally rots during the incubation and larval period, causing heavy losses of eggs and larvae. Good results have been achieved using nests constructed from plastic fibres.
1 Based on material contributed by Dr L. Horváth, Agricultural University, Gödöllö, Hungary
In extensive conditions, nests or feeding fry are introduced into ponds already stocked with carp or other carp-related species. This results in recovery of a few thousand wels fingerlings when the ponds are fished in autumn. In a more advanced method, fry are reared in ponds in monoculture for varying lengths of time. However, various diseases, most commonly ichthyophthiriasis, can cause high mortality rates in monocultures without continuous veterinary control. Because of this, the more extensive rearing method has hitherto proved to be more reliable in large-scale farming.
In recent years, scientifically-based techniques for large-scale propagation of warmwater fish have been established. The natural-type propagation system can now no longer meet the increasing demand for wels fry, making the introduction of induced propagation in hatcheries more and more desirable. During the development of such propagation techniques for wels, some essential changes had to be made to the general carp-hatchery procedures.
The hatchery technique for the propagation of wels can be summarized as follows:
preparation of the broodstock;
transport of broodstock to the hatchery, and anaesthetization;
tying the mouth closed;
injection of carp pituitary extracts to induce ovulation;
ripening of the sexual products;
determination of the exact stripping time at the ambient water temperature;
stripping, and fertilization by the dry method;
egg incubation in vertical Zug jars;
treatment of eggs with a proteolytic enzyme;
rearing of fry in tanks and ponds.
Preparation and care of broodstock
Wels broodstock properly managed can be propagated more successfully than wild fish captured from natural waters. Fish ponds are generally shallow, and warm up easily. This favours the development of eggs in the ovary. Proper feeding is also an important precondition. The farmer can ensure a good food supply by providing enough forage fish. Breeders need to consume 3–5% of their body weight per day.
Throughout the year, breeders can be kept in small ponds at a stocking density of 200–300 fish of 5–10 kg per 1 000 m3 of water, with a water flow of 100–200 1/min. Wels almost stop feeding during winter, though it is wise to stock a few hundred forage fish in broodstock ponds for possible consumption on warm days. In the spring, when water temperature reaches about 10°–12°C breeders should be separated by sex. The sexes can be differentiated on the basis of the size and shape of the genital papilla. In females this is big, broad and protruding, often with a red tip, while the papilla of males is pointed and flat. In addition, skin colour is generally darker in males, and the head of old males is more angular than that of females.
The sexes are kept separately in ponds until the weather becomes suitable for spawning. Adequate feeding is very important during this pre-spawning period, when the breeders can consume 30% of their annual ration. Relatively small (5–7 kg) breeders spawning for the first time are most suitable for hatchery propagation.
The maintenance of a broodstock in good health and good condition is one of the most important tasks of the fish farmer. Chloramphenicol is a good antibiotic against bacterial diseases. It is first diluted to a concentration of 3 g/1 of distilled water; the stock solution is then used at the dose of 1 m1/kg body weight.
White spot disease can be prevented or eliminated in ponds by treating with malachite green at concentrations of 0.4 mg/l, provided water flow is good.
Induced spawning can be started as soon as the pond water temperature reaches 20°C. Fish showing the most obvious signs of maturity should be selected for spawning first, and these are generally the smaller individuals. The spawning season in Hungary is from May to the end of June.
After transfer to the hatchery, breeders are anaesthetized in a 1:10 000 solution of MS 222 (i.e., 10 g in 100 l of water). Fish should be left anaesthetized only for the minimum time necessary. Gill cover movements should be closely monitored and additional oxygen provided, because oxygen deficiency can occur when the same water is used to hold many fish. When fish have been narcotized, their mouths can be closed with a piece of string passed through holes drilled in the nasal and chin bones. This procedure does not disturb either respiration or the sexual processes.
For hypohysation, acetone-dried pituitary glands of common carp are normally used, at a dosage of 4–4.5 mg/kg body weight in two injections for females, and 3–4 mg/kg body weight in one injection for males. The procedure is the same as for common carp. At 23°–24°C, ovulation occurs 12–15 hours after the second injection, within 260–280 degree hours.
Wels spawners produce smaller numbers of eggs than cyprinids, a maximum of 200 000–400 000 eggs per female.
Eggs of wels can be easily stripped from the ripe female after anaesthetization, in the same way as carp. However, wels sperm are less abundant, and therefore special care must be taken in collection of milt. To make maximum use of the sperm available, it is often necessary to sacrifice the male, dissect out the testes, and squeeze them through fine-mesh, synthetic gauze onto the eggs.
