KUZNETSOVV.V. Russian Federal Research Institute of Fisheries and Oceanography, Russia

The intensity of development of the hydrocarbon resources (oil, gas) on the shelf of the Arctic region steadily grows resulting in a danger to fish resources (Kudersky 2000).

Minimization of harmful consequences of this development may be made only on the basis of fishery monitoring for which it is necessary to identify the main processes in the life cycle of the commercial species inhabiting the shelf, areas and periods optimal for conducting the systematic observations.

The high-arctic seas are characterized by the low productivity level. The proper marine ichthyofauna in the Russian Arctic seas (the Kara Sea, Laptev Sea, East Siberian and Chuckchi Seas) is poor both qualitatively and quantitatively (Zenkevich, 1963). The main fish resources of the freshen shelf areas with the development of the oil-gas resources are the brackish water whitefishes. In some extent these areas are inhabited by fresh water populations.

The favorable conditions for existence the relatively high productive fish stocks in the coastal regions are created due to the high river discharge determining the freshening, warming of the coastal areas, influx of nutrients and driving away and melting of ice.

When characterizing the hydrology of the coastal areas, where the river "meets" the sea, it is often said about the joint of the river and marine waters that does not exactly correspond to the real situation. In real, the high-Arctic seas have the specific secondary water mass (brackish) formed from the Arctic sea surface and river waters. It characterized by the specific euryhaline fauna of invertebrates and fishes. This water mass is very variable both in space and time regarding to its volume and configuration. Its food resources are used the such valuable species as arctic cisco, muksun, Siberian cisco, nelma, Siberian sturgeon, smelt, char, burbot. In the periods of the higher river discharge the fresh waters are extended widely in the sea bringing in the sea the freshwater species (Siberian whitefish, broad whitefish, peled, taimen, etc.) As brackish species are generatively the freshwater ones the feeding of their juveniles occurs in the rivers and the delta arms during the downward migrations and also in the fresh and brackish waters of estuaries and coastal marine areas.

The coastal freshened areas are characterized by the large seasonal ecosystem changes which in large extent determine the fish distribution. This is very important for fishery assessment of the fishing grounds and organization of fishery monitoring. The main factors determining the ecosystem state are associated with the large seasonal fluctuations of the river discharge, wind mixing of the water masses and wind-induced setups, regular tides, ice cover which decrease the impact of the atmosphere on water masses (Alexandrova and Kuznetsov 1967a, b), and in the Ob Bay the formation of the suffocation zone (Moskalenko, 1971; et al.).

The river discharge determines in large extent the direction of the coastal currents (Antonov, 1961). The many rivers with different flow mouth in the marginal seas of the North Ice Ocean. The degree of freshening of the coastal areas and the aquatory used by the brackish and freshwater species depend on the volume of discharge. The environmental conditions of the brackish fauna in the estuary areas of the different rivers are differed depending on their discharge. The similar differences associated with the volume of discharge are observed among the estuaries of the delta arms of the large rivers. The small rivers in winter are completely frozen and do not have the discharge. The winter discharge is rather small even in such large rivers as Yana, Anabar, Omoloi, etc. The discharge of the greatest Siberian rivers (the Ob, Enisei, Lena) in the autumn-winter period decreases sharply.

Thus, from November to May in the Lena River it is observed only 9% from the annual discharge. Correspondingly, the volume of the brackish zone which is the biotope for the brackish water fauna including fishes is reduced.

In the estuaries of the small rivers the freshening zone is lacked in winter. In the estuary areas of the large

r rivers the brackish area reduces in autumn due to the decrease in the discharge, and the fish concentrations migrate toward the river mouth. This regards both freshwater species which distribute outside the rivers with the summer river discharge and brackish water species. By this, the latter feed actively on invertebrates of the brackish water complex under the conditions of the reduced freshened zone (Kuznetsov, 1971a,b; 1975). Later in those flows where the winter discharge is low, the reduced brackish water zone shifts upward the river together with the inherent fauna, for example this is observed in the downstream of the small-water Tumatsky arm of the Lena river delta (Dormidontov, 1961), and in the downstream of the Anabar River (Moskalenko., 1971 and others). In the areas where the river discharge is insufficient for the keeping of the freshening zone in winter, the death of arctic cisco was observed. (Probatov, 1950). But normally the brackish water fishes leave the regions in time.

In the areas of the high river discharge (e.g., in the areas freshened by the waters of Trofimovskaya delta arm of the Lena River mouth, on which in the winter low flow comes 80-85% of the river discharge) the autumn decrease in the discharge together with the wind mixing, wind setups and regular tides also results in the reduction of coastal freshened zone, and then to advection of the brackish waters to the estuary (Alexandrova and Kuznetsov, 1967a). In that period the most densest concentrations of the commercial species are formed at the confined aquatory. By this, the active feeding of fish mainly by the brackish water plankton is observed (Kuznetsov and Alexandrova, 1966).

