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TROUT HUSBANDRY MANAGEMENT IN THE U.S.S.R.

by

V.V. Lavrovsky
Senior Scientific Worker,
Candidate of Biological Science

Along with the rapid growth of pond carp culture in the U.S.S.R. there are all the preconditions for the development of trout culture.

Until recently no special attention has been given to increasing trout production as the requirements for the valuable salmonids were mainly satisfied by catches from natural waters.

During recent years the demand for trout has increased, particularly in areas of health resorts and big industrial centers.

At present there are 27 trout farms in operation, 3 in the Russian Soviet Federated Socialist Republic, 17 in the Ukranian Republic, 1 in the Belorussian Republic, 3 in the Armenian Republic, 1 in the Georgian Republic, 1 in the Kirghiz Republic and 1 in the Estonian Republic.

The total water surface of these pond fish farms is about 27 ha. The reproductive capacity in terms of incubation exceeds 70 million eggs (Sadlaev, 1964).

There are three main types of fish farms:

  1. Fish farms dealing with trout reproduction in natural water bodies and hatcheries.

  2. Food fish farms producing commercial trout.

  3. Pond fish farms where a small amount of trout is bred in carp ponds as auxiliary fish.

Fish farms specializing in production of food fish can at the same time also rear young trout for other fish farms as well as for stocking natural water bodies.

Trout Culture in Natural Water Bodies

During the postwar years some lakes in the Karelian isthmus area, rivers in the transcarpathian Ukraine and other water bodies have been stocked with trout (mostly rainbow trout Salmo g. irideus Gibb.).

Trout yearlings and two-year-olds have been introduced into Karelian lakes inhabited mainly by Perca fluviatilis L. and Rutilus rutilus. In such lakes three-year-old trout average 400 g and four-year-olds 1,000 g. Rainbow trout start feeding on fish from the age of three years. Fouryear-olds live exclusively on fish, mostly Rutilus rutilus and Perca fluviatilis L. (Gorbunova, et al., 1965).

Water reservoirs of the Crimea are also favorable for rainbow trout habitation. It is worth mentioning that in Crimean conditions rainbow trout 6 to 7 years old reach a weight of 7 to 8 kg, whereas their weight in other water bodies usually does not exceed 3 to 4 kg.

Every year the State Crimean Fishery Reserve stocks various water bodies with tens of thousands of trout fry.

Since 1963 there has been trout stocking in the storage lakes of the Ternopol area of the Ukraine, Sevastopol and Simpheropol. Spawning of rainbow trout has been observed in a river discharging into the Sevastopol reservoir. Fingerlings have grown here to a size of 9 to 10 cm and reached a weight of 20 to 25 g. (Melnikov, Chaplina, 1963; Zhuravlev, 1965). It is interesting that in southern districts of the U.S.S.R. (the Crimea and transcaucasian zone) the spawning times of rainbow trout have changed considerably. In central and northwestern districts of the country the fish spawn in spring (March, April, May), in southern districts spawning takes place in December, January and February.

The artificial propagation of trout has increased in scope in Sevan Lake, the biggest high altitude lake in the Soviet Union. Sevan Lake is at an elevation of 1,916 meters in the Armenian Republic. Its water surface area is 1,416 km2, the length being 75 km and the width 56 km. Its maximum depth is 98.7 m, the average depth being 41.3 m (before water decrease). This lake is the richest in the world in trout stock.

The annual trout catch here amounts to about 3,000 centners of trout of the four endemic races: Salmo ischchan typicus Kessler, Salmo ischchan gegarkuni Kessler, Salmo ischchan aestivalis Fortunatov and Salmo ischchan Danilewskii Jakovlev. Fish culture has been practised in Sevan Lake since 1924. At present all races of trout but Salmo ischchan Danilewskii Jakovlev are the objects of breeding, as well as: Varicorhinus capoeta sevangi Filippi, Coregonus lavaretus maraenoides Bolshakov and Coregonus lavaretus ladoga Poljakow which are acclimatized here.

A good sized stock of Coregonus lavaretus makes it possible to transfer their fertilized eggs for hatching into different water bodies of the country for the purpose of acclimatization.

