Often, in the studies for the evaluation of fish stocks, the abiotic and biotic habitat characteristics of the environment are very briefly described, but a good knowledge of this environment is indispensable for the ecological interpretation of the observations made on fish populations and then for the development of appropriate management measures. The aquatic ecosystems investigated and described in the 4PS / 2017 project are presented in this paper, with the name of: the Danube Study Area (ZS Danube), the Siret Study Area (ZS Siret), the Prut Study Area (ZS Prut). The ZS Danube stretches between the confluence with the Siret River and the Bendul Cobului area. This sector has about 26 km long with an average width of 800 m, such that the total area covered is 20.8 km2. ZS Siret stretches between Sendreni village and the confluence with the Danube. This sector of approx. 12 km long, with an average width of 100 m, has a total area of 1.2 km2. ZS Prut, stretches between Cîşliţa village and the confluence with the Danube. This sector of about 10 km long, with an average width of 50 m, covered a total area of 0.5 km2.