CHEMICAL METHODS OF LAKE REHABILITATION AND PREPARATION OF NEW FISH FAUNA IN LAKES

by

E.V. Burmakin, D.Sc. (Biol.)
Chief of the Laboratory on Acclimatization of the State Research Institute on Lake and River Fisheries, Leningrad, U.S.S.R.

In small lakes (area less than 1,000 ha), especially in the temperate zone of the U.S.S.R., such species as perch (Perca fluviatilis), roach (Rutilus rutilus), ruff (Acerina cernua) and others are abundant, their production being of little value.

Long time experience shows that it is not so easy to improve the specific composition of fish in fresh water bodies, particularly in small lakes.

Between 1763 and 1957, 2,467 introductions of commercial species were made into 1,398 bodies of water in the U.S.S.R. with a view to acclimatizing them.

Only in 63 cases did the introduced fish survive in the new conditions and propagate to enable fishing on a commercial scale, i.e., the desired effect was achieved in one of 20 cases at an average (Burmakin, 1963).

The expansion of acclimatization work as well as the progress made in the biological techniques of introduction, which was especially great in the last decades, have produced a notable effect. The catch of fish transplanted into fresh water bodies of the U.S.S.R. doubled within the last eight years: rising from 8,450 to 16,470 t, i.e., from 4.0 to 7.9 percent of the total U.S.S.R. catch in such water bodies. According to preliminary data, in 1963 the catch of transplanted fish continued to grow and surpassed 8.5 percent of the total U.S.S.R. catch.

Water bodies in the Kazakh SSR yielded 92.7 percent of the transplanted fish catch. The share of the remaining bodies of water in the Russian, Armenian, Byelorussian, Georgian, Kirghis, Turkmen and Ukrainian Republics was only 7.3 percent, although the acclimatization measures taken in the Kazakh SSR were on a far smaller scale than in the other republics.

These good results may be accounted for by the conditions conducive to the survival of the transplanted fish in the water bodies of the Balkhash zoogeographical subarea, where, for certain historical reasons, the fish fauna has been impoverished and, therefore, the unfavorable effect due to the presence of the indigenous fish is but slight.

For instance, out of 12 fish species now in Lake Balkhash, six species are autochthons and the other six are species which have been acclimatized and are well established in the lake. These are Cyprinus carpio, Stizostedion lucioperca, Abramis brama orientalis, Acipenser nudiventris, Barbus branchicephalus and Leuciscus baicalensis.

In 1962 the acclimatized species accounted for 83 percent of the catch in Lake Balkhash and for 86 percent in Alakol Lakes.

Rather favorable conditions for the survival of transplanted species are also observed in some transcaucasian water bodies (Lakes Seven, Tabatskury, Taparavany and others) and in the Turkmen SSR (the Murgab River). The survival rate of some valuable species stocked in a number of transural lakes (Mysyash, Shartash, Tavatuy, etc.) is relatively high. Many of the lakes are periodically subject to oxygen deficiency in winter, some undesirable fishes perishing or their numbers dropping considerably. In most of the other regions of the U.S.S.R. the unfavorable effect of the indigenous fish fauna is stronger.

The results of the introduction of larvae of Coregonus spp. into the lakes of the northwestern part of the U.S.S.R. are indicative of this, repeated introductions having been conducted in the course of several decades with no substantial success.

The analysis of many years experience indicates that, in order to improve the species composition of fish in small lakes, it is necessary to observe many conditions, primarily elimination of the unfavorable factor of indigenous fish which, to some extent, are nearly always predators, competitors and disease bearers.

The control of undesirable species by reducing their number calls for taking simultaneous measures against predators and competitors. These measures should be taken annually and should produce the required selective effect. Should any of these requirements not be met, the control will prove ineffective.

Taking into account the labor-consuming character of these measures, it is more expedient to completely eradicate the undesirable species rather than to reduce their number.