In 1940 Sevan Lake waters started to be used for power and irrigation. That negatively affected natural propagation of trout. Owing to dropping of the lake water level, the majority of spawning grounds were drained.

Trout of different races used to spawn almost all year round (9 to 10 months) from October till July (Pavlov, 1951) both in the lake itself and in the rivers entering it. Because of the change in the lake's hydrologic regime, measures were taken to reproduce trout stocks by means of artificial propagation.

Special attention has been given to improving the operating efficiency of the three Sevan hatcheries. Formerly they used to stock the lake and its tributaries with trout larvae as soon as they reached the age of 20 to 30 days. Now a new method of rearing young trout in concrete tanks till they are 5 to 6 months old has been developed and put into use.

While in the tanks fingerlings reach a weight of 0.8 to 3.0 g.

The quantity of eggs to be incubated was increased up to 70 million. In 1963 more than 46 million larvae and 2 million fingerlings of Salmo ischchan gegarkuni Kessler and Salmo ischchan aestivalis Fortunatov (Rizkov, Gzranian, 1964) were introduced into the rivers of the Sevan basin. At present the stock of Salmo ischchan aestivalis Fortunatov has reached the same amount as during the period before the decrease of water and the stock of Salmo ischchan gegarkuni Kessler has even exceeded it.

The biotechnique of rearing fry of Salmo ischchan aestivalis Fortunatov, which is the most valuable race of the Sevan trout, has been also worked out. The fry is being reared in ponds and concrete reservoirs. During the period of 5 to 6 months while taking natural food, the young trout grow to a weight of 3 to 3.5 g. Due to the measures taken, the hatcheries provide 80 percent of the total trout catch.

To collect eggs, water in the river where the spawners run is dammed, the lower reaches of the river being provided with solid screens with traps.

Eggs are fertilized by the Russian “dry” method and, after swelling, are transferred to incubators. Incubation can be performed in a Williamson apparatus holding 400,000 to 1 million eggs.

Due to good care and high water quality, the loss of eggs does not exceed 8 to 10 percent.

Immediately after hatching, larvae are transplanted into rectangular concrete tanks with 7 to 10 m2 floor space.

Larvae are kept here for 3 to 4 weeks. The tanks are stocked at the rate of 5,000 to 10,000 per m2. The majority of the larvae, at the age of 20 to 30 days, are introduced into the Sevan tributaries in quantities to accord with their natural fecundity. The commercial return of larvae is considered to be 1.3 to 1.7 percent.

The larvae left in the concrete reservoirs and ponds for rearing are at first fed live food (Daphnia magna, Daphnia pulex, etc. and Enchytraeus), which is grown in special structures at the hatcheries. Later a combined diet also is started which will be the object of further discussion.

While rearing the young in ponds, considerable effort is required to fight filamentous seaweeds (Spirogyra, Mougeotia, etc.).

The Sevan trout is a promising subject for acclimatization in other water bodies.

Stocking of Issyk Kul Lake with Salmo ischchan gegarkuni Kessler in 1930 and 1936 serves as a good example of such acclimatization.

Issyk Kul Lake lies at an elevation of 1,621 m in the northeastern part of the Kirghiz Republic. Its water surface area is 6,206 km2, its length is 184 km, and its width is 60 km. Its depth averages 279 m and the maximum depth is 702 m.

Fertilized eggs (ova) from Sevan Lake were given additional completing treatment in the incubators in one of the rivers. Over a million fry of 19 to 24 mm length at the age of 22 to 45 days were introduced into Issyk Kul Lake (Luzhin, 1956).

Fingerlings were found in 1947 in the rivers discharging into the lake.

Under the influence of the new environment, the biology and morphology of trout in Issyk Kul have considerably changed. This fact made it possible for ichthyologists (Luzhin, 1956) to refer to the fish as a new subspecies, Salmo ischchan issykogegarkuni Luzhin. The author singles out the following considerable changes in the biology of the trout introduced in Issyk Kul.

The growth rate of the trout increased considerably. They grow to a size of 89 cm and reach 10 kg and even 12 kg in average weight, whereas in Sevan Lake their length goes up only to 60 cm, the weight being 4 kg. The fecundity has increased five or six times (the average production is 7,460 eggs).

