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.