The investigations of dynamics and composition of plankton, bottom fauna and fish community have been done at three localities (1-inflow, 2-the middle, 3-dam) at the Barje water reservoir (Serbia, Yugoslavia) during 1997. During winter period diatomea Asterionella formosa was dominant at all the localities. At the same time among zooplankton species Rotatoria group was dominant, with the emphasis of Polyalthra dolicoptera at the locality 2, while Keratella cochlearis was dominant at the localities 1 and 3. The present species are oligo- and betamezosaprobionts. Among the investigated bottom fauna species Oligochaeta and Chironomidae were present. The number of oligochaeta was between 1665-7878 ind./square m. By qualitative analyses the domination of Tubificidae family was present. It is known that the presence of this group of organisms and it's high number indicate the organic pollution of ecosystem. 30 individuals of the bighead, (Hypophthalmichthys nobilis) with the average weight of 5.6 kg was captured by net.

During the period 1990-1997, the content of 11 trace metals (Fe, Cr, Co, Al, Mn, Cd, Cu, Ni, Pb, Zn, Sr) was studied in muscle and other organs of fish from DTD (Danube-Tisza-Danube) channel system (Yugoslavia). According to obtained and analysed data, the trace metal bioaccumulation can possibly act as enviromental indicator in biomonitoring. DTD (Danube-Tisza-Danube) Channel System can, therefore, be classified as the 3rd class, concerning analysed trace metal content.

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.

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.

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.

In order to carry out the program of protection and rational exploitation of Kovilj Marsh (Serbia, Yugoslavia), which is a part of special natural reserve called the Marsh of Kovilj and Petrovaradin, we have done an ecological analysis of the fish community. The research was conducted during the period of "low water", in the autumn 2000, on the following locations: Slajz, Tonja and Arkanj, and we have established the presence of 12 species from 6 different families. The saprobic index of the majority of bioindicative species shows that the water belongs to the beta-mesosaprobic type (the saprobic index according to Pantle-Buck is 2.1). The presence and abundance of certain ecological groups of fish is examined in relation to migration, type of food and type of substrate on which the fish lay their eggs. The results of this examination point to main features of certain localities in this floodplain, as well as to the necessity of taking appropriate measures of protection of this valuable water resource.

Since the year 2004 the water reservoir Kraljevac has been recognized as a Special nature reserve. Investigations of the water reservoir were done during the autumn (November) of 2009 at three localities. on the basis of the results obtained in this study Kraljevac reservoir water is moderately polluted and correspond to classes 2nd and 2nd-3rd based on physical, chemical, microbiological and hydrobiological parameters.