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FISHERIES RESEARCH BOARD OF CANADA Translation Series, No. 414

BASIC FACTORS IN THE FLUCTUATIONS OF THE STOCKS OF AND LESSER FISHES IN THE SEA OF AZOV

By E. G. Boiko Azov Research Institute [AzNIIRKh]

Original Title: Osnovnye faktory kolebaniiâ zapasa chastikovykh i osetrovykh ryb azovskogo morn

From: Trudy Soveshchanii Ikhtiologicheskoi Komissii Akademii Nauk SSSR, No. 13, pp. 147-157. 1961. [Soveshchanie po dinamike chislennosti ryb, 1960.3

Preliminary translation by W. E. Ricker

Distributed by the Fisheries Research Board of Canada, Biological Station, Nanaimo, B. C.

January, 1963 The principal objective of this article is to demonstrate from factual material the significance of the most important factors that affect the fluc- tuations in numbers of Azov sturgeons and other commercial fishesl. Hence I will not dwell on a consideration of the present level of the stocks of these fishes or on their future prospects.

What is basically responsible for fluctuations in the commercial stocks of Azov anadromous fishes? (By "commercial stock" I mean a stock of fish of sizes and ages suitable for capture.)

Up to very recent times the commercial stocks of these fishes was utilized very intensively (65-70% per year of the stock of the non-acipenserid fishes, or even more, have been captured). In a situation of this sort fluc- tuations in the stocks have been determined by the recruitment: when the was using abundant year-classes, the stock was large; when the scarce year-classes of the "improductive years" came along, the fishery declined, anedeclined the farther, the weaker were the year-classes recruited and the more frequently such year-classes followsd one another.

It is generally known that fluctuations in abundance of individual year-classes of Azov fishes are very large (Fig. 1 and 2). Special interest attaches to the extremely rich year-classes of the "very productive" years. Such year-classes provide the basis for a good fishery for a long period of time. For example the 1932 and 1933 year-classes of the Kuban zander were cap- tured in large numbers for 8-9 years, and the 1939 year-class of the Don zander for 11-12 years, and even at the age of 14-15 years it stood out noticeably among the other year-classes. Hence it is among the abundant year-classes that we will be able most clearly to recognize the role of the basic--governing--factors determining the fluctuations in numbers of fishes. Unfortunately such year- classes are very rare.

Data collected over many years lead to the conclusion that conditions for reproduction are of decisive importance in determining fluctuations in numbers. Conditions for the survival of the offspring in the sea play a secondary, sub- ordinate, role in this regard. There is a direct relationship betwsen the com- mercial yield [return - vozvrat] from a year-class and its abundance when young. As a rule, the greater the abundance of the fingerlings migrating down to the sea, the greater is the commercial yield of any year-class (Fig. 4). [page là2] Prediction of the size of the commercial stock [in a given year] simply amounts to an evaluation of the probable yield to the fishery of the year-classes which are being newly recruited to the stock, and of the carryover of the year-classes which have previously been subject to the fish- ery. A preliminary evaluation of the probable yield to the fishery of each year- class is made on the basis of a census of the fingerlings migrating downstream to the sea (using lampara nets in August, and trawls in October and April). Subsequently this evaluation is adjusted year by year [on the basis of its observed contribution to the fishery, presumably]. For an abundant year-class,

. 1["Sturgeons" include the (Huso huso) as well as of Acipenser; the "other fishes" or "lesser fishes" chastikovye ryby - include zànder, bream,roach, shad, , Clupeonella, etc.] - 2 - observations are made throughout the whole course of its life, from the time of its birth until it has been fully caught out by the fishery.

Correlations between the abundance of offspring and the abundance of spawners among the fishes under consideration have not been observed in the great majority of cases (Fig. 1-3). This factor as a rule is not a limiting one. The number of spawners, provided it does not deviate too much to one side or the other, commonly does not determine the abundance of the offspring. For example, the tremendous production of young zanders in Tsimllinreservoir in 1952 (the first year of the reservoirls existence) was obtained when there was not more than one zander for every 5 to 10 hectares of spaWning area. The very abundant production of zanders in Veselov reservoir was obtained from a comparatively small number of mature zanders in this reservoir; and in more recent times, as the numbers of mature zanders have increased, the production of young has decreased.