The Issyk Kul Lake trout mature to 1 to 2 years earlier than those of Sevan Lake.

Males attain sexual maturity at the age of three years (2+), females at five years (4+).

According to data by Fortunatov (1927), the Sevan gegarkuni usually mature at the age of six years.

Their migration into the rivers for spawning begins in the second half of November and continues until the end of February, so it is one month later than in Sevan.

The character of adult trout feeding has changed completely. In Sevan 60 to 100 percent of the food of adult gegarkuni consists of Gammaridae and only large size specimens take fish food at times. But in Issyk Kul trout have become typical predators. During the fattening period the main component of their food is fish (81.8 percent), mainly of genus Nemachilus.

The basic food of the young is Gammaridae, Ephemeridae and Tendipedidae larvae.

At the present time there is a hatchery in operation on Issyk Kul Lake. As on Sevan Lake, it is supposed that trout stock will be reproduced by artificial propagation. The natural propagation of gegarkuni will play a secondary role. It is also proposed to stock pond fisheries with young trout from Issyk Kul for rearing to commercial weight.

Trout Rearing in Pond Fisheries

Salmo g. irideus Gibb. (rainbow trout) are the main object of commercial pond fish farming. Salmo trutta m. fario and Salvelinus fontinalis are reared in small amounts and only in some pond fish farms.

The biotechnique of intensive trout pond farm management differs only slightly from that used in other countries.

As a rule, trout hatcheries are located by springs, which provide a natural water supply for trout rearing.

The oxygen content should be no less than 5 mg/l, pH 7.0 to 7.2; oxidation should not exceed 11 mg O2/l; water should be hard, about 15 German degrees; the ferrous oxide content should not exceed 1 to 2 mg/l.

Water containing hydrogen sulphide or other noxious gases such as methane is, of course, entirely unsuited for trout culture. So is that mixed with industrial wastes.

A common type trout farm complex usually comprises the following set of structures:

  1. Spawner stock (4+ to 6+) ponds and stock replenishment (2+ to 3+) ponds.

  2. Incubator shop with water filter, where water is cleared of suspended materials.

  3. Larvae reservoirs or series.

  4. Rearing ponds for fingerlings, mostly used for wintering the young.

  5. Feeding ponds for rearing of food fish.

  6. Live food shop where Daphnia magna, D. pulex and Enchytraeus albidus are cultured.

  7. Feed preparing house with a storehouse for dry food.

  8. Cold storage for perishable fish food (fish and slaughterhouse refuse).

  9. Dwelling houses for the staff and service facilities.

Various ponds, such as spawning, rearing, feeding, etc. are usually rectangular, the length to width ratio being 1 to 5, or 1 to 10.

The water area of each of the ponds should be within the following limits: spawn ponds, 7 percent of the total surface of the fish farm; rearing ponds, 18 percent, and feeding ponds, 65 percent. The rest of the fish farm area should be occupied by other service premises (Martishev, 1958).

Usually earthen ponds are used but on all modern fish farms the bottom and walls of the ponds are made of concrete or faced with reinforced concrete slabs. This makes it possible to ensure good sanitary conditions.

Surface area of the reservoirs of different types ranges from 50 to 1,000 m2, their depth ranging from 1.0 to 2.0 m.

An intensive trout fishery requires an abundance of pure water. The production of 100 kg of commercial fish requires a water consumption rate of about 1 l/sec. One hectare of pond surface should be fed 500 to 1,000 1 of water per second.

Spawner stock of rainbow trout commonly consists of 3 to 7 year old males and females.

The common belief has been that it is best to obtain eggs from 4 to 7 year old females and to obtain sperm from 3 to 6 year old males.

Old spawners should be rejected as defective if the eggs are slightly colored and easily broken or the sperm is thin and has a bluish tint.

Eggs of good quality should be yellow-orange or bright orange, their size when hardened being about 5 mm in diameter. They should have a regular ball-shaped form. For fertilization of eggs, males secreting white-yellowish milt of creamy consistency should be used.

