J. From
Danish Institute for Fishery and Marine Research, Danish Trout Culture Research Station, Broens, Denmark
ABSTRACT
A detailed survey is given of the trout culture practices in Denmark. The eggs are obtained in February – March and the fry are reared in indoor tanks up to a weight of 3 g to avoid problem with myxosomiasis and sunburn.
After this the fingerlings are grown in earthen ponds up to marketable size. Feeding and cleaning practices and the most common diseases in the different phases and their treatment are discussed.
RESUME
Un résumé détaillé est présenté de la pisciculture de truites au Danemark. Après la production des oeufs en février–mars, les alevins sont maintenus en bacs à l'intérieur jusqu'à ce qu'ils aient atteint un poids minimum de 3 g afin d'éviter la myxosomiase et des brûlures de soleil.
Les truites sont ensuite élevées en étangs jusqu'à ce qu'elles aient atteint le poids de commercialisation de 180 – 200 g. Les méthodes d'alimentation, les mesures sanitaires, les maladies les plus générales et leur traitement sont indiqués.
The Danish production of Salmo spp. is practically speaking based on rainbow trout Salmo gairdneri Richardson, 1836. When the fish have reached table size (180–250 g, 1–2 years old) they are sold for human consumption. The methods used in the very limited production of other Salmo spp. (S. salar Linné, 1758 and S. trutta Linné, 1758) used for stocking of natural waters do not differ from the methods used in the rearing of rainbow trout. For this reason there will be no distinction between the different Salmo species in this report.
The rearing of fry takes place indoor in concrete tanks or fibre glass basins. The main reason for using tanks or basins is to avoid whirling disease, myxosomiasis (caused by the myxosporidian Myxosoma cerebralis (Hofer, 1903). It is especially the rainbow trout that is affected by the disease. In the tanks and basins the contact between the young susceptible fry and the infectious stages of the parasite, which are found in mud, is prevented. When the fry have reached fingerling size (about 6 centimeters, 3 g) they can be kept in earthen ponds heavily infected with Myxosoma without being damaged by the parasite, because it only can penetrate into the skeleton when this still is cartilaginous.
The most important prerequisite for a plant for rearing of fry is that the water is free of Myxosoma spores. Therefore, it is not advisable to use water that has been used for fish farming earlier. If this condition is fulfilled, water from a river or brook can be used. Often ground water is used, provided that it is sufficiently oxygenated and that the iron content is not too high (less than 0.5 ppm ferro-iron). The iron can be precipitated by oxygenation or by raising the pH by means of adding lime to the water.
Since the temperature of ground water is often rather low, it is not advisable to use it if the temperature is not at least 10°C in the summer. For rainbow trout the optimum temperature is 16–18°C, the maximum temperature a little more than 20°C and for brief exposure times 25°C. For the other Salmo spp. the optimum and maximum temperatures are a few degrees lower.
The reason why the tanks and basins are placed indoor is mainly to avoid sunburn of the young fry. But of course also to avoid snow and frost. Further, fin rot is not so common and serious a problem indoor as it is under the open sky.
For rearing of fry concrete tanks of 6 m × 60 cm × 60 cm are mostly used. Because of the cleaning and to avoid injuries to the fish it is important that the sides and bottoms of the tanks are as smooth as possible. To facilitate the daily cleaning a slope of the bottom of 8–10 cm in a 6 m long tank is preferable.
New concrete tanks are treated with soda because the raw concrete liberates base which cauterizes skin and gills. If the water is acid the tanks must be protected by paint. If this is not done porous surfaces will result from the cauterization of the concrete and this will among other things impede the cleaning. A very suitable paint which also prevents the growth of algae is ship paint which contains copper.
In most hatcheries the eggs of rainbow trout are taken in February and March (the other Salmo spp. in November and December). The fish are anesthetized before being stripped and the dry method of fertilization is used. The eggs are hatched in rectangular trays placed in long troughs. The frame of the trays is made of wood or fibre glass, the bottom of perforated aluminium. After hatching and absorption of the yolk sac, the fry are placed in the tanks or basins (from March to June).
The number in each tank or basins naturally depends on the quality of the water and on how long time you want to keep the fish in the tanks before they are placed in earthen ponds. A concrete tank of 6 m × 60 cm × 60 cm with a water depth of 40 cm is normally stocked with 40 000 fry. The water supply is about 1 l/sec. When the fish have not been thinned, a water supply of 21/sec is normal when the fish have gained a length of 5–6 cm. (This can be given from a pipe with a diameter of 1½ inch).