Similarly since the regulation of the flow of the Don River, the production of young by Don shad has been the less, the greater the number of shad which have reached the places of spawning. An inverse relationship between production of young and abundance of spawners on the spawning grounds has been observed also aneng the Don River chekhon [Pelecus cultratus] since the Don has been regulated.

An analysis of material over 30 years shows that the magnitude of the production of young in a majority of cases is basically determined by the effect- iveness of reproduction. As an index of effectiveness of reproduction I use the magnitude of the number of offspring (the yield to the fishery of the year-class or else the abundance of the downstream-migrating fingerlings, which amount to the same thing) per spawner. The index of effectiveness of reproduction of the zander, for example, varies more than a hundredfold. The ratio of the recruitment of zanders (as commercial yield) to the abundance of the parental stock which produced it [page 150] is in certain cases up to 23:1, average 3.5:1, not com- monly 1:1, and sometimes as low as 0.2:1.

A high level of effectiveness of natural reproduction is a rare phenomenon. Usually it is low; it suffices for the existence of the species in the absence of a fishery or when the fishery is light, but it is not sufficient to support a high level of abundance of the species when there is an intensive fishery. In a great majority of cases a high level of effectiveness of repro- duction for the Azov zander was observed only when food conditions were good and consequently there was good survival of zander larvae during the time of their changeover to active feeding. When there is a large biomass of zooplankton suitable as food for larvae on the zander spawning grounds, and when there are few other plankton-eating fishes there (and in fact few zander larvae there), the recruitment of young zanders is always large. When the plankton during the period the zander larvae are hatching is insufficient on the spawning grounds, or when the spawning grounds are crowded with other plankton-eating fishes, recruitment of zanders is always inconsiderable.

For the Kuban zander, the favourable conditions for reproduction just indicated exist in estuaries that have just become fresh, while for the Don zander it occurs when the fish on the Don flood-plain in the warm flood waters. In all other cases the effectiveness of reproduction of the zander is always very poor: namely, when reproduction of the Kuban zander takes place in continuously fresh Kuban estuaries where plankton is scarce and other fishes are very numerous, or for the spawning of the Don zander in years of small floods when it spawns not on the flood-plain, but in the river channel and other permanent bodies of water--shallows, flood-plain lakes--or in the Don delta, all of which have a large population of other fishes. The increase in the progeny of zander following a freshening of con- siderable areas of the Kuban estuaries which previously were continuously strongly saline, or when the Don valley flood-plains are covered with water, is caused not by any increase in spawning area but solely by an increase in the effectiveness of spawning. The increase in effectiveness in these cases results from the qualitative difference in the spawning areas: highly productive, rich in plankton, and poor in other fishes. When the Kuban estuaries have been in a freshened condition for a long time they lose their favourable characteristics, and zander spawning then becomes ineffective. For the last decade and a half the Kuban estuaries have been fresh, but nevertheless the recruitment of zanders from them has been very inconsiderable. The extremely high effectiveness of reproduction of zanders in the Tsimllân reservoir in the first year of its existence is also to be explained by the fact that under conditions prevailing then this reservoir resembled the Don flood-plains. Early flooding, a warm early spring and absence of plankton - eating fishes resulted in the development of an abundant spring plankton and con- sequently a high effectiveness of zander spawning in the reservoir in 1952. Large biomasses of spring zooplankton and the relative scarcity of plankton-eating fish in Veselov reservoir after the Kuban water was run into it through the Nevinnomyssky Canal was responsible for the very high effectiveness for zander spawning in this reservoir. Once the Don River bream used to spawn in the same places as the zander, and at the same time. Hence the fluctuations in production of young bream in general were synchronized with fluctuations in production of young [page 151] zanders (Fig. 4). In a general way too the fluctuations in production of Kuban roach [taran--Rutilus rutilus heckeli] were synchronized with those of the Kuban zander. Freshening of the Kuban estuaries in the past has periodically increased the effectiveness of reproduction of both the zander and the roach. Howsver, it is not only freshening of the estuaries which increases the effectiveness of reproduction for Kuban fishes. In a very hard winter (for example in 1953-1954) when the estuaries freeze over and all the fish in them are killed, the effectiveness of zander and roach reproduction also increases sharply. This is quite understandable, since survival of the progeny of these fish, as I have said, is determined by conditions for larval feeding as indicated by the biomass of the spring zooplankton, while [competitor] fishes that eat plankton naturally reduce the production of young [zanders and roach]. In addition, these fish also consume larvae of zander and roach. Regulation of the flow of the Don by the Tsimlfân Dam has radically worsened conditions for natural reproduction of a number of Don River [page 152 ] fishes, zander and bream in particular. The Don River flood-plains in 5 years out of 8 from 1952 through 1959 have not been flooded over at all, while in the other 3 cases the flooding was-of short duration, was not everywhere simultaneous, was interrupted, and was insignificant in extent. During these years the progeny of zander and bream from the whole of the Don River spawning grounds was very small, which has led to an unprecedented decline in the Don bream stock. Its catch, which in the past used to go as high as 460 9 000 centners, now.remains at the level of 20-30 thousand centners. Reproduction of the Don zander during this period has occurred mainly in Veselov reservoir, where very favourable conditions exist for zander reproduction, whereas from the Don River spawning grounds proper there has been no recruitment of zanders.

