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Fauna
Composition of the Lake Ladoga zooplankton includes 378 species (Dengina, Sokolova, 1968) with 141 taxa found in litoral zone (Telesh, 1996). The species composition of zooplankton in the lake changed little during 80 years of investigations. Simultaneously our knowledge of taxonomic structure of the Ladoga lake zooplankton is incomplete as in course of latests taxonomic studies over one fourth of the species list appeared to be the first records of species for the Lake (Telesh, 1996). At the same time, the size structure of the community in the shallow areas of Lake Ladoga has changed (Ogorodnikova, 1995) due to eutrophication and enrichment of the water with nutrients and allochthonous organic matter. About 600 species of invertebrates dwell in the bottom communities of Lake Ladoga. These forms, also including near bottom fishes, are called - benthos (Fig. 3.3.2.1 and 3.3.2.2). Comparison of data of 1975-1985 results of studies in 1989-1991 revealed some modifications in the benthic communities. Firstly, total biomass of bottom invertebrates grew considerably. Secondly, not only the quantitative but also the qualitative characteristics of benthic animals were found to change in some areas of the lake: the species which used to dominate were replaced by other species. Thirdly, direct anthropogenic pollution lead to the formation of local "dead" zones (for instance in the Shchuchy bay near Priozersk, not far from the Pitkaranta municipal sewage discharge area, in the estuary zone of the Janisjoki river near the town of Laskela). The areas with a notable increase of benthos biomass induced by man-made eutrophication concentrated mainly in the near-shore zone: near the towns of Priozersk, Sortavala, Pitkaranta, Salmi. Two areas with abnormally high biomass values were found at some distance from the Western Ladoga shore. Benthos biomass was the most stable is the Central-Ladoga depression and in the Valaam area. According to the most recent data Lake Ladoga is populated with 42 species of fish and pisciformes not including those introduced into the lake for acclimatisation (Kudersky, 1996). Total of 58 species and interspecific taxa of fish inhabit water bodies of the Ladoga region. The species occurring in Lake Ladoga are ecologically non-uniform. They can be grouped into two major categories: freshwater - 35 species (81% of the total composition), anadromous and semi-anadromous - 8 species (19%). Many of the cyclostomes and fishes inhabiting Lake Ladoga (23 species or 54% of the total) are of commercial importance. The bulk of the harvests however is constituted by just a few mass species. These are whitefish, vendance, smelt, pike-perch, bream, roach and perch (see "Analysis of fish resources" and "Description of fisheries) . Owing to intensive research we now possess relatively detailed information about the main commercial fish species in the lake. The abundance of most commercially valuable species (trout, grayling, asp, vimba, eel, whitefish, vendace) is not high. It decreased even more in the last decades in response to the deteriorating environment, destruction of spawning grounds and poaching. As a consequence, the catches of many formerly commercially valuable species dropped as well. Populations of especially valuable fish species are not in a good shape. Scarce shoals of the Svir and Vuoksa whitefishes are on the verge of extinction. Volkhov whitefish (Red Data Book species) abundance now depends on how effectively the Volkhovsky fish farm works. Freshwater forms of salmon and brown trout are partially maintained by the Svirsky fish farm. The fish species and forms living in Lake Ladoga included in the Red Data Book of the Russian Federation are: river lamprey Lampetra fluviatilis, common sturgeon Acipenser sturio L., sebago salmon salar morpha sebago, brook trout S. trutta morpha fario, lake trout S. trutta morpha lacustris, Volkhov whitefish Coregonus lavaretus baeri. Lake Ladoga has just one representative of large water mammals - the Ladoga subspecies of ringed seal (Phoca hispida ladogensis Nordq.). The occurrence of a marine relict in a freshwater body can be explained by the lake genesis, that was formed by isolation from a sea. The seal abundance in the lake in the recent years is estimated as 4000-5000 individuals, but this estimate resulted from extrapolation of the data on the abundance in the Sortavala skerries and Valaam archipelago areas. Data on the distribution and seasonal migrations of the Ladoga seal are quite contradictory. From August to May most animals are believed to dwell in the southern and south-eastern parts of the lake (spreading throughout the lake water area in cold winters, when the fast ice area is extensive). From May to June the majority of the animals stays in front of the estuaries of the southern and south-eastern shore rivers, and for the period from June to August the animals migrate to the northern areas of the lake to form temporary summer herds of up to 200 individuals on island shores or on rocky shoals (Fig. 3.3.2.2). Most researchers attribute the seasonal migrations to the corresponding fish migrations. Generally speaking, the water of Lake Ladoga pre-determines the living standards in St. Petersburg and the state of the Baltic Sea marine environment, and the Ladoga itself is of great interest as a refugium of rare and relict species. Simultaneously, both the non-living environment and the natural component of the lake itself are exposed to intense detrimental human impact (see "Ecology"). |
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