The liability to catch whirling disease depends on the size of the fish and not upon their age. Therefore it is necessary to grade the fish at adequate intervals so that the fish have a length of at least 5–6 cm before they are placed in earthen ponds. The fish are now called fingerlings and it is about 8 weeks after they were placed in the tanks or basins.
For the feeding of fry only dry feed is used. The amount of feed given is based on appetite and feeding tables provided by the food manufacturer. Depending on the fish size and water temperature the amount of dry feed will vary from about 2–10% of the fish weight. As it would be much to laborious to hand feed the fry, automatic feeding is used. The feed is given all the light hours (from 4 a.m. to 7 p.m.), 3 times per hour. The conversion ratio is 1–1.3. The feeding of the fry is started before the yolk sac is completely used up. In this way all the fish will learn to feed. Naturally, the feed supply must in the beginning be very moderate but nevertheless it is so abundant that inevitably there is a waste of feed. Apart from the first weeks untill the fish have started to eat well, there ought not to be an actual feed waste. If you want an optimal growth a minor waste cannot be completely avoided but it can be reduced to a minimum, because during the cleaning it can be detected if there is food on the bottom. It is not recommended to feed with such small amounts of feed that there is no waste at all, because in this case the weaker individual will not receive sufficient.
A condition for mass rearing of fry in tanks or basins with a density more than 100 times as high as in a pond, is daily cleaning. In the beginning when the fry are small and delicate it is necessary to be very cautious. By drawing the outlet gate a little and at the same time cautiously sweeping through the tank with a soft broom it is possible to get most of the faeces and food waste out through the screen at the outlet. If there are dead fish these will be placed on the screen from which they can be swept away with a brush. When the fry are bigger the gate can be drawn more at the bottom and the bottom in the tank is easy to clean with a broom and the fish themselves will contribute considerably to the cleaning since they will gather at the outlet due to the slope of the bottom.
Naturally, the intensive rearing of fry in tanks and basins involves an increasing risk for diseases. The diseases are the same as in ponds but the frequency and strength are often bigger. On the other hand a disease will often be recognized earlier in tanks than in ponds. Furthermore, the treatment with chemicals is easier in tanks where the volumes of water are limited and the water is easy to change if the concentration of the chemicals should be too strong.
Whirling disease (myxosomiasis)
The rearing of fry in tanks and basins takes place, as mentioned, to avoid whirling disease. The disease normally only occurs if the water has been used earlier for fish farming.
Viral haemorrhagic septicaemia (VHS, Egtved disease)
No means of treating infected fish or to remove or alleviate the severity of the disease is available. The only effective control measure is avoidance, by ensuring that water supplies and eggs are free of virus.
Infectious pancreatic necrosis (IPN)
There are no means of treating infected fish to eliminate the virus. But if the fish are placed in ponds (at a much lower stocking density than in the tanks and basins) the death rate will decrease and the fish will be non-symptomatic carriers. It is believed that this is due to the fact that the stress upon the fish is heavier in the tanks than in the ponds. However, if the fish are smaller than 5–6 cm they cannot be put into the ponds, because they will catch whirling disease. Again, avoidance is the only effective control measure.
To prevent the virus diseases prophylactic precautions are taken, e.g. cleaning the tanks and tools in formalin or jodophores. Jodophores and formalin can also be used to disinfect the eggs. By this process it is only the virus at the surface of the eggs that is inactivated. As IPN virus is more inclined to penetrate from the surface and into the egg than the Egtved virus is, the effect on the IPN virus is very limited.
The most effective control aganst the virus diseases is a stamping out programme. All the fish in the trout farm are slaughtered and holding facilities adequately disinfected.
Bacterial gill disease complex.
Bacterial gill disease is very common. The disease is cured with chloramine-T, sodium paratoluenesulphonchloramide, (CH3)C6H4SO2(Na)Cl. 3H2O in the concentration 1:100 000 (volumes of chloramine-T powder to volumes of water). Time 1–8 hours.
Fungal infections of skin and gills (saprolegniasis).
Fungal infections are treated with malachite green. The comparitive non toxicity of malachite green has caused that its usage is based rather on experience and estimate than on exact concentrations. However, the dosage is about 1:5 millions. Time 1–8 hours.
Of the skinparasites the flagellate Costia necatrix (Henneguy, 1884) (Ichthyobodo necatrix (Henneguy, 1884), is the most common on fry. The costiasis is cured with formalin bath. Concentration 1:4000. Time 1–8 hours.
Fin-rot
Fin-rot is treated with a malachite green-formalin mixture consisting of 1 litre 2.5% malachite green solution mixed with 1 litre formalin. This solution is added to the water on estimate and after testing in one tank or basin.