The delta of the Don, both in the past and in its present condition of regulated flow, has not played an important role in the production of zanders, although these fish spawn in the delta in very-large numbers. This fact is explained by the massive washing out of zander larvae from the spawning grounds of the delta into the Gulf of Taganrog, where conditions for their sUrvival are extremely unfavourable. The latter is again associated with the fact that in spring not only the delta itself, but also the parts of the gulf near the delta, are poor in zooplankton and in addition are inhabited by various plankton-eating fishes--tiulka [nupeonella delicatula], young Percarina demidoffi, Knipovich's goby [Knipowitschia longicaudatai,and many others.

Washing-out of the young fish at early stages of development (larvae and embryos) in the delta region and in Taganrog Gulf is also one of the causes of the failure of reproduction of Don River Russian Clapemer g_1.2u1L_ii.lcti]Ict: and beluga [Ras° huso]. After construction of the TsimEra..n Dam these two species were forced to spawn below the dam, 200-300 km 0 from the mouth of the Don, whereas formerly their spawning grounds were situated much farther upriver. Under the changed conditions there was an increase in the washing- out of larvae of sturgeon and beluga into the delta and into the sea, where con- ditions for their survival at early stages of development are unfavourable. The fact is that the embryos of sturgeon and beluga adopt a half-pelagic manner of life for some time after they hatch from the egg, and only after the lapse of a certain interval of time, which is characteristic for each species of the stur- geon family and which depends on water temperature, do they descent to the bottom, and only then change over finally to a demersal mode of life.

After construction of Telm112n Dam on the lower Don, conditions were established such that embryos of sturgeon and beluga were washed out by the water current to the Don delta. and the Gulf of Taganrog earlier than the stage at which they settle to the bottom, and as a consequence the reproduction of these fishes has now become ineffective.

In spite of the passage of large numbers of sturgeon and beluga spawners up to the places of reproduction, there has been no production of fully- developed downstream migrating young of these species since the regulation of the flow of the Don, whereas in the past the downstream migration of these young was quite important. At the present time 99% of the offspring of sturgeon and beluga are washed into the delta in the embryonic stage of development.

The increased washing-out of embryos of sturgeon and beluga is caused by the fact that these fish now spawn considerably closer to the sea, as a result of which their progeny (embryos) "do not succeed" in finishing their semi-pelagic stage of existence in the time during which they move from their , place of spawning to the sea. A contributory factor is the fact that current speeds in the places of spawning of acipenserids are now greater than in the past. Washing-out of the embryos is also associated with the fact that the temperature of the water in the lower Don River, since the Tsimlfân reservoir was built, is much lower in spring and summer [page 153] than it was formerly; as a result, the period of development of the embryos (up to the time of absorption of the yolk sac and settling to the bottom) has been lengthened.

Among young Don River sevriuga (Acipenser stellatus) the period of semi-pelagic existence is shorter, therefore these unfavourable factors have less effect on the magnitude of its recruitment than on the Russian sturgeon and beluga. Under the regulated flow conditions, in some years there have been quite consider- able, or even large, productions of young sevriuga. This can be explained by the fact that in the past the principal spawning places of sevriuga were situated below the spot where the Tsimliân Dam is situated. However, the increased current speed and the generally lower water temperatures in the Don during the spring and summer have resulted in a decrease in the effectiveness of spawning, that is, in a large washing-out of the embryos even of the sevriuga. Up to the time of regulation of the flow of the Don only about 1% of the larvae were washed out, whereas today it is about 17%.