With the increase in the ice cover the influence of the wind on the water masses decreases, and usually the clear water stratification by the salinity and temperature is observed. Under the high river discharge the salted waters move outside the estuary, and the fresh waters also extend to the sea. The main fish concentrations shift outside the estuary following the retracting brackish zone. The environmental conditions of fish in the estuary worsen. Later in the winter period the slight migrations of fish in the estuary are observed due to the strong onset winds and syzygial tides usually without of the repeated salting in the estuary. With the lower, however the still rather discharge (Bykovskaya delta arm of the Lena River) the all processes in the estuary are the similar excepted that due to the smaller coastal freshening the repeated advection of brackish waters in the estuary with the corresponding fauna of food organisms and fish occurs simetimes (Alexandrova and Kuznetsov, 1967a; Kuznetsov, 1971a).

The widely-spread opinion that the Lena River muksun stock as the other brackish water species winters in the delta arms of the Lena River (Pirozhnikov, 1953 and others; Moskalenko, 1971) does not correspond to the reality. Only relatively small concentrations of the brackish water fishes winter in the delta in the low-water arms.

The most complex processes are in the mouth area of the Ob River. Besides the processes associated with the fluctuations of the river discharge, wind impact and regular tides the very important factor is the formation of the suffocation zone in winter (Moskalenko, 1971; and others) which extends to the southern Ob Bay from the river. The other important feature of the Ob River basin is the flood plain very rich by the food resources. If in the mouth areas of the other Siberian rivers the main food resources are represented by the brackish water fauna, that areas have two important resources, i.e. the brackish water and freshwater fauna developing in the summer in the flood plain of Ob River. The development of the suffocation zone in the river and its extension to the southern Ob Bay determines the fish wintering outside this zone. The rich food base of the lower Ob River determines the so-called "piercing" fish run from the bay into the flood plain after the feeding followed by the migration for the spawning in the upper Ob River and its tributaries.

The "piercing" fish run are preceded by the formation of the dense fish concentrations in the pre-suffocation zone in May-June.

The main investigations on the fishery monitoring on the shelf must be done in the time of the formation of the densest fish concentrations during the periods of the large ecosystem changes. The areas of the formation of such concentrations are very valuable for fishery and need to be conserved. The scientific samplings obtained from the densest concentrations representing the essential part of population are characterized by the most representativity.

In the all estuary regions the observations in period of formation of the dense fish concentrations in October-November are of the great interest. Because the observations in the time of ice cover formation are hard it is reasonable to begin them immediately after the formation of the rather sustainable fast ice. By this, the most representative parameters of the population density, age-size composition and biological condition of fish together with information on their food base and feeding may be obtained.

In some coastal areas where the freshening is rather low, the observations on the fish concentrations in these regions under the conditions of the river discharge decrease are very interesting. For example, such observations were made over the arctic cisco concentrations in the Oleneksky Bay of the Laptev Sea near the mouth of Oleneksky arm of the Lena River delta (Kuznetsov, 1971a). Near the mouths of relatively small spawning rivers (the Omoloi River) in August-September the large feeding concentrations of pre-spawning Siberian Cisco are observed (Kuznetsov, 1975). In the conditions of the Ob Bay the observations on the fish concentrations inhabiting the pre-suffocation zone in the "pre-piercing" period (May-June) are of the great interest.

In summer the main feeding and growth of the whitefishes are observed. Despite the density of commercial fish concentrations distributed over the large freshened coastal aquatories in that time is usually low, this period is very efficient for the study of immature non- spawning individuals, biology and assessment of mature fish not entering in the river of spawning. The accounting of the abundance of the growing under-yearling fish migrated in the estuary from the river is the most reasonable in August-September. Those accountings of the migrated juveniles is conducted by SevPENRO in the lower Pechora River in the forecasting purposes (Kuznetov, 1987).

References
Alexandrova E.N., Kuznetov V.V. To the study of the biology of the anadromous fish of the River Lena in the autumn-winter period // Vestnik MGU, Biology and Soil-Science. 1967a, No. 3. P. 12-22 (in Russian).
Alexandrova E.N., Kuznetsov V. V. The ecology of the anadromous whitefishes of the Lena River in the autumn-winter period // Voprosy Ichtiologii. 1967b. Vol.7(l-42). P. 46-58 (in Russian).
Antonov VS. The influence of the river discharge on the freezing of the river mouths and the coastal zone of the Laptev Sea // Trudy AANII. 1961. Vol.213 (4). P. 5-37 (in Russian).
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Kuznetsov V.V., Alexandrova E.N. The autumn-winter feeding of anadromous whitefishes of the Lena River // Vestnik MGU, Biology and Soil-Science. 1966. No. 4, P. 43-48 (in Russian).
Moskalenko B.K. Whitefishes of the Siberia // M. Food Industry. 1971. 184 p. (in Russian).
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