The protected natural goods of the swamp Zasavica is situated in the north part of Macva (Serbia, Yugoslavia). It is parallel to the flow of the Sava river, which leads to the fact that it used to be the riverbed of the Sava and also of the Drina river. It is interesting for its variety of the plant and animal species. The investigations have been done during autumn period 1997, when the investigated groups were the most heterogeneous. It included algae, Rotatoria,Cladocera, Copepoda and Hydracarina. Among algae, Bacillariophyta were dominant. Among all the investigated groups Rotatoria were the most heterogenous, with great number of the species that are rarely present. Also, some Cladocera species were rare. The estimation of the water quality is given according to the bioindicator species.

Biosystematic research of fish population from the river Nisava (Serbia, Yugoslavia) is placed from the mouth of Jerma river and Nisava river to the mouth of Nisava river and Juzna Morava river (Serbia, Yugoslavia). The next part of the research is about determining the methodology for counting of particular fish species in the river Nisava. Hematological analyses of some species were done at the same time in order to establish the relation between water quality and number of erythrocytes, leucocytes and quantity of haemoglobin. Two autochthonous and allochthonous species were analyzed. The method used in this research are explained as well.

Amplified and accelerated eutrophication process is a characteritic of a whole variety of small and medium-sized water-bodies, such as channel systems, microreservoirs, ponds, lakes and bank regions of many even bigger ecosystems. During the process, in relatively short period of time, the upper capacity limit of a certain biotop is far too exceeded, which leads to reduced ability of ecosystem's autopurification. As a consequence of such degradation, ecosystem faces a serious discrepancy between members of a biocenosis e.g. in ration producers/consumers/decomposers, which, in the end, causes destruction, e.g. dystrophy of the hydroecosystem. The introduction of diverse fish communities, can, therefore, be one of the basic measures for enabling a sustainable trophic stability, in a sense of keeping a dynamic balance. This measure includes carefully planed fish community - it should consist of three basic components of a trophic food web (planktivore and macrophytovore, bentivore and carnivore). Apart from enabling certain biodiversity considering particular fish species (both quantitative and qualitative), care must be inevitably taken of age structure, as well as fish stock and fishing dynamic, concerning the number, quantity and age.

Mathematical modles and adequate investigation works enables successful water quality modeling. These models includes all relevant abiotic components (concentration of dissolved oxygen, temperature regimes and temperature stratification, concentration of organic and inorganic components of physical-chemical water conditions), while the biotic components are present only with phyto- and zooplankton. In new water reservoir, ichthyofauna mostly undergo mancontrolled successions, starting with planned stocking a water reservoir with fish. The paper presents analytical approach for mathematical modeling of ichthyofauna and application of Lotka - Volterr's equations.

In this paper are presented autochthonous and all other alochthonous famous fishes in Lake Ohrid and its catchement area (The Former Yugoslav Republic of Macedonia). Introduction of new species in lakes and rivers in the Former Yugoslav Republic of Macedonia is prohibited, especially in Lake Ohrid and catchement area. The most probable period and possible way of introduction are presented and the reason of that introduction, as well. It is necessary, in the future, more attention to be paid on Lake Ohrid protection from introduction of new species, which may cause big changes in Lake Ohrid dynamics.

During the period May - September 2000, the structure of quality and quantity of Oligochaeta community was investigated on three locations of "Kapetanski rit" fish pond (Serbia, Yugoslavia). At same time were determined some physical and chemical parameters of water temperature, pH value and concentration of dissolved oxygen. In that period of time, the location of Divlje jezero there was no presence of Oligochaeta community. The location of Novo jezero was characterized by 6 species from two families - Naididae and Tubificidae, and the location of Veliko jezero was characterized by only three species. The results from Veliko and Novo jezero show differences in proportional presence and quantity characteristics.

The river Timok with its watershed area is one of very imperilled running water on the territory of Serbia (Yugoslavia). By several years investigations of colonies bottoms (phytobenthos, microzoobenthos and macrozoobenthos) and plankton we have found the following: great diversity and specific phytobenthos and bottom fauna (frequency of certain taxa is considerably larger in relation to the other areas, and new taxa have been recorded). Stated specific structure of phytobenthos and bottom fauna have considerably reflected to right estimation of water quality of tested area.

In the period from May to August in 1996, the samples for hydro-biological and inchthyological from the following localities were collected: the Klaicka river, the Radevacka river, the Sumanka river and the Lipovacka river (Serbia, Yugoslavia). The water quality was determined n the basis of bioindicator values and the abundance of particular species of phytoplankton, zooplankton, bottom fauna and fish community. According to the portion of plankton organisms, the saprobic index for the Klaicka river was between 2.01 and 2.57, for the Radevacka river it was between 2.83 and 2.93, for the Sumanka river between 2.59-2.78 and for the Lipovacka river this index had a value between 2.72 and 2.83. Among the botton fauna, 10 zoobenthic groups were determined. The highest frequency and abundance was registered in the groups of Ephemeroptera and Trichoptera, and the water quality is between oligo- o-beta-mesosaprobic (the Lipovacka river), -beta-mesosaprobic (the Klaicka river), o-beta to alfa-mesosaprobic (the Radevacka river) and beta to alfa-mesosaprobic (the Sumanka river). The ichthyofauna of the Klaicka and Sumanka rivers include the species like southern barbel, chub and spirlin. Basing on the fact, the water quality was determined as beta-mesosaprobic (1.65). The majority of results point to the satisfactory biological quality of water in this hydroecosystem, which is, therefore, considered to be one of the potential sources of water supply.

A complex ichthyological analysis of the individual sections of the DTD (Danube-Tisza-Danube)-Backa canal system (Serbia, Yugoslavia) performed during 1996 vegetation period 23 fish species from 7 families. On the basis of the occurrence and abundance of indicators and limnosaprobic parameter, water quality of the analyzed canal sections was found to be beta-mesosaprobic index ranging from 2.15 to 2.21. The obtained results show that water quality of this DTD (Danube-Tisza-Danube) canal sections (Bajski Kanal, Odzaci-Sombor, Vrbas-Bezdan and Novi Sad-Savino Selo canals) corresponds to its purpose. The fish community representing an important factor of a whole aquatic biocenose, in addition to its undoubtedly indicator values also representing an active factor in the process of selfpurification and quality preservation of whole hydrosystem.

The investigations have been done in Krstonosica shaft at 3 points, in February, May and October 1995, and in January and April 1996. They included algae, Rotatoria, Copepoda, Cladocera, and Hydracarina. The species from divisio Bacillariophyta were presented most among the algal species, with 56 species, then from divisio Chlorophyta, with 26 taxons, Euglenophyta 11, Cyanobacteria 9, Pyrrophyta 3, Xanthophyta 2 and Chrysophyta 1. Among animal groups the highest number belonged to the Rotatoria group, 111 taxons have been determined. The highest number was found during autumn season (86), then in spring (73), and only 14 species in winter. The species that mostly prevailed were phytophyl species. The Cladocera group was present with 18 taxons. The phytophyl species from overgrown plants again prevailed. The Copepoda group was present with 10 species and the phytophyl species were prevailed. According to saprobiological characteristics the greatest number belongs to beta-mezosaprobionts, then oligosaprobionts. A small number of species indicates to the eutrophication process and organic pollution of water.