After construction of the Tsimliân Dam the transparency of the water increased considerably in the portion of the lower Don situated 100-150 km. from the dam, that is, in the spawning places of acipenserid fishes, and this favours the consumption of eggs and larvae of the sturgeons by other fishes. So also does the greatly increased abundance of fish (mainly Pelecus and shads) in the part of the Don near the dam.

Consumption of their larvae apparently was the most important cause of the sharp decline in the effectiveness of reproduction of sevriuga from 1953 to 1958. Mass consumption of sevriuga embryos by the Don shad has been confirmed by direct observation. As the number of shad and Pelecus on the sturgeon spawning grounds has increased, the production of young sevriuga has fallen off (see Table 1).

In 1953 when shad and Pelecus were scarce on the spawning grounds, the production of young sevriuga was so great that a commercial return of the order of 25-30 thousand centners should be taken from it; while in 1958, with great accumulations of shad and Pelecus on the spawning grounds of sevriuga, the latter scarcely produced any young at all, and the commercial return from this year-class will scarcely exceed a few hundred centners.

The increase in abundance in shad and Pelecus in the upper portions of the lower Don adjacent to the Tsimlfân Dam has not only had an unfavourable effect on the production of acipenserids, but has also decreased the effectiveness of reproduction of the shad and Pelecus themselves. The young produced by these two species in recent years has fallen off at the same time as their abundance on the spawning grounds has increased.

Other unfavourable factors associated with the constrUction of the Tsimlfâh Dam (increased speed of the current in summer and low temperature in spring and summer) have also reduced the reproduction of shad and Pelecus. The reason is that now their young are washed out into the delta and into Taganrog Gulf at the larval stage or even at the stage before the embryonic development of the egg is complete, whereas in the past young shad and Pelecus mainly went down to sea at the stage where the young fish were fully formed. 6 -

[page 154] In the paste foraging conditions have not limited the abun- dance of the stocks of Azov anadromous fishes. Their commercial stock then was regulated mainly by the production ordownstream migrating young, that is, in the final analysis by the effectiveness of reproduction. Only in a few cases, as a result of extremely high effectiveness of spawning, did the stock of Azov fishes increase to unprecedented levels, producing record catches: zander-- 719-715 thousand centners 71,900-71,500 metric tons] (1936-37); bream-464 thousand centners (1936); roach-235 thousand centners (1935); acipenserids--up to 65-73 thousand centners (1936-37). In other years, both before and sine the regulation of the flow of the Don, the food supplies of the Sea of Azov have been incompletely used by the rather mall stocks of sturgeons and other fishes.

As a result of the increased salinity of the sea caused by regulation of the flow of the Don, and also the low flow of the rivers in the years immediately preceding regulation, the feeding range of a number of fishes has considerably decreased: for zander from a maximum of 32,000 km. 2 down to 22,000 km. 2 ; for bream from 22 down to 5 thousand km. 2 ; for Pelecus from 12 to 5 thousand km. 2 (the feeding range of the Azov sturgeons has not decreased, and conditions for their existence in the sea have not worsened with the changes in the water regime there). In years of increased salinity of the sea (1952-1955) bream have lived only in the Gulf of Taganrog, scarcely entering the sea proper at all. This has greatly worsened their foraging opportunities and has led to a considerable decrease in rate of growth (to 40% below average).

Speaking in general terms, the following relationship between variations in the area inhabited and the rate of increase in weight of bream has been observed: when the foraging range decreases as the salinity of the sea increases (and the flow of the Don River decreases) the rate of growth of adult bream falls off, while with increase in their range (increased flow of the river) growth increases. In very recent years the rate of growth of the bream has improved, in association with some improvement of the water regime of the sea.

