Over the years, to maintain navigation along the Danube, the implementation of a series of hydrotechnical works was imposed in order to ensure the continuity of economic activities. Between 2011 and 2014, one such project was built in the vicinity of the county of Braila, on a secondary branch of the Danube - The Caleia branch. The main goal was to redistribute water discharge from the secondary channel to the main branch by construction of a bottom sill. The bottom sill and subsequent discharge redistribution led to hydrodynamic alterations caused by increased water current velocities. These modifications of the hydrodynamic regime may affect the migration of anadromous sturgeon species that swim from the Black Sea to spawn upstream, therefore making the monitoring of the impact paramount. Considering the fact that the anadromous migrating species are internationally protected species, by a series of agreements and conventions, and that the stocks of the species are still in decline, a complex monitoring program was implemented in the affected area. This study presents important information regarding the site utilization during spawning migration and an evaluation of the behavior of sturgeons tagged with ultrasonic transmitters that migrated upstream over the bottom sill. During the monitoring period, the studies show that five sturgeons passed the bottom sill during their upstream migration, swimming against maximum water current velocities of 2.5 m/s.

Evaluating the state of ichthyofauna at both the Lower Danube level and at the national level contains knowledge gaps regarding species dynamics, with the most complex studies regarding species composition being undertaken more than 50 years ago. Over time, the Danube River - an important navigation route that connects Western Europe with Asia - has suffered a series of anthropogenic interventions that led to river discharge regularization, interruptions of longitudinal/latitudinal connectivity and reductions in floodplain area. These anthropogenic activities may negatively impact suitable fish habitats leading to demographical effects. The Danube is regarded as a river with high species richness that provides a source of income for the local population by the practice of commercial fishing. The area of interest for this study was selected taking into account the fact that, in the last decade, it was subject to hydrotechnical works that aim to redistribute the river discharge to improve navigation conditions. The ichthyofauna population dynamics is analyzed using an 8 year-long dataset that includes baseline data before the project started and a monitoring period after the project ended. The results indicate the presence of 38 fish species (excluding anadromous fish species – sturgeons and shads). The identified fish species are classified in two categories: 1) species of commercial interest and 2) species of Community interest. This study provides evidence that the high mobility capacity of the fish species is the main factor affecting species dynamics as support of the national efforts in action to stop the degradation of aquatic habitats and biodiversity, in response to goal 15 “Life Earth” of the UN 2030 AGENDA for sustainable development.