Prophylactic treatments with chloramine-T, malachite green and formalin are very common (Monday, Wednesday, Friday).
Hexamita truttae (Shmidt, 1920) (Octomitus truttae (Shmidt, 1920)). This gut flagellate is treated with acenitrazole = 2-acetamido-5-nitrothiazole, of which there is given 1 gramme in 25 kg food for 4 days.
Internal bacterial infections
These are treated with sulphonamides and nitrofurans. The mostly used are sulphamerazine and furazolidone. The first is given in 0.2 gramme/kg fish/day for 3 days, then 1 day with no treatment and then again treatment for 2 days. The latter is given in 50 mg/kg fish/day for 10–20 days.
Starting from the middle of April up to October the fry reach a size of about 6 cm (about 3 grammes) and are then called fingerlings. They are stocked into earthen ponds because they no longer are liable to catch whirling disease. The fish are called fingerlings up to a size of 16–18 centimeters (45–60 grammes).
A common size of a trout pond is 30 × 12 meters with a water depth of 0.7 m. Inlet and outlet pipes are made of wood. The ponds are supplied with water from a river or brook. A middle-sized Danish trout farm will have 35–60 ponds.
In the ponds the fish are fed minced sea fish or dry food. Earlier only the minced fish was used. But by now as better dry feed formulas have been developed at the same time as the trout farms have come in the limelight of the environmental authorities many trout farmers have started to use dry feed. The food waste from dry feed can be negligible in contrast to minced fish where the waste is about 50%. Further the feeding with dry feed can be automatized and self feeders are often used.
In December the first fingerlings will have grown to a size between 16–18 cm (45–60 g) and are no longer called fingerlings but (translated from Danish) ‘undersizers’. In the succeeding May or June, the fastest growers will have reached a marketable size (180–250 g). The majority of the trout, however, will not be big enough for marketing before they are 1.5 years old, and some will be 2 years old before they are marketable.
At adequate intervals the ponds are emptied, cleaned from mud and the fish are graded. A common way to clean the ponds is by using a vacuum slurry tanker.
Parasitic diseases in ponds
The diseases are the same as for fry except for IPN and whirling disease. As mentioned the fish will have the IPN virus but the fish are non-symptomatic carriers. And the Myxosoma will penetrate the skin of the fish, but they can not penetrate the skeleton and therefore Myxosoma will not harm the fish. Bacterial gill disease is not so common because this disease is favoured by the crowding of fish in tanks and basins and by the dry feed which for fry consists of fine dust. This dust is often retained by the gills and here it constitutes an excellent fertile soil for bacteria.
Among the skin parasites Costia necatrix (Henneguy, 1884) is not so common as on fry. In the ponds the ciliate Trichodina sensu latiore is very common. In the summer the ciliate Ichthyophthirius multifiliis (Fouquet, 1876) is common. Trichodina and Ichthyophthirius are treated with malachite green.
Diplostomum spathaceum (Rudolphi, 1819)
(Synonyms: Diplostomulum spathaceum (Rudolphi, 1819), Diplostomum volvens (Nordmann, 1832), Proalaria spathaceum (Rudolphi, 1819). The eye fluke was formerly rather common, but now it is laid down by law that the trout farms must have wire over the ponds, so most of the gulls are kept away. Further the snails that are intermediate hosts for the fluke can be killed with formalin if the ponds are drained. There is used 1 l formalin to 40 m2 pond bottom.
Nutritional diseases
If the trout farmer has used herring as food for a long time, he has to add vitamin B1 (thyaminhydrochloride) in a water solution to the food, as herring contain an enzyme (thiaminase) which destroys vitamin B1. Dosage: 2.4 gramme B1 dissolved in 2 1 water is added to 100 kg minced fish.
Lipoid liver degeneration can be a problem in the winter, especially if the quality of the sea fish for food is bad. (Deficiency of tocopherol (vitamin E) in the diet results in rancidity due to lack of protection against oxidation). There exist no treatment for lipoid liver degeneration. But, by adding α-tocopherol acetate to the diet, lipoid liver degeneration may be prevented. For food with a fat content of 8–14% we have in Denmark used 25 mg/kg dry food and 75 mg/kg wet food.
Environmental diseases (poor water quality)
Poor water quality, in most cases, results from a high iron content in the water, which may suffocate the fish by covering the gills with iron compounds. The iron can be precipitated by aeration or by raising the pH by means of adding lime to the inlet water.
Deficiency of oxygen, resulting from repeated use of water and overstocking, also occurs. It is counteracted by aeration with atmospheric air or pure oxygen.