Theoretically, a complete eradication of undesirable fish in small lakes may be effected by various means, such as draining the water, creation of unfavorable conditions in winter (due to a considerable oxygen deficiency), electro-shocking, shocking with explosives, chemical poisoning, etc. A panacean method of eradication of fish in small lakes has not been developed yet. Ichthyocides, though not applicable everywhere, are the most promising means.

Fish toxicants of plant origin have been known and used in fishing from time immemorial. They were frequently used in tropical and subtropical areas where there are many species of plants containing strong ichthyotoxic substances. Seeds of the liana Anamirta cocculus used as a medicinal drug and fish poison were brought from India to the Arab countries at one time.

In the middle ages, the plant was brought to Europe by the Crusaders. Anamirta cocculus and other fish toxicants are toxic to people consuming the fish poisoned with them so their use was prohibited as early as the sixteenth and seventeenth centuries (Calning, 1902).

From the middle of the last century A. cocculus began to go out of use and now the toxicant is nearly forgotten. Yet, fish poisons are still used in some areas even now.

For instance, Mexican Indians still use water extracts from the leaves, bark and roots of Gilia macombii, (Gajdusec, 1954).

Especially well known are certain tropical and subtropical plants of the Leguminosae family producing a strong toxic effect on fish due to their poisonous rotenone content. This toxicant was discovered in specimens of the Derris, Lonchocarpus, Tephrosia and other genera. Finely crushed tissues of the plants containing 4 to 5 percent rotenone serve as raw materials. The toxicant is used in the form of dust, emulsions or special preparations (Rounsefell and Everhart, 1953; Bonn and Holbert, 1961, et al.).

The 0.025 to 0.050 mg/l concentration of rotenone at a temperature no lower than 9°C is usually lethal for most fish. Lakes treated with rotenone remain toxic from 4 to 6 weeks to four months or even for one year. In the U.S.A. rotenone was first used for collecting fish for scientific purposes in the early twentieth century and also as a means of removing undesirable species for specific composition amelioration purposes approximately in the thirties.

Besides the U.S.A., rotenone is used for the same purposes in Canada, Sweden and Finland (Rounsefell and Everhart, 1953; Öquist, 1956; Bonn and Holbert, 1961, et al.).

Fish specific composition amelioration is carried out by means of selective or total eradication of undesirable species. Total eradication is far from being always effective and is effective in very small lakes only. Selective eradication is based on the different susceptibility of fish to the toxicant and on the difference in levels and areas of the ranges of the protected and undesirable species in the water body at least during some part of the year (Rounsefell and Everhart, 1953).

Rotenone and its preparations are very expensive. In the U.S.A. the chemical required treatment of a lake (area 1 ha; average depth 3 m) costs $23 to 70 (Bonn and Holbert, 1961); in Sweden, 150 Kr¹ (Svärdson, 1957). By way of comparison, we can say that the Soviet made ichthyocide - polychlorpynene (described below) required for treating the same volume of water costs only 1.5 rubles.² It is understandable why fish are eradicated with rotenone primarily in small lakes, the area usually being under 40 ha and seldom over 100 ha.

¹ 1 US$ = 5 Kr.

² 1 US$ = 90 rubles

Not only plant preparations are toxic to fish. Certain concentrations of various inorganic acids, alkalies and salts, as well as organic compounds obtained from oil, coal and other raw materials are toxic to fish. It is impossible to point out a compound to which fish would not react. Every significant change in the normal environment brings about a reaction and with still greater changes the fish die.

Numerous investigations conducted in the U.S.S.R. and other countries (Kononova and Shpet, 1947; Berezina, 1956; Burmakin, 1958; Hess and Keener, 1947: Doudoroff, Katz and Tarzwell, 1953; Mayhew, 1955; Truelle, 1957; Hooper and Grzenda, 1957; Workman and Neuchold, 1963, et al.) revealed some preparations highly toxic to fish, with their toxicity approaching that of rotenone. Toxaphene appears to be most effective among foreign-made chemical and polychlorpynene (PCLP) among Soviet-made preparations. They are similar but not identical.