Fluctuations in rate of growth in weight of zander are large, but they have not been associated with changes in the water regime of the Sea of Azov and basically they have been determined by the feeding conditions for the zander, which latter depend on fluctuations in temperature during the autumn- winter period. When there is a warm prolonged autumn the rate of growth in weight of zander is considerably greater than when the season cools off early and there is a hard winter, under which conditions the length of their feeding period is contracted. In years since the regulation of the flow of the Don the rate of growth in weight of zander has been 40% greater, on the average, than before the regulation, because during this period years with long warm falls have predominated.

Food supply has never, during the period of observation, limited the rate of growth o f adult zander in the sea, because the fish they feed on (gobies, Clupeonella, , Percarina and others) are always sufficient for them in the Sea. of Azov. However, the great retardation of the growth of fingerling zanders in Tsimliân and Veselov reservoirs during recent years, associated with an inadequate supply of suitable foods there especially for the larger year-classes, has gradually led to a nearly complete elimination of sufficiently large finger- ling zanders (5-10 am.). In these cases they die gradually from malnutrition (Tsimliân reservoir in 1952, and in certain other years) or from being consumed by predators (Veselov reservoir). This circumstance has had a decisive effect on the abundance of the recruitment of young zanders to the sea. 7

Year-to-year fluctuations in growth in weight of adult zanders and bream in the sea are very great. They are used in predicting the commercial supplies of these fish.

[page 155] During the last 4 decades, excluding the several years during World War II, the Azov fishing industry has been very intensive. On the average about 65-70%, and in a number of cases more than 80%, of the commercial stock of zanders has been caught during a year, and 43% or sometimes 78% of the stock of bream, and 65% of the roach. Formerly about 90-95% of the total numbers (commercial return) of a year-class of zanders had been utilized by the end of its 5th year.

It is obvious that under such circumstances natural mortality among adult zanders, which are capable of surviving to 16 or 17 years of age, cannot have been important.

Computations which have been made show that the most effective fishery, if it is evaluated over a 15-year interval of Urne, is obtained when zanders are caught not earlier than their 4th year of age and when the fishery takes not more than 50% annually from the commercial stock. With more intensive utilization of the stock (60% or more), or with earlier capture of the zander--e.g., beginning in their 3rd year--the total catch of zanders over a 15-year period is considerably less; this decrease is explained by the inadequate utilization of the natural weight increase of the fish. With a less intensive fishery (using 40% of the stock of 4th-year and older zanders) the total zander catch over 15 years also becomes less than at 50% utilization, because under these conditions the loss from natural mortality increases.

The high intensity of the Azov fishery is confirmed also by marking data. For example, from a total of 6709 roaches marked in 19409 27.3% were recaptured in less than a year. Of 396 zanders marked in August, 1958, 17.4% were recaptured over 10-12 months, of which 10% were recaptured up to the end of 1958, that is in 3-4 months. Of 297 bream marked in the Don River in the spring of 1959, 28% had been recaptured up to the end of the same year, and half of thia number were taken by anglers and poachers, that is, catches that don't appear in the official statistics. Of 800 large fingerling Don River acipenserids marked in the Don during the summer of 1959, 13% had been recaptured up to the end of the year as accidental components and undesired species taken by river seines in the Don and by fixed gill nets in the Gulf of Taganrog.

The capture of the young of valuable fishes in the Sea of Azov is well known to have been large for many years. For example the fishery using raspornye nets, which were widely distributed during the early thirties and were suppressed in 1938, was almost wholly (about 80%) based on the capture of small-sized 2nd- or 3rd-year bream. This fishery considerably depressed the Azov bream stock, reducing the commercial return of some of the productive year-classes of those years by 20-68% (average 43%). The total loss from the catch of mature bream for the 8 years of the existence of the raspornye fishery amounted to more than a million -centners.

Important losses of catch of valuable fishes have occurred as a result of the fishery for young Clupeonella and anchovies by.'fixed trap nets. Shad, roach, Pelecus and Vimba have suffered especially severely from this fish- ery. The annual loss of catch of these fishes,resulting from the Clupeonella 8 fishery, has been computed as 17 to 40 thousand centners (1935-1947). During the period of abundant year-classes of zander (1952-54) a very large incidental capture of their young by the Clupeonella trap nets was observed. Abolition of Clupeonella fishery in 1955 had a favourable effect on the condition of the stock of a number of fishes, primarily shad and roach.

