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The seal

Source:  The problems of Lake Baikal / Ed. by G.I Galazi, K.K. Votintsev. Novosibirsk, 1978. pp. 251-258. The Academy of Sciences Transactions. Siberian branch, Vol. 16 (36).

 

As it has been already said, a highly specialized mammal lives in the Baikal, a valuable hunting object – the seal, or the nerpa, as the local people call it. The Baikal seal – Pusa sibirica Gmel. – can be rightly referred to the number of the most interesting representatives of the unique fauna of that lake.

Many famous researchers, zoologists, zoogeographers and even geologists, were interested in the taxonomy and comparative morphology of the Baikal seal, its origin and coming to the Baikal, almost in the centre of the Asian continent, connected with the problems of the origin and history of the lake itself. But until now the question about the nerpa’s origin, about its appearance in the Baikal, is the theme of discussions and the proposed solutions are not beyond more or less grounded but still hypothetic considerations.

There are two main points of view on the problem. According to one of them, the nerpa came to the Baikal long ago (in the Quaternary period), from the Arctic basin along the system of rivers (Georgi, 1775; Cherski, 1877; Nordgvist, 1899; Vereshchaghin, 1930, 1940; Kozhov, 1947, 1962; Lamakin, 1964; Anbinder, 1969; and others). According to the second one, the nerpa comes from one of the generic branches of the Tertiary seals of the inner Sarmat-Pontian basin (Ghernes, 1898; Ognev, 1935; Chapski, 1955; Kondakov, 1961; and others).

The comparative analysis of craniometrical characteristics of the present-day species of the Pusa genus shows greater proximity of the Baikal seal and the seal from the Caspian Sea (Ognev, 1935; Yablokov, 1966; Pastukhov, 1969a), the descendants of the seals that lived there in the late Tertiary time (Bogachev, 1927a, b; Chapski, 1955), than the hispid seal of the Arctic and Far East seas. There is an opinion (Chapski, 1955) that it came from a later (than the Caspian one) original form, common for the Baikal and hispid seals.

It is known that the present-day inter-continental spreading of seals of the Pusa genus (P. caspica, P. hispida, P. sibirica) in Europe and Asia is limited with the Caspian Sea, the Ladoga, the Saima (Finland) and the Baikal. The above mentioned lakes are quite spacious, deep, have rather powerful photosynthecising layer and, what is especially important, inhabited by short-cycled small pelagic fishes with high reproductive and productive potential. In the Baikal they are golomyankas and bull-heads, in the Caspian Sea – sprats, aterinas, bull-heads, in Ladoga Lake smelts, stickle backs, ryapushka, in Lake Saima ryapushka.

 Evidently, relatively stable conditions can be preserved for quite a long time only in big lakes with great physical and chemical and bioproductive momentum (independence). They provide reproduction and survival of individuals, i.e. the complex of outer and inner conditions supporting the population homeostasis.

The nerpa’s settling in the Baikal should be referred either to the time when pelagic fishes, golomyankas in particular, had already been formed there, or which is not less probable, the forming of the “golomyankas-nerpa” complex was going on simultaneously. Whatever happened, one thing is quite clear: if there were no mass pelagic fishes in the Baikal now, there would be no nerpa either.

It is quite possible that the present-day spreading of the Pusa genus seals in a very limited number of European and Asian continental lakes should be considered from this point of view (i.e. taking into consideration the trophic factor, first of all).

Research of recent years gave the opportunity to reveal a number of new ecological and morphological peculiarities of the Baikal seal which distinguish it from the nearest relatives – the hispid seal and the Caspian seal. It is distinctive with its bigger body and skull sizes, very strongly pronounced signs of sex dimorphism, the peculiarities of teeth structure, bigger eyeballs, very developed claws of the fore flippers, high rate of linear and especially (2-3 times) weight growth of the offspring during their first year of life, the lactation period twice as long, smaller death rate during their first year of life, and higher reproduction and production potential. These and other peculiarities are conditioned by long lactation period, stable food basis, duration of the ice period. Such is the simplified scheme. In a more orderly way it is a system with direct and reverse relations, determined by the geographic situation, climate, orography, the size and bathymetry of the lake, water and heat exchange, physical and chemical properties, and lastly, the bioproduction properties of the Baikal ecosystem itself.

Whereas the hispid seal is extremely weakly differentiated in the morpho-population way (Pastukhov, 1969a), but inhabits an immense area (the Ladoga, the Saima, the Baltic Sea, the Arctic and Far East seas) with very different environmental conditions, the Baikal seal has evidently passed the longer way of independent evolution. The proof of that is, on the one hand, some primitive (ancient) osteological characteristics (Chapski, 1955), on the other hand, acquiring a number of above mentioned progressive adaptation morpho-physiological and ecological properties (evidently, in the period of the deep water Baikal).

It is characteristic that the forms of the hispid seal which used to be isolated in the Tertiary period in the Baltic Sea, the lakes of Ladoga and Saima, do not have the morphological differences from the Arctic and Far East forms, P. hispida, which the Baikal nerpa possesses (Chapski, 1955; Pastukhov, 1969a).

All this taken together does not let exclude the idea of the Baikal seal as a relic of the Tertiary fauna (Ghernes, 1898; Ognev, 1935; Chapski, 1955; Kondakov, 1961) which continues its progressive evolution (Pastukhov, 1972a,b). The question of the Baikal seal origin is an integral part of the more general problem – the origin of the Baikal fauna, which in its turn grows into the global problem of fresh water fauna development not only in Asia, but also in the whole Golarctic, as G.F. Mazepova (1974) justly points out.

Detailed research of the morphological peculiarities (craniological, osteological, exterior and interior), i.e. mostly the population’s phenetics, now seems to be especially necessary, and first of all for more definite sentences about the taxonomy and the origin of the Baikal seal, for establishing the degree of relations among the representatives of the Pusa genus, and lastly, for understanding the evolution (and especially microevolution) of this species in the closed freshwater Lake Baikal.

The next range of problems concerns biology and population ecology.

Studying the animals’ spreading showed that the nerpa keeps mostly far from the shore round the year and it can be referred to typical pelagic animals with full grounding.

The nerpa can be found all over the Baikal, but it prefers its northern part.

Clearly expressed migration nature, yearly movements of the animal around the lake in accordance with the ice situation and a number of other peculiarities denote the fact that this isolated species has been formed by a single panmixic population. At the same time a number of observations show that forming short-time spatial groups of individuals is typical of the nerpa.

After exact age diagnostics (by dentine layers and the cement of the fangs) the age and sex structure of the population was deciphered for the first time. The age line (starting from the age of one) looks like decreasing exponent. According to the data of 1961-1973, the number of animals from 1 to 9 years of age is within 76.0-78.9 per cent, from 10 to 19 – 17.8-18.9 per cent, from 20 to 29 4.0 per cent, from 30 to 39 0.5-2.1 per cent, more than 40 not more than 0.4 per cent. The Baikal seal is distinguished by its potential longevity among other seals of the Phocidae family: the age of females is determined as 56, of males as 52 years. But the average population age is not over 7-8. The number of grown-up (mature) animals is about the same as immature ones (without cubs). The offspring is 18.4-21.3 per cent of the total number at the beginning of biological year. There are more females (57-58 per cent) than males (42-43 per cent) in the population.

As a result of recent research the former ideas of the Baikal nerpa’s reproduction had to be changed almost completely (Dybowski, 1873; Svatosh, 1926; Ivanov, 1938). The females reach productive maturity (mating with the following pregnancy) at the age of 3-6, and they first give birth to the young when they are 4-7 years old. The males become mature at the later age. The nerpa is characterised with yearly cubbing. But because of different reasons about 10-20 per cent of grown up females stay dry in different years. The period of cubbing is from late February until early April. Most nerpa cubs are born within two weeks, in the middle of March. Nerpas give birth to their cubs in snow dens; mating is under ice (in April). Pregnancy is not 9 months long, as it was believed before, but 11. At that there is embryo diapause, the first 3-5 months long. The Baikal seal is closer to polygamist species in their breeding relations.