The ichthyotoxic property of toxaphene was revealed quite by chance. In the late forties toxaphene was used for aerial treatment of crops and found its way into the ponds of a fish farm in Arizona, causing mass mortality of fish (Hemphil, 1954; Workman and Neuchold, 1963). Special investigations corroborated the high toxicity of toxaphene to fish. This property aroused great interest on the part of foreign fishery biologists.

Toxaphene dissolves in organic solvents but is insoluble in water. It is manufactured in the form of concentrates at prices considerably lower than rotenone. Toxaphene has been used for fishery purposes since the early fifties particularly, for complete eradication of fish (Hemphill, 1954).

It is used in the U.S.A. and Canada (Hooper and Grzenda, 1957; Fukano and Hooper, 1958, et al.).

Rotenone and toxaphene seem to be used on a rather large scale in complete or partial eradication of undesirable species, though the treated areas are not extensive. Abroad, the lakes rehabilitated with the aid of the above fish toxicants are used mainly for the culture of food fish.

Research work aimed at development of Soviet ichthyocides had been conducted in the U.S.S.R. since 1957 and revealed the ichthyotoxic properties of PCLP. It has been tested in lakes since 1959 (Burmakin, 1958, 1963). By the end of 1964 over 200 lakes with a total area of about 16,000 ha had been treated for removal of undesirable species in various parts of the U.S.S.R.

PCLP belongs to the group of chlorinated terpenes and is manufactured in the form of concentrates. It does not dissolve in water but dissolves well in organic solvents. PCLP is used as an insecticide for pest control of beet crops and lately as an ichthyocide for total eradication of fish in lakes.

PCLP is toxic not only to fish and certain invertebrates but also to man and warm-blooded animals. The latter becomes intoxicated if the preparation finds its way into their respiratory and gastrointestinal tracts or enters through the skin. PCLP is highly toxic when penetrating through the respiratory tract (Burmakin and Voitenko, 1964), and moderately toxic when introduced through the gastrointestinal tract or skin. The permissible concentration of PCLP in production rooms is 0.0002 mg/l; 0,0005 mg/l for short-term operations in the open air, and 0.20 mg/l for bodies of water.

In fishery practices PCLP concentrations of 0.05 to 0.15, sometimes up to 0.20 mg/l, are used depending on the species to be removed, the depth of the water body and water temperature.

PCLP is a stable ichthyocide. The period of detoxification, i.e., the duration of the process wherein PCLP loses its toxicity, is determined by many factors. In air the emulsified chemical is destroyed rather quickly. In the lakes at Leningrad's latitude this process lasts much longer, about one year, sometimes longer.

The rate of the detoxification of water treated with PCLP depends on the concentration of the chemical, water temperature and alkalinity, depth, extent of mixing of water masses and possibly some other factors.

The fry of fish and especially their larvae are most susceptible to toxicants. Among adults, ruff, pike and perch (Acerina cernua, Esox lucius and Perca fluviatilis) are affected most by the poison (Burmakin, 1958, 1963). Crustaceans are less susceptible than fish but various species are characterized by different resistance. For example, Daphnia magna and Eurycercus lamellatus are less resistant to PCLP than Chydorus sphaericus or Cyclops viridis (Mantelman; 1963). Most resistant are worms (Vermes) and molluscs (Mollusca), being unsusceptible to PCLP concentrations applied in fishery. Since the resistance of various species of invertebrates to PCLP is different, many fish populations in the treated lakes recuperate even before these bodies of water become suitable habitats.

Landlocked lakes are used for lake rehabilitation purposes or, in certain cases, lakes with a small outlet which can be regulated by fish screens to prevent undesirable fish from entering the body of water under treatment. Shallow lakes are preferred, with an average depth of 3 to 5 m. Very seldom are water bodies with a maximum depth of 15 to 20 m and deeper selected for chemical treatment.