To obtain high quality milt and eggs, the reproduction and spawner stock are kept in running water ponds. The rate of stocking is 3 to 4 specimens for 10 m2. They are fed twice a day with fresh slaughterhouse refuse (spleen, lungs, etc., cut in small pieces) or with fresh lean trash fish fed whole or diced. Daily diet is at a rate of 2 to 5 percent of the fish body weight.

To avoid degeneration of trout resulting from close inbreeding, the spawner stock or fertilized eggs are periodically brought from other fisheries of the country as well as from abroad. Some fish farms maintain their spawner stock, which was bred from the eggs brought from Czechoslovakia and Denmark. In this case inbreeding is practised.

It has been already mentioned that the spawning times of rainbow trout are different for various climatic zones of the U.S.S.R. A few days before spawning it is necessary to stop feeding the adult fish. Males and females should be placed in separate ponds or tanks with running water. When the water temperature becomes suitable for spawning, females are examined for the ripeness of sexual products frequently, the procedure taking place daily or every other day.

If a slight pressure on the belly makes eggs flow freely from the genital vent, the fish is ripe and should be placed into a separate tank with pure water. A small number of ripe males are placed into another tank.

The eggs stripped from 4 to 5 females are placed into a clean enamel pan to be fertilized with the milt of 3 to 4 males.

Trout males can be stripped again in 5 to 7 days.

Impregnated eggs should be washed free from milt and then set aside to harden for 2 to 4 hours in clean and periodically changed water. Then the eggs are counted by either the volumetric or displacement method and after that placed in incubation apparatus. Trout hatcheries use Williamson incubators, trough type incubators, Schuster, California apparatus and Ropsha apparatus.

Their capacity is different. The Schuster, Ropsha and trough apparatus not only incubated eggs but also keep larvae to develop.

In the case of the Williamson apparatus the eggs, before hatching, are transferred on meshed frames into the larvae nursery. There the larvae rest for some time and then switch to active feeding when they begin to swim up to the surface of the water in search of food. Water consumption rate in this apparatus should be between 0.15 to 0.20 1/min. per 1,000 eggs.

The length of the incubation period ranges from 30 to 60 days according to the water temperature (330 to 360 degree days), 8 to 9°C being considered the optimal temperature.

The eggs must be handled with extreme care as they are highly susceptible to mechanical influence (shock, impact and light).

Dead eggs are removed every 2 or 3 days. During the so-called “critical” period it is the practice at hatcheries to leave eggs undisturbed until this delicate stage has passed (cleavage, gastrulation, “eyed stage” when eyes become pigmented, and the period of formation of blastophore).

To prevent fungus (Saprolegnia) infection, in case of extreme necessity the routine treatment of eggs with malachite green is used at the 1 to 100,000 rate of concentration.

If eggs are covered with a layer of silt they should be showered during the period of low susceptibility. When first hatched larvae or sac fry are allowed to rest on the bottom of the apparatus for 8 to 15 days, depending on the temperature.

The water circulation rate during the hatching period should be doubled.

As soon as they migrate upward, they are gradually conditioned to light by decreasing the covered surface of the apparatus.

After the yolk sac appears to have been absorbed by two thirds of its initial volume, the larvae are offered food consisting of daphnia ground into very fine particles. Chicken egg yolks, omelette, followed by “screened” spleen with pellicle removed are also used to feed larvae.

Food should be given in abundance (20 to 30 percent of the body weight), 5,000 to 15,000 larvae (sac fry) being planted on 1 m2 of the apparatus or larvae nursery.

Gradually as the sac fry grow into advanced fry, the population density decreases to 5,000 fish per m2. Larvae which are active in search of food and weigh 100 to 200 mg are transferred into rearing ponds.

According to the routine adopted at different hatcheries, fingerlings are reared either only in ponds or first in tanks and later in ponds.

Fry are planted in tanks at the rate of 2,000 specimens per m2. During 1.5 to 2 months they grow to a weight of 2 to 3 g, the optimal water temperature being 15 to 16°C. Water consumption, depending on the fry weight, varies from 0.3 to 0.6 l/min. per 1,000 fry.