Linear growth of the nerpa is over by 17-19 years of age, and its weight increases some years more. The average population weight is about 50 kg. From spring until autumn the weight characteristics change considerably, they differ within 17-32 per cent in accordance with ages and sexes. The average weight of hypodermic fat which plays an important role in thermoregulation is about half of the total body weight. The researches of the Baikal seal in its natural habitat (Gurova, Pastukhov, 1974) and in a long-term (half a year) experiment (Pastukhov, 1969) show clearly that these animals are typical ichthyophags. They mostly transform golomyankas and bull-heads not used by man. There are clear seasonal changes in their food: in ice period it is very poor in quality, in the iceless period it is most varied; the animals eat least during mass rookeries on ice in spring and autumn, and most nutritious period is during fattening (July-September). Yearly differences in their nutrition are conditioned by fluctuations in their quantity and in the number of golomyankas and bull-heads, which is expressed in changing sizes and weights, quantity of fish consumed and in the frequency of the latter.

First the route-band and then more perfect area methods of defining the nerpa’s quantity by the dens of pregnant females were worked out and applied (Pastukhov and others, 1973) in accordance with ecological peculiarities of the Baikal seal, taking into consideration spatial and temporal character of breaking the ice cover of Lake Baikal in spring. The total quantity of offspring or propagating females (each of them usually gives birth to one cub) in 1972-1974 was 15.4 thousand head with the statistic error of 10 per cent.

In different years about 19-28 per cent of offspring (or females) are concentrated in the Southern, 27-49 per cent are in the Middle, and 23-54 per cent are in the Northern Baikal. The total quantity of the flock by the end of biological year calculated on the basis of the data about the number of the year offspring, age and sex structure of the population, fertility and frequency of females’ participation in reproduction at this or that age, was about 68 thousand head in 1972-1974.

The problem of estimating of the nerpa’s role in the Baikal biome needs further development. The first steps are made in this direction (Pastukhov, 1965, 1967a, 1973, 1974; Votintsev, Pastukhov, Popovskaya, 1969). But they were only first steps. The many-sidedness of the problem is in the fact that such estimation should be done in appropriate quantitative expressions not only for the nerpa, but also for other components of the biome. It is necessary to say that the idea of the optimal standard for many phenomena and states of these or those populations (and for the whole biocenosis) is still very poorly worked out. We cannot measure many things with the necessary accuracy yet, to say nothing about the spatial and temporal average indicators of the received results. It can be referred to the population of the Baikal seal, too. Here broad complex researches of the main components of the environment are necessary, in their indissoluble connection and conditionality. To estimate the role of the nerpa in the Baikal biome is at least to answer the question about its optimal quantity (density) necessary for vital functions and production of the population itself and for optimal functioning of the biocenosis as the single whole. Another extremely important aspect of this problem is estimation and maintaining of the nerpa’s optimal quantity from the point of view of the complex rational usage of the Baikal biological resources. Evidently, from the point of view of man, well-being of the nerpa’s population and the biohydrocenotic activity as the whole, the optimal quantity of the nerpa combines different values, the estimation of which (what’s more, finding some “average” compromise) is somewhat difficult.

The question of the nerpa’s optimal quantity is closely connected with the question of the Baikal environments capacity as applied to that animal. We can suppose confidently enough that the environment capacity is not the constant value, but it changes in accordance with cyclic fluctuations of the whole ecosystem of the lake under the influence of hydroclimatic, biotic and anthropogenic factors.

For the time being the maximum quantity of the nerpa in the years of reduced total bioproductivity of the Baikal can be estimated more exactly. For example, the researches of 1961-1974 show that there were no considerable changes in the population’s age structure. But in those years true reduction of the populations reproductive ability was noted; it was expressed in the females later beginning of reproduction and higher dryness among wittingly grown up animals (18 per cent in 1917-1973 against 10 per cent in 1961-1969). Throughout some years the general weight of the animals and their fatness reduced. There were also some changes in their nutrition: the total quantity of the consumed food reduced (especially that of the dominant type – the small golomyanka), there are also some changes in the main nutrition objects correlation.