The removal of indigenous fish, generally inferior species, and restocking of the water body with valuable species promotes better utilization of feed supplies. Fish parasites also usually perish with the fish, thus contributing to sanitation of the water body. But the human rehabilitating factor does not end here.

A highly valuable fish species composition creates economic prerequisites for bringing fertilizers into lakes. Favorable conditions are formed in the lakes for the development of both indigenous and transplanted species of food organisms. All these factors taken together, contribute to the alteration of the type of the lake and result in a sharp increase in its productivity.

Integrated rehabilitation measures ensuring a sharp rise in the fish productivity, a radical improvement in the fish species composition and sanitary measures aimed at removal of parasites are referred to as the chemical method of lake rehabilitation (Burmakin, 1963). The method is based on the use of chemical (PCLP and fertilizers), biological (substitution of some fish species by other ones and improvement in the composition of food organisms) and technical (preparation of the bed, construction of dikes, etc.) means. The chemical method of lake rehabilitation greatly affects their biocenoses.

Some forms are eliminated and new forms are established. Populations of certain fish species are depressed, others develop rapidly. Four successive stages may be distinguished in this process, each being characterized by its own pecularities, qualitative and quantitative changes of biocenoses included (See Table 1).

Table 1. Stages in Lake Rehabilitation Process

These stages are apparently typical of lakes under rehabilitation but they become distinct stages only with application of large doses of PCLP.

Longer observations of Lakes Okunevets and Zhemchuzhonje have been carried on since 1959 and 1960, respectively. The results obtained confirm the regular character of changes in the standing crop of food organisms in the rehabilitation process (See Table 2 and 3).

At the first stage, indigenous fish perish and the number of invertebrates decreases, especially when larger doses of ichthyocide are applied. The toxicant can completely eradicate insect larvae as well as crustaceans such as Heterocope appendiculata, Diaptomus graciloides and possibly some other species.

At the second stage the situation changes. Abiotical and biotical conditions favorable for mass development of invertebrates arise, the toxic effect of PCLP becomes weaker, the hydrochemical conditions improve, the fish “press” is absent and competition among the invertebrates slackens. This stage is favorable not only for the development of the indigenous forms that have survived but also for those that are being acclimatized. According to the data by Maximova (1963), in Lake Okunevets where zooplankton was enriched at the second stage, more than half of the crustacean species are translocated ones.

The increase in the standing crop of zooplankton and zoobenthos during the second stage is sometimes rather considerable and assumes the character of a real ecological “explosion”. For instance, according to Vladimirova (1963), a year after Lake Zhemchuzhonye had been treated with PCLP and fertilized, the standing crop of coastal crustaceans in the littoral area amounted to 195 g/m³, i.e., increased nearly 500 times, while the standing crop of the mollusc Radix pereger rose to 130.5 g/m², a 720-fold increase!

This sharp increase in the number of organisms may be undoubtedly accounted for by their high reproductive ability, the effect of mineral fertilizers, and the absence of fish that feed on them the first year after the application of PCLP.

The effect of organic fertilizers, such as decomposing fish and some sunken macrophytes, perishing due to lower water transparency, seems to be of some importance too.

Mineral fertilizers introduced into landlocked lakes accumulate in the water masses and bed soil and, after a certain period of time (the duration is not yet known), the amount of fertilizers required will apparently be reduced.

The third stage is the best time for introducing fish. By this time the water is no longer toxic to fish, rich food supplies are provided, there are no predators or competitors and sometimes no parasites.

Such conditions result in a very high survival rate of the stocking material, even when it is introduced at early stages, as larvae for instance.

As a result of the considerable stocking rate and the intensive fish feeding on food organisms, the standing crop of food diminishes greatly.

The amount of the standing crop of zooplankton and zoobenthos at the third stage is sometimes less that it was before the rehabilitation work began. (See Tables 2 and 3). This contributes to an erroneous conclusion that fish food supplies in the water body have run out. In reality, the water body continues to produce large quantities of feeds sufficient for a 6 to 10-fold and more increase in fish productivity, compared with the original productivity figures for the same body of water.

