Influence of Hydrological and Physico-Chemical Variability on Length-Based Recruitment Signals of Abramis brama (Linnaeus, 1758) in the Lower Danube River (2021–2025)

This study investigates the population dynamics of the freshwater bream ( Abramis brama ) in the Lower Danube River between 2021 and 2025, focusing on growth parameters, mortality rates, length-based recruitment estimates, and the influence of hydrological and water physico-chemical factors. A total of 685 individuals were collected, with an average total length of 31–32 cm and a balanced sex ratio. Growth parameters estimated using the von Bertalanffy Growth Function (VBGF) revealed an asymptotic length (L∞) ranging from 39.9 cm (2021) to 55.7 cm (2024) and growth coefficients (k) between 0.80 and 1.40 year −1 . The total mortality (Z) varied from 2.19 to 5.24 year −1 , while the exploitation rate (E) reached a maximum of 0.73 in 2025, indicating increased fishing pressure. Length-based recruitment analyses showed a unimodal seasonal pattern, with peak recruitment occurring between June and October and maximum monthly values recorded in September 2025 (29.89%). Pearson correlations indicated that recruitment was positively related to water temperature (r = 0.65) and negatively to average water level (r = –0.63). Recruitment estimates are derived from length-frequency back-calculation and reflect proxies of cohort entry into the exploited stock rather than direct juvenile abundance. These results indicate a consistent seasonal pattern of cohort entry within the exploited component of the population and highlight the role of temperature and river discharge in modulating length-based recruitment signals under variable hydrological conditions.