A shed covers the tank from direct sunlight.

Running water tanks are protected with meshes to prevent fry jumping out of water.

The trout fry are fed frequently, every 1.5 to 2 hours during the daylight period. The most suitable food during this period is spleen with pellicle removed and spleen based mixed diets, for instance, a diet comprising the following percentage components:

spleen, 85;
rye meal, 12;
hydrolized yeast, 2;
cod-liver oil, 1

Vitamins and antibiotics may also be added.

At present a dry mixed diet is widely used to facilitate preservation of initial diet components and to exclude perishables from them.

The KPT-III formula, for instance, includes the following components (per 1 kg):

blood meal - 250 g
fish meal - 150 g
silkworm chrysalis meal - 150 g
dried seaweed (Fucus) - 170 g
hydrolized yeast - 40 g
cod-liver oil - 10 g
vitamin A - 15,000 IU
vitamin D - 7,500 IU
antibiotics - 150 g
water - 230 g

In the KPT-III diet a mixture of antibiotics is used consisting of penicillin, biomicine and furasolidon. (Malikov and Kotov, 1961; Rimsh, 1963).

Mixed diets are continually improved. Finely ground zooplankton, ground gammaridae and molluscs are added the diet of the fry.

Food is prepared on feeding racks (metallic net or tiling) located in a tank close to the water flow at the rate of one feeding rack per 1,000 fish. The daily diet ranges from 15 to 20 percent of the fish body weight, depending on water temperature, food composition and the state of fry. Food consumption depends on the rate of eating.

Rearing of fry is labor-consuming and tedious work. Every day ponds are cleaned of excrements and food remnants. Periodically trout fry are placed into vacant tanks with a table salt solution for disease prevention while residential ponds are thoroughly disinfected with a solution of lime chloride or potassium manganate.

During transplantation fry are partially sorted and large-size fish are transferred into separate tanks in order to prevent cannibalism.

Later fingerlings are grown in ditch-like ponds, the stocking rate being 25 fish weighing 2 to 3 g each per m2 of the pond water surface.

In fisheries where the water quality is good the stocking rate may be increased. Every 1.5 to 2 months fingerlings are sorted into two or three size lots and planted in different ponds. The fingerling is still fed mixed diets composed mainly of spleen but now also introduced into the diet is forced-fish (5 to 10 percent), fish meal and bran.

The advanced fry are conditioned to get food “out of hands”, the latter being distributed over the surface of the pond.

The daily diet for fingerlings ranges between 7 to 15 percent of the body weight.

Bright lamps or light traps are sometimes installed over the ponds to attract “flying food” or insects in the night time. Dams are planted with trees to protect the water surface from direct sunlight.

On some fish farms in the northwestern regions of our country, trout fingerlings are reared only in ponds which are stocked with 25 to 30 day-old larvae, averaging 0.1 to 0.2 g.

In this case, at the beginning of the rearing period, larvae are offered no artificial feeding. These rearing ponds have a larger surface (up to 0.2 to 0.5 ha).

To preserve zooplankton as a stable feeding base, water circulation is brought down to 10 to 12 l/sec. per ha.

Ponds are fertilized with horse manure at the rate of 57 t/ha, the fertilizer being placed along the water edge of the pond. No water circulation should be started in the pond before stocking, as slack water is favorable for the development of zooplankton.

Only in case of oxygen deficiency and at a water temperature of 16°C should water circulation take place. The stocking rate in this case should be 5 to 6 fish per m2. (Gracheva, 1955).

At the end of July and beginning of August the young grow enough to be satisfied with natural food only and are fed in the routine manner, spleen constituting the base of their diet. Feeding troughs are placed at a depth of 50 to 60 cm, close to water flow in places where fry would concentrate.

As trout get accustomed to take food, the number of feedings should be increased from one to two or three feedings a day.

Food consumption is regulated by the rate of eating, the number of feeding places being increased accordingly.

Rearing ponds are fished in late October and early November when the water temperature is below 6°C. The trout caught are immediately placed into tanks with fresh clean water. After having been counted and sorted, they are transferred into ponds for wintering.

