Habitat-Driven Population Parameters Insights for European Eel Anguilla anguilla (Linnaeus, 1758) in Croatian Waters

Key parameters were estimated separately for the European eel, Anguilla anguilla (Linnaeus, 1758) subpopulations across freshwater and brackish environments within the Eastern Adriatic Eel Management Unit (EMU: EA). Between 2023 and 2024, European eel sampling was carried out at 23 locations along the Croatian coast (N = 678). Ages ranged from 1 to 13 years in freshwater and 1 to 8 years in brackish waters. The population structure was dominated by undifferentiated (42.8%) in freshwater and females (46.3%) in brackish habitats. Eels in freshwater exhibited a significantly higher b-coefficient in their length–weight relationship and better condition. Based on the otolith annuli patterns, age classes 3 to 5 predominated in both groups. A slightly longer asymptotic length and lower growth rate were found for the freshwater group compared to a shorter length and higher growth rate in the brackish habitat. Natural mortality was estimated at 0.174 ± 0.09 year−1 and 0.191 ± 0.133 year−1 for brackish and freshwater habitats, respectively. Total mortality was higher in freshwater (0.86 year−1) in comparison with the brackish (0.83 year−1) habitat. According to obtained results, more than 50% of eels aged three years are under exploitation. The maximum Yield per Recruit (Y/R) was 0.082 g/recruit in brackish at Lc = 44.88 cm, and a current Lc is 19.4 cm in the samples. In freshwater, Y/R peaked at 0.042 g/recruit at Lc = 55.49 and a current Lc 11.1 cm. It is recommended, following a precautionary approach to management, that the current fishing practices change in order to increase the minimum landing size (MLS), at least to 45 cm (above the current official MLS of 35 cm), to increase the fishing yield, and directly enhance population resilience. Findings emphasise the need for sustainable eel management policies considering different subpopulation parameters along the freshwater/brackish gradient at a small spatial scale when proposing and implementing stock management measures.