Abstract
This paper is an outline of the current state (2020-2023) of the fish fauna of the fluvio-maritime environment still in formation of the Danube Delta, the youngest territory of Romania. The lakes of the Rosu-Puiu complex have become more fluvial in the course of time, due to the deltaic area and the breaking of the connection with the sea, while the sea areas of the Musura and Sacalin Bays have become fluvio-maritime freshwater areas, very slightly saline in the dynamics of sediment deposition. The collection of fish fauna was carried out with two complementary sampling methods: Nordic/commercial gillnets for open water and electrofishing for vegetated shorelines. The results are expressed in relative abundance and biomass as Catch Per Unit of Fishing Effort (CPUE standardisation is done by a calculation system so that catches can be compared over time or between lakes or complexes), but also ecological indicators such as ecological significance, biodiversity index Shannon-Wiener, evenness and ecological status using ECOFRAME methods and Biological Integrity Indicator (IBI) are used for comparative/complementary characterization of ecological status. The IBI index was adapted for metapotamal and hypopotamal region (bream and ruffe/carp fish region) where missing or weak-represented are Salmonidae family, replaced by Percidae family motivated. With few exceptions the ecological status of fluvio-maritime area was between moderate and good status (native fish gene pool affected to the limit by numerical and areal reduction, recovery capacity is not affected).