[page 156] For a long time an excessive fishery has exploited the stock of Azov acipenserids. Therefore, as a start (in 1938), the fishery for fish of this kind was prohibited in the western parts of the Sea of Azov in the places where accumulations of immature and non-breeding sturgeons occur. Sub- sequently the sturgeon fishery by means of self-catching hooks [samolovnymi krinchyami] was prohibited, and later all kinds'of fishing in the sea. These measures also considerably.improved the condition of the stock of acipenserids.

An intensive fishery for anadromous Azov fishes, under conditions where their natural reproduction has not been successful as a result of the unfavourable condition of the spawning grounds, has led to a situation where the fishery depends on the young age-groups. For example the capture of bream in 1956 con- tained 82% of age 3 and age 4 fishes, whereas in the past bream were caught mainly at the ages of 7 and older. In the catches of zander in 1955 9 instead of 4-year-old and older age-groups, the 3rd-year and 2nd-year fishes strongly predominated (93%) and of these the 2nd-year fish amounted to 67%. The shift to the utilization of younger ages was caused not only by the extrsmely intensive fishery and the small magnitude of the natural recruit- ment of anadromous fishes, but also it was to a considerable degree associated with changes in the water regime, and consequently changes in the behaviour and distribution of the fish. For example, the decrease in stock and in the catch of older age-groups of bream can be explained partly by the fact that after regulation of the flow of the Don all the bream lived exclusively in the Gulf of Taganrog where the intensity of the fishery is high, whereas in former years the older age-groups of bream (4 years old and older) were always found in the open part of the Sea of Azov (where the intensity of the fishery is much lower) except during the period of their spawning migration.

The increase in temperature of the Don River during the late autumn and winter period, caused by construction of the Tsimliân reservoir, has resulted in an accumulation of young age-groups of bream in the Don delta greater than what occurred in the past, and hence this too has increased the exploitation of bream during this season at this point.

A number of limitations on the fishery which have been imposed during recent years, and also the establishment of quotas for the principal acipenserid and other fishes in the Azov basin, which have been in effect since 1957, have had a favourable effect on the condition of the commercial stock of these fish. As a result of the general decrease in intensity and rationalization of the fishery the older ages have accumulated again, and consequently there has been a noticeable increase in the commercial stock of zander, bream, roach and sturgeon. However, this increase in the stock is not yet a steady one, inasmuch as the reproduction of these fish remains inadequate because of the unfavourable condition of their spawning grounds, and the inadequate scale of fish-cultural efforts.

In addition, a number of important inadequacies have appeared in the 9 system of limiting catches. However, these had been predicted in advance. In particular, very large and in practice almost unsurmountable difficulties have arisen as a result of the fact that almost all the commercial fishes whose catch is being limited, are as a rule caught at the same time and in exactly the same places, and by exactly the same kinds of fishing apparatus, as many species whose catch is not restricted. In this kind of situation, establishing a quota for the catch of one or several kinds of fish, especially those whose level of exploitation must not be very large, excludes the possibility of continuing the fishery for other species whose limiting rate of exploitation is considerably greater and therefore are not as yet fully utilized, or species whose capture is not limited at all. [page 157 ] In such cases fish whose quota is nearly filled or has already been exceeded are not delivered by the fishermen to the fish collecting points, although they do occur in the catches. This circumstance is the main cause of the excessive increase, in recent years, of "leakage" of fish into non-commercial channels [flutechki" ryby na storonu]. This difficulty must be overcome.

In addition, I would like to direct the attention of this Conference to the Kuban roach (Fig. 5). The size of the commercial yield of its year- classes, the abundance of its fingerlings in the sea (as estimated by catches of lampara nets in August) and the number of the young which were liberated by the Kuban fish hatcheries (mainly after improvement of the Beisug spawning ground) have varied in parallel fashion during 1950-57. The greater the release of young roach from the nursery establishments, the more of them there were in the sea and the higher was the commercial yield of the year-class in question. This indicates that the establishment of fish-cultural and improvement measures has not been without effect. Some part of the production of young roach, of course, has come from reproduction on natural spawning grounds, but obviously it is not very large, since the curve of abundance of fingerlings in the sea coincides with the curve of hatchery production. Hence we may proceed to an evaluation of the size of the commercial return of young roach if we consider that all of their production has been obtained as a result of the fish-cultural measures.