Table 2. Stages of Rehabilitation Process in Lake Okunevets
(pelagic layer biomass figures according to I.I. Mantelman and G.D. Maximova)

Table 3. Stages of Rehabilitation Process in Lake Zhemchuzhnoje (Biomass average weight figures according to T.M. Vladivirova and A.A. Salazkina)

Under the influence of the fertilizer accumulations the eutrophication of the water body continues at the fourth stage. In spite of the density of the fish population consumers of these feed supplies, an increase in standing crop is recorded.

The little experience gained from observations shows that fish productivity of lakes at the fourth stage continues to grow from 20 to 300 percent per year. The rate and duration of this increase in lakes of various types have not yet been established. One can suppose that fish productivity in landlocked lakes will grow at a faster rate, under the influence of fertilizers introduced, than in drain ponds which, though fertilized every summer, lose part of these fertilizers when they are carried away by the water.

Thus, the three first stages of the rehabilitation process, from the time PCLP is applied to introduction of the fish, last for about a year and constitute in fact the period of preparation of the body of water. The duration of the fourth stage is not known.

Lake fertilizing is carried out on a moderate scale to avoid an acute deficiency of oxygen which is harmful to fish. The lakes intended for fingerling rearing are an exception. They are fertilized lavishly with a view both to maximum fish raising productivity and to causing oxygen deficiency in winter to eradicate fish not caught during the previous autumn. This is done to prevent cannibalism, which is very often the case among fish, even “peaceful” species such as Coregonus peled.

According to data by Gulin (1963), ten yearlings of Coregonus peled per hectare eat 2 percent of released larvae of its own kind, 100 yearlings eat 18 percent of the larvae and 1,000 yearlings eat 87 percent.

Depending on the possibility of level regulation, duration of fish rearing period, fish species composition and some other factors, rehabilitated lakes can be utilized to good advantage for various fishery purposes, mainly for rearing stocking material and food fish (See Table 4).

Culturing of stocking material is more promising than that of table fish. This is because not every lake is suitable for rehabilitation and the number of lakes which can be used for this purpose is rather limited. Table fish cultured in such lakes are the final product, whereas the stocking material is a kind of semifinished product. It is restocked for further rearing from rehabilitated lakes into bigger lakes, rivers and other water bodies which cannot be treated chemically and where the grown fish is already inaccessible for the majority of ichthyophags.

Such feeding area is immeasurably larger than that of lakes suitable for rehabilitation. Besides, the fish productivity of lakes when fingerlings are raised is considerably higher, approximately double, that in the case of year-round or several-year rearing. Higher fish productivity of lakes in the case of one-summer raising is chiefly due to the possibility of intensive fertilizing which is impossible under the conditions of long-term rearing.

In constant-level lakes fingerlings are raised during one season, yearlings during one year or table fish during several years. The utilization of such lakes may be effective, provided their beds are cleaned thoroughly to enable use of seines.

Partial discharge shallow lakes are used for rearing fingerlings of the Coregonus genus and sometimes of other genera which react well to running water and shallowing.

The chief requirement for such lake utilization is regulation of the water runoff to raise the level. At the time of the fish introduction the accumulated stock of water is suddenly released by removing dike gates, which causes a mass descent of fish from the lake. The descending fingerlings are caught and their number estimated at the outlet with the aid of a fish trap.

This makes labor-consuming clearing of the lake-bed, collection of fish population and transportation to the places of release unnecessary.

Natural propagation of fish can take place in rehabilitated lakes, avoiding the necessity of stocking the water body with fry. And yet, it is undesirable because it is difficult to control the progeny numbers in the case of “wild” spawning.

To be able to control the progeny population, it is preferable to rely on periodic stocking of fish larvae, rather than on natural propagation of the stock, though in a number of cases the latter can hardly be avoided.