In central and southern areas of our country trout stay in rearing ponds all the year round, as the water temperature usually does not fall below 8 to 9°C. As has already been mentioned, fingerlings stay for winter in rearing ponds.

In the central and northern districts of our country yearlings spend the winter in small ditch ponds with running water, their depth being no less than 1.5 m. Water is commonly supplied from natural springs to prevent a temperature fall below +3°C. This allows intensitive trout feeding in winter.

The stocking rate of fingerlings is 50 to 100 specimens per m2 and more, depending on water quality and size of fingerlings. Before being planted for winter, they are sorted into three lots of approximately the same size: large size, 10 to 15 cm; medium size, 8 to 10 cm, and small size, 5 to 8 cm.

Size standards for each group may be different.

Trout yearlings usually reach a weight of 20 to 25 g.

The amount of food and the number of feedings are set in accordance with the rate of eating. If water temperature is 5 to 6°C, trout are fed twice a day. If the temperature goes down to 3 to 4°C, feeding should take place once a day or every other day.

Sometimes trout are left for winter in conventional type carp ponds with a very low water temperature (below 1.0°C).

Trout continue feeding a little even at a water temperature of 0.3°C. The number of yearlings that survive after wintering is about 70 percent (Suhoverhov and Pisarenkova, 1963).

By the end of 1.5 to 2 years rainbow trout attain marketable weight.

The optimal temperature for rearing too-year-old is 16 to 20°C.

The stocking rate for yearlings to be reared in ponds with adequate water depends on the rate of water flow. Water flow being 100 to 200 l/sec. per ha, the stocking rate would be 10 to 25 yearlings per m2. If the flow equals 500 to 600 l/sec. per ha, the stocking rate may amount to 100 and even more yearlings per m2.

Two-year-olds are fed at water temperatures of 5 to 10°C once or twice a day. At a temperature of 16 to 18°C they are fed 3 to 4 times a day at fixed hours. Trout soon get conditioned to regular feeding and when the time comes they gather near feeding places.

The average amount of the daily diet approximates 5 percent of the fish body weight. It varies, depending on water temperature, from 2 to 10 percent.

Usually two-year-olds are fed a pulpy mixed diet, the food being distributed over a wide area of pond surface.

On some trout farms granulated food is prepared by running it through powerful grinders. In this case the food is given to the fish in granulated form, each grain being 3 to 4 mm in diameter and 3 to 10 mm in length.

This method ensures lower loss of water-soluble components and minimizes the waste of valuable food constituents.

To provide for 1 kg of trout weight gain, it is necessary to use 5 to 7 kg of pulpy food, whereas while using granulated fat food only 3 to 4 kg is needed.

Mixed diet composition may be different on different fish farms. It depends on the availability of animal food used as diet components, such as fish, slaughterhouse refuse etc.

Examples of formulated mixed diets:

I

Slaughterhouse refuse-50  percent
Fish meal-15  percent
Silkworm chrysalis-  5  percent
Bran-20  percent
Sweeped-off meal-  5  percent
Hydrolized yeast-  5  percent

II

Fresh fish-60  percent
Meat and bone meal-10  percent
Blood meal (albumen)-  5  percent
Silkworm chrysalis-50  percent
Bran-15  percent
Hydrolized yeast-  5  percent

Green plants may also be used as additional components of the diet, (they include stinging nettle and lucerna, making up to 10 percent of the diet), as well as synthetic vitamins, microelements and antibiotics.

Trout which have attained their commercial weight are placed into small troughs and feeding is stopped two days before they are sold. The consumer gets only live trout.

Fish productivity of trout farms is 30 to 40 t/ha, depending on the quantity and quality of water. The results obtained under laboratory conditions are better.

