Translation Series No

!AR C.1:11YE FISHERIES RESEARCH BOARD OF CANADA Translation Series, No. 414 BASIC FACTORS IN THE FLUCTUATIONS OF THE STOCKS OF STURGEONS AND LESSER FISHES IN THE SEA OF AZOV By E. G. Boiko Azov Fisheries 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 fluctuations 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 fluctuations in the stocks have been determined by the recruitment: when the fishery 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 captured 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 commercial 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 fishery. 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 beluga (Huso huso) as well as species of Acipenser; the "other fishes" or "lesser fishes" chastikovye ryby - include zànder, bream,roach, shad, anchovy, 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 effectiveness 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 commonly 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 reproduction 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 spawn 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 consequently 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.

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