Table 4. Specific Features of Fish Rearing in Rehabilitated Lakes

The rehabilitated lakes are put into service gradually.

The results of the observations are as follows.

Pattern 1 was tested by rearing fingerlings of Cyprinus carpio and Coregonus peled.

Carp fingerling rearing was conducted by V.V. Erick in the Pskov region in landlocked Lake Okunevets (6 ha). Until treated with PCLP, it was an acidotrophic carp water body with the ichthyomass of 43.4 kg/ha and a fish productivity of about 14 kg/ha (Rudenko, 1962).

During 1961–63 Coregonus peled fingerlings and commercial carp were reared (Burmakin, 1965). Lime and fertilizers were introduced into the lake and the carp were fed.

The yearly average fish productivity was 200 kg/ha (Cyprinus carpio), 100 kg/ha and Coregonus peled, 100 kg/ha) but the yearly average catch was only 119 kg/ha (Coregonus peled, 98 and Cyprinus carpio, 21).

The rest, mainly Cyprinus carpio, perished due to the oxygen deficiency in the winter of 1963–64 but their number had been estimated.

In June 1964 Cyprinus carpio larvae were introduced into the lake at the rate of 7,700 ha. The fish were collected in the autumn of the same year. The average weight of the Cyprinus carpio fingerling was as much as 63 g (711 specimens), the returns were 49 percent, the fish productivity 238 kg/ha. The latter figure was approximately 17 times bigger than the initial one. The carp had not been fed. Its parasite fauna was impoverished.

Rearing of Coregonus peled fingerlings was conducted by E.P. Popov in three small landlocked lakes: Sukhlets, 2 ha; Glushak, 1 ha and Chernyshok, 1 ha.

Until treated with PCLP, they were perch water bodies, their natural characteristics being similar to those of Lake Okunevets.

In May 1964 Coregonus peled larvae were stocked into the three lakes at the uniform rate of 16,000 specimens per ha. Lake Sukhlets was treated with optimum doses of lime and fertilizers, Lake Glushak was deliberately treated with excessive doses of lime and optimum doses of fertilizers, Lake Ghernyshok was neither treated with lime, nor fertilized, for checking purposes.

In autumn of the same year the fish in the lakes were collected. The best results as one might expect, were scored in the case of Lake Sukhlets. The average weight of the Coregonus peled fingerling was as high as 124 g (400 specimens); the weight of an individual specimen reaching 174 g; the returns were 19 percent; the fish productivity, 399 kg/ha, which was approximately 28 times as much as the initial figure. In the case of Lakes Glushak and Chernyshok, the weight of the fingerling was 13 g (200 specimens) and 23 g (100 specimens); the returns, 44 and 11 percent; fish productivity, 94 and 36 kg/ha, respectively. The low percentage of the returns of Sukhlets is accounted for by the larvae of C. peled having been eaten by the yearlings of the previously stocked larvae. According to the conclusion by the ichthyopathologist U.A. Strelkov, these lakes were free from C. peled parasites.

Pattern 2 was tested by V.V. Erick who bred carp yearlings (Cyprinus carpio) in Lake Plavuschee (5 ha) in the Pskov region. The characteristics of the lake before it was treated with PCLP, the composition of its fish and fish productivity were similar to those of Lake Okunevets.

In July 1963 the fry of Cyprinus carpio were introduced into the lake at the rate of 5,300 specimens per ha. By early June 1964 the fish were collected. The average yearling weight was 40 g (427 specimens); the returns amounted to 41 percent; the fish productivity was 96 kg/ha.

The next stocking with Cyprinus carpio larvae was done in June 1964 at the rate of 7,800 specimens per ha. By autumn the average fingerling weight was 30 g.

In both cases the carp had not been fed. Its parasite fauna had been impoverished. The lake was fertilized moderately to avoid an oxygen deficiency.