A semi-physiological solution (0.3% NaCl) is used for fertilization. The dry-stripped eggs and milt are mixed with the fertilization solution. After a few minutes the fertilized eggs can be further stirred while more fertilization solution is added. After stirring for 4–5 minutes, the eggs are transferred to Zug jars. There, they stick either to the glass surface or to each other. Oxygen demand of eggs during the first 12–15 hours after fertilization is very low, and a water flow of one litre per minute through each jar is enough. The fixed, static eggs are well suited to survive the morula stage, very sensitive to movement or mechanical damage.
From 16 to 20 hours after fertilization, unfertilized eggs become white and fungi can easily multiply on them. At the same time, their oxygen demand increases as embryogenesis progresses. After the morula stage, therefore, it is important to ensure more space for the eggs to float freely in an ascending water current. To this end, a new procedure has been elaborated to separate wels eggs.
Eggs sticking to the sides of the jars can be detached 12–15 hours after fertilization by using a plastic tube covered with soft fabric. The water flow is stopped for 5 min. The clumped egg-masses are treated with proteolytic enzyme solution. Two hundred millilitres of a 1% alkaline protease solution is added to each Zug jar. During treatment, eggs should be continuously and slowly stirred with a plastic stick, so that the enzyme solution can reach every egg and dissolve the external protein layer responsible for stickiness on its surfaces.
During embryogenesis, the growing embryo expands the egg shell, and 8–10 hours before hatching the egg swells to about double its original volume. It is customary to treat the eggs with a 1:200 000 malachite green solution 3–4 times to prevent infection by Saprolegnia during this period.
Freed from the egg by the combined effect of their movement and the hatching enzyme, newly hatched larvae immediately try to find a stable surface to which to attach themselves.
When the first hatched larvae can be observed, the water flow should be gradually reduced to 0.1–0.2 1/min. Since the unfertile eggs are heavier than the fertile ones (unlike cyprinid eggs) they will now sink to the bottom of the jars. The majority of the fertile eggs will hatch within a few minutes after the water flow is reduced. The larvae and the unhatched floating eggs can then be removed from the hatching jars.
This is a very delicate operation. Using a plastic tube, both the hatched larvae and unhatched-but-fertile eggs are syphoned off separately. When subjected to this slight suction, the egg shells of the unhatched eggs burst. At this stage, there is no water exchange through the jars, and consequently the hatching enzyme accumulates and hatching becomes complete. The hatched larvae are distributed either into 30 x 40 x 60 cm frames covered with 0.5 mm mesh cheese-cloth, standing in a water flow of 2–4 l/min; large plastic Zug jars of 200 l volume may also be used, as for common carp.
When the larvae become sturdier in their oxygen-rich environment, they seek shelter in dark places. From the second day after hatching, good quality larvae start to take on a grey colour and become more and more active. On the fourth or fifth day they gradually swim up to the water surface around the walls of the vessel and begin searching for food. At this time the larvae still have yolk sacs, but their digestive tracts are capable of assimilating exogenous food.
The delicate wels larvae require a protected environment. This can best be provided in hatchery troughs or outside in small nursery ponds.
Rearing larvae in troughs. After first feeding, larvae are transferred into shallow troughs in the hatchery. The essential requirements are:
The most appropriate foods for wels larvae consists in Tubifex (red worm) cut into sections a few millimetres long, and complete pelleted feed. Trough rearing lasts for only 2–4 weeks, in which time the fish reach 2.5–4 cm. At this stage they can be transferred to ponds for further rearing.
Rearing in troughs requires a good technical backstopping and uses a lot of labour. Consequently, it is economical only for mass production.
Rearing larvae in small ponds. In small ponds, stocking is carried out with feeding larvae (early fry) or with fry first kept in tanks for 2–3 weeks. During 3–4 week pond rearing, the following important conditions have to be provided:
Rearing larvae and fry in larger ponds. This method combines fry rearing with rearing of fingerlings, because it is inconvenient to harvest from large ponds after one month only. Wels are reared in polyculture with the fry of cyprinids and stocking with early fry is carried out simultaneously. Ponds are treated before stocking to produce dense populations of rotifers.
Ponds should be provided with:
In such ponds, wels is an additional fish, and stocking rates are 10 000– 20 000 per hectare. Survival rates vary greatly, but are often very poor. Wels are not artificially fed, their food requirements being covered only by the natural food organisms present in the ponds.