For the year-classes 1950-55 the commercial return varied from 2 to 9% and on the average was 4.7%. In 1956-57 it increased to 13.6-24.4%. However, these year-classes were not very accurately censussed and it is possible that their numbers are overestimated.

Thus the fish-cultural measures have been one of the factors respon- sible for the change in abundance of the stocks of commercial fishes in the Sea of Azov. We have pointed out, for example, that young beluga. and Russian sturgeon did not occur in the Don after regulation of the flow, but since the sturgeon hatcheries have gone into operation they have appeared in large nuMbers as incidentally-caught fish. Table 1. [page 153] Production of young sevriuga [Acipenser stellatus] and number of shad and Pelecus on the spawning grounds of Don River acipenserid fishes

Catch of fingerling No. of shad and Pelecus stellate sturgeon Year per' 100 trawl catches Millions Thousands in the Don, in pieces of pieces of centners

1953 3500 3.5 6.9

1955 590 6.9 10.9

1956 260 10.3 17.4

1957 4 ' 12.4 17.2

1958 22 14.1 22.7

1959 17 8.9 15.5 d4

Jo 28 25 k a 24 k li 22 ke 20 e, 18 .% „ feu 15 t 7 14 Ô 12 W ›

1 8 C, 4 › 3 5 2 4 I , f 114 I III! I ii Iii IL Il te:e.›..ebefffleAceelie effleem

Fig. 1. [page 148] Year-class abundance and number of spawners of Kuban zanders [left-hand ordinate - number of spawners, in millions of pieces; right- hand ordinate - abundance of the year-class, in millions of pieces]. Columns - commercial yield of the year-class; curve - number of spawners; the years are arranged in the order of decreasing abundance of spawners.

q., 22 e- . 20%, .9- 18 ,k k- --; ,t 8- i51 o 7 _\\N„...... \ C ... 14 k t,. 12 % o(.., .„_ 4 a - 10% 4 8 C ,c„.. 4 j - %) c % 2 _ % • 1 4 › l 1 o t,°4 I- o • II à Ilk la ka a H ..-.N.,-. ,...-,-...„.,„...,...,„.0,„.,„.....,..,..,...... a...!III ...h...... ,,,,,,,

Fig. 2. [page 148] Year-class abundance and number of spawners of the Don zander. Scales and symbols as in Fig. 1. MOO

q12 11

, ve % s O 8 V o

*go 4 e 3 O, 2

9 lee MI fee xue 1119 1.98, 1854

Fig. 3. [page 149] Number of spawners and production

of fingerlings of the Don stellate sturgeon [left -

hand ordinate - spawners, in thousands of pieces;

right-hand ordinate - number of fingerlings, in pieces]. The columns show the catch of fingerlings per hundred hours of trawling in the Don River; the curve is the number of spawners.

oreIt.i.uM pi 22 t 201 I 18 16 LI 1111Mili11 t "121 111 MIMIlla a 2 1 1 II/1 II II IMINIIIII IZIMMIllk % a latimaimmul / wawa& 84 reran lorammurAu 2 moos M I ll I I I I à0 LI!!!! !NI MAIM N--J 0 42

481

44 i 2 40 1 I 1 35 1 •472 I II 28 L / 1 t I 1 e 1 i zo Ill I 1 Is ..., IBM Illf IIIIMI\ : 3 L WalLirr.,- leilnie ‘ r . i \ ' 4 \i \- -.‘, / \ é I V %... 18)8 I8M 1M8 12)9 1840 1941 1942 184) 1944 /948 1846 1947 lee 1949 MO 1931 1932 fad&

Fig. 4. [page 15 ]. J Production of young and commercial yield of successive year-classes of the Don River zander (above) and bream (below) (in millions of individuals). 1 - commercial yield of the year-class; 2 - abundance of fingerlings in the Gulf of Taganrog.

1 v. . ---• -si „ 1 .1,' , 2\ î

1 1 I if . \.,, wa 1.911 1912 191) I914 1911 1916 eve

Fig. 5. [page 157] Indices of abundance of Kuban roach. l--commercial yield of the year-classes, millions of pieces; 2--number of young fish reared in the hatcheries and on the improved spawning grounds, millions of pieces; 3--number of fingerlings in the sea (catches of a lampara net in August, number of pieces).