FRESHWATER SNAILS (MOLLUSCA: GASTROPODA) OF FISH PONDS IN SILESIA (SOUTHERN POLAND)

aBStract: The paper presents results of studies on the snail fauna of 22 fish ponds of Silesia and its vicinities. 20 species were found. The results suggest that neither the size and depth of a reservoir nor the bottom type effect directly particular snail species. It seems that the impact of these factors is overlapped or modified by cultivation which limits the number and species diversity of snails. As regards ponds in the Upper-Silesian Industrial Region, another limiting factor is industrial water pollution.


INTRODUCTION
The malacofauna of fish ponds in the Upper Silesian Industrial Region and its vicinities is poorly known, since there is only one paper dealing With it (Kownacka 1963).Some fragmentary data on the subject were given by other authors who dealt with the bottom fauna of particular ponds (Ky-Sela 1958, Za~wilichowska 1965, Zi~ba 1967, Krzytanek 1973).The malacofauna of other types of artificial reservoirs was studied by Strzelec and Serafinski (1984) and Strzelec (manuscript).

MATERIAL AND METHODS
Samples of the snail fauna of 22 ponds were taken every two weeks from May to October, by means of common hydrobiological qualitative methods.In each pond snails were collected close to the bank, along 100 m of the water margin.In some ponds the snails were' eollected on several stations, because of the mosaic distribution of plant patches.142,931 specimens were collected altogether.The nomenclature of the species is acknowledged after Piechocki (1979).

CHARACTERISTICS OF THE STUDIED PONDS
The studied ponds were situated in three voivodships in South Poland: Katowice (eight single ponds and a complex of two ponds in Kutnia Nieborowska), CzQstochowa (two complexes of ponds in Ciasna), and Bielsko-Biala (two single ponds).The data on each pond or pond complex are given in Table 1.The plant species found there are listed in Table 2.
Two species occurring in very small numbers and in no more than one pond (Valvata piscinalis and Bithynia tentaculata) .wereconsidered acciden tal faunistic elements.The remaining 18 species occurred permanently and their constancy index value ranged from 15.4 to 100\.Three species (Lymnaea stagnalis, Gyraulus albus and Planorbarius corneus) occurred in all the studied ponds, being constant but not always dominating elements.The occurrences of species in particular ponds were variable.It was impossible to indicate the dominant species for all the ponds.The following species were founrl to predominate in particular ponds: It is noteworthy that from among three species of 100\ constancy only §xraulus albus was a dominant in some ponds, while the other two were more or less abundant, though none of them being the most numerous in any pond.A comparison of the gastropod faunae of two neighbouring fish pond complexes (Ciasna I and II) is worth of attention.Those ponds supplied with water from different sources and cultiv~ted in various way differed somehow also in respect of their gastropod faunae in which the same species occur~ ed in different proportions.
The results do not suggest any clear relation between the number of snail species and environmental conditions in the studied ponds.The malacofauna there seems connected neither with the area and depth of a pond nor with the type of bottom.However, human impact is the only factor that appears to affect the snail fauna qualitatively and quantitatively.In particular, mowing water weeds and the periodical drainage of ponds affect the snail fauna negatively, whereas liming has an opposite effect.In general, several years of such activities lead to a remarkable reduction of the number and diversity of snails.

DISCUSSION
The above results show that the snail fauna of th'e studied fish ponds is differentiated depending on pond.Thisis due to variable local conditions in particular ponds, especially the intens~ty of the fish production: the more intensive the pond cultivation the poorer the snail fauna.On the other hand no apparent relation between the snail fauna and the bottom type and area has been found.
The fact that the snail species number .found in the laziski I pond near Grodziec (0.8 ha.) was the same (11) as in the much larger (51 ha.) pond Maciek Wielki is in disagreement with the opinion of Boycott (1936) that more snail species occur ~n larger water reservoirs, owing to a greater possibility of introduction there than in smaller ones.On the contrary, the observation of Aho (1966) that the density of snail populations is greatest in ponds of the highest plant production and does not depend on vegetation diversity has been •confirmed by the present results.
The studied ponds seem in general to have their snail fauna richer than i:hat of the fish ponds in Golysz and Landek studied by Kownacka (1963), who found up to 5 -9 snail species per pond and 12 altogether, among them Lymnaea stagnalis and Gyraulus albus as dominants.Planorbis planorbis was absent in the ponds studied by Kownacka.It was probably due to the very soft water of those ponds.In Upper Silesia! however, P. planorbis occurred abundantly in 75 -100\ of water bodies of various kinds.