Pattern 3 was tested by joint rearing of commercial Coregonus peled and commercial Cyprinus carpio in Lake Zhemchuzhnoye (69 ha) with an outlet. It is in the Leningrad region. Until treated with PCLP, there were nine fish species in the lake, perch (Perca fluviatilis), roach (Rutilus rutilus) and ruff (Acerina cernua) populations being most abundant. The ichtyomass was about 60 kg/ha.

The lake was treated with lime and fertilizers. In spring 1962 Coregonus peled larvae and Cyprinus carpio yearlings (average weight 17 g) were released into the lake. By the autumn of 1963 they had reached commercial size: the weight of the two-year-old Coregonus peled was 470 g (1, 107 specimens) and the three-year-old carp was 994 g (500 specimens). The fish were collected with seines in October 1963 and later, till the end of September 1964, with other types of fish nets. The total catch, mainly by the seines, was as much as 232 kg/ha (Coregonus peled, 157 kg and Cyprinus carpio, 75 kg).

Pattern 4 was tested by N.N. Malashkin by joint rearing of Coregonus lavaretus ludoga and Coregonus albula infra-species ladogensis fingerlings in the partial discharge Lake Zalynschick (129 ha) in the Leningrad region.

In autumn 1963 after the lake had been treated with PCLP, the outlet was barred with a dike. In spring 1964 the larvae of Coregonus lavaretus ludoga and Coregonus albula infra-species ladogensis were stocked in the lake. By autumn of the same year when the level of the lake had risen by about 40 cm, the gates were removed, causing a water level drop and the descent of the fingerlings. They rushed down the stream into the Pasha River and further on into Lake Ladoga for which they had been intended. The number of descending fry was estimated in the stream where a net trap was installed. From 24 September to 4 November the estimated number of descending fish was 1,097,000. The total weight was 27.1 t, i.e., 210 kg/ha. By 27 October the weight of the Coregonus lavaretus ludoga fingerling had reached 28 g (25 specimens); the average weight of the Coregonus albula infra-species ladogensis fingerling had reached 21 g (25 specimens).

The returns of Coregonus lavaretus ludoga amounted to 48 percent and Coregonus albula infra-species ladogensis to 35 percent.

It should be noted in conclusion that application of ichthyocides permits obtaining sufficiently precise data on the ichthyomass, the numbers and age composition of the populations in lakes (Burmakin, 1960; Burmakin and Zhakov, 1961; Rudenko, 1962; Menschutkin and Zhakov, 1964). When the numbers and age composition of the population are known, it is possible to estimate both the actual fish productivity potentialities of the water body and the potential catches. Application of ichthyocides can help approach the solution to a number of theoretical problems in ichthyology, such as establishing the regularities of intra- and interspecific relations of fish, as well as regularities governing the volume of fish biomass and the fish numbers and fish productivity characteristics of various types of lakes.

The above data shows that application of the chemical method of fish management rehabilitation of lakes permits us:

1. To utilize effectively many landlocked and small low-discharge lakes as rearing water bodies instead of ponds. This also refers to lakes unused commercially and no fishery value unless rehabilitated.

2. To cultivate combinations of valuable fish species in rehabilitated lakes and to increase their productivity. In the northwest U.S.S.R. where considerable experience has been already accumulated, fish productivity of rehabilitated lakes, without use of artificial feeds, is being increased 6 to 10 times (in some cases 28 times) and productivity reaches 0.2 t/ha (sometimes 0.4 t/ha) with the one-summer rearing cycle and 1 t/ha when fish are raised throughout the year. These figures do not seem to be limiting.

3. To raise the survival rate of fish larvae introduced into rehabilitated lakes by about 40 to 50 percent. In similar but not rehabilitated lakes of the northwest U.S.S.R., larvae of introduced fish generally perish completely.

4. To cultivate fish free from parasites or with an impoverished parasite fauna.

5. To solve some theoretical problems in ichthyology, to disclose the regularities governing the amount of fish biomass and the numbers of fish and fish productivity characteristics of various types of lakes.

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