TABLE 3. AVERAGE MONTHLY NUMBER OF CRUSTACEANS AND TOTAL ZOOPLANKTON STANDING CROP IN THE VOLGOGRADSKOYE RESERVOIR IN 1960 (VIUSHKOVA, 1962)

ZoneJuneJulyAugustSeptember
1,000 indiv/m3Weight g/m31,000 indiv/m3Weight g/m31,000 indiv/m3Weight g/m31,000 indiv/m3Weight g/m3
  up to up to up to up to
Upper102.5  4.219.41.934.52.39  3.90.44
Intermediate60.11.341.92.586.53.1121.10.52
Lower42.81.637.54.943.21.2011.90.26
         

TABLE 4. WATER LEVEL AND AVERAGE ZOOPLANKTON STANDING CROP IN THE TSYMLYANSKOYE RESERVOIR IN DIFFERENT YEARS

Year19551956195719581959196019611962
Water level, m35.335.635.135.633.534.533.532.8
Standing crop g/m32.22.51.72.81.11.21.2  1.0

Trout Rearing in Carp Ponds

At some pond fisheries rainbow trout are reared as auxiliary fish in ponds stocked with two-year-old carp.

The stocking material, trout yearlings, is bought at specialized trout hatcheries. There is a common belief that trout may inhabit only running waters but they grew fast in carp ponds where there was abundant natural food. Ponds with a surface area of 10 to 20 ha and depths of 2.5 to 5 m are quite suitable for two-year-old trout. Oxygen content in this case should be no less than 2.5 to 3 mg O2/l. Two-year-olds wipe out trash fish (Leucaspius delineatus, Alburnus alburnus) and other small fish in ponds. The basic trout food consists of aquatic insect larvae, beetles and mosquito larvae. They also feed on frogs and tadpoles with the trout food being exclusively natural, l ha of pond surface yields 20 to 35 kg of these valuable fish, without any food expense. Trout do not compete with carp in feeding and even contribute to the increase of pond productivity.

It is planned to increase trout production in the U.S.S.R. in the near future.

Some big trout farms are being designed and constructed on the Black Sea coast, in the Baltic republics and in the republics of central Asia. The distinguishing feature of the farms being built and remodeled is their high capacity of 100 to 200 t of commercial trout per year.

BIBLIOGRAPHY

Arnoldi, L.B., 1929 Materials on the study of the capacity of Sevan Lake. Transactions of the Sevan Lake Station v.II. part 2.

Dadikian, M.G., 1955 Feeding of the Sevan trout. Papers of the Sevan Hydrobiologic Station, v. 14.

Eggert, V.B., 1963 Trout culture in the Latvian S.S.R. Fish-Culture and Fishing No. 6

Fortunatov, M.A., 1927 Fishing on Sevan Lake. Transactions of the Sevan Lake Station, v.1, part I.

Gorbunova, L., U. Dmitrienko and A. Zabolotsky, 1965 Rainbow trout in Karelian Lakes. Fish-Culture and Fishing No.1.

Gracheva, M.N., 1955 The biological factor in rainbow trout rearing. Summary of the thesis. M.

Lavrovsky, V.V., 1963 Progagation and rearing of pond trout. Manuscript All-Union Research Institute of Pond Fisheries.

Luzhin, B.P., 1956 The Issyk Kul trout gegarkuni. Frunse.

Malikova, E.M. and N.I. Kotova, 1961 Application of antibiotics in artificial rearing of salmon fry. Transactions of the Latvian Research Institute of Fisheries, VIII, Riga.

Martishev, F.G., 1958 Pond fish culture. Published “Soviet Science”.

Melnikov, G.B. and A.M. Chaplina, 1963 On planting of (Salmo ischchan Kessler) Sevan trout into Crimean water-reservoirs. Scientific Reports of the Higher School, Biological Series No.3,

Pavlov, P.I., 1951 Materials on biology of the Sevan trout. Transactions of the Sevan Hydrobiological Station, v.XII.

Rimsh, E., 1963 The KPT mixed diet in trout culture. Fish-Culture and Fishing No.1,

Ryzhkov, L. and P. Gzranian, 1964 Artificial propagation of the Sevan trout. Fish-Culture and Fishing No.5

Sadlaev, K.A., 1964 The prospects of trout culture. Fish-Culture and Fishing No.1.

Sukhoverkhov, F.M. and A.S. Pisarenkova, 1963 The experience of rearing of rainbow trout yearlings and two-year-olds in carp ponds. Transactions of All-Union Scientific Institute of Pond River Fisheries. VII.


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