According to G.I. Popovskaya’s data, 1969-1973 were characterised by weak development of the spring phytoplankton. We have every reason to think that in those years the negative influence of the phytoplankton depression touched the whole chain of heterotrophs including the nerpa, too.

The display of the above mentioned negative symptoms of the Baikal nerpa’s populations state against the background of the age and sex structure and the quantity of the mother total number stabilized within several years, gives the opportunity to suppose that the stock of the nerpa are now close to the level determined by the Baikal environment capacity in the condition of its present-day lowered productivity. To estimate the role of the nerpa in the Baikal hydrocenosis, besides knowing its quantity, it is necessary to understand clearly the quantitative and qualitative characteristic of the animals’ nutrition throughout the year. The research of the nutrition of many hundreds of animals hunted in different seasons, years and in different parts of the Baikal showed that the nerpa does not do much harm to fish. The weight of marketable fish in its nutrition is not more than 2 per cent; the number of fish makes up per cent   decimal parts. At the same time there is a reason to say that the nerpa eats mostly weak and sick fish which is also important for those species of fish from the point of view of natural selection. Of course, the role and significance of the nerpa in the Baikal ecosystem cannot be limited only with estimation of its direct effect on the marketable ichthyofauna representatives. We have already mentioned that the nerpa transforms the most mass (but spread in the thick of water) small pelagic fishes – golomyankas, inaccessible for man, making them the accessible product, the product of the nerpa’s hunting.

 Moreover, the biocenotic significance of the nerpa is evidently in the fact that it is a specific regulator of biohydrocenosis as the whole, reducing the amplitude of inter-yearly fluctuations of its most important representatives, especially golomyankas, i.e. stabilizing the level of production of heterotrophs row. How it is displayed and how it affects the marketable fish (connected with non-marketable ones through food competition and preying) are the questions which stay open to a considerable extent. For example, high degree of similarity of the food composition of the omul and golomyankas and bull-heads shows their very close food interrelations. At that the omuls of different races (populations) stay mostly crayfish-eaters (epishura, macrohectopus, ground amphipods and others) consuming this or that quantity of the fry of pelagic bull-heads and golomyankas; they compete with typical aborigines – the bull-heads and golomyankas (Gurova, Pastukhov, 1974). We think that the nerpa not only eats the golomyankas and bull-heads, but also plays the role of a very important natural biological improver releasing the food resources for the main marketable fish, the omul. From this point of view it goes without question that it is necessary to preserve the high quantity of the nerpa.

From times immemorial and until present-day time the nerpa has been a valuable object of hunting; it is hunted for food and technical fat, meat and skins. The official hunting amount has been considerably different throughout 70 years (from 1 thousand to 7-8 thousand head) which was caused by different demand for these or those kinds of products received from the animal, on the one hand, and by fluctuations of the population’s quantity as a result of the hunting pressure and the natural environmental factors.

Now about 2.5 thousand head of nerpa are hunted a year. Evidently, the amount of planned hunting can be up to 4-5 thousand head in the nearest years.

The analysis of nerpa hunting let us make some proposals about its reconstruction.

The main recommendations have been already introduced. For example, since 1967 hunting has been based only on one-year-olds that have already shed their hair; they produce skins, fat and meat of highest quality. Giving up killing cautious grown up and half-grown up animals gave the opportunity to avoid their unjustified deaths as a result of frequent deadly wounds. The method of nerpa hunting with the help of under-ice nylon nets is widely used; there are no wounded animals in this case. The hunting dates and the hunting regions are legalized. Hunting is allowed at the end of the cub-bearing period (from April 25 until May 15). Hunting is in the northern and partly middle Baikal. The southern part of the lake has been made a kind of reserve for the nerpa.

But it is very necessary to introduce more effective measures of the animal’s protection. It is necessary to work out and introduce new forms of organizing the work, technological schemes of raw materials processing and grounded methods of the nerpa’s quantity regulation.











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