Coastal lagoons and adjacent marine areas form dynamic habitat mosaics, providing essential ecosystem services shaped by temporal and spatial fluctuations of local biotic and abiotic factors. The present study investigated the spatiotemporal variation in the abundance of Lithognathus mormyrus (Linnaeus, 1758) in Faro Lake (Capo Peloro Coastal Lagoon, Sicily), and two adjacent permanently or seasonally connected areas, the Strait of Messina and the Tyrrhenian Sea, respectively. The investigation was performed over two annual cycles (2022/2023 and 2024/2025), by combining monthly underwater visual census (UVC) data with in situ measurements of temperature and salinity. Generalized Linear Modelling (GLM) was applied to evaluate the environmental factors driving the abundance of L. mormyrus juveniles and adults in the three sampling sites (Faro Lake, Strait of Messina and Tyrrhenian Sea), fitting species abundance as response variable, and temperature and salinity as explanatory ones. A total of 10391 individuals were recorded across 48 UVCs performed in the three sites. Juveniles were more abundant during colder periods, especially in Faro Lake, whereas adults increased during warmer periods and were more associated with the adjacent marine sectors. Model selection based on AIC identified GLM with negative binomial error distribution as the most appropriate model for abundance data. The selected model showed a strong response to temperature and more heterogeneous salinity effects. Overall, the results highlighted that Faro Lake serves as an interconnected ecological mosaic rather than an isolated habitat, with environmental gradients shaping L. mormyrus distribution differently according to life stage. The marked segregation between juveniles and adults suggested the ecological relevance of Faro Lake as a key shallow habitat for early stages, while adjacent marine sectors appeared more exploited by adults during warmer periods. This study provides a non-invasive framework to evaluate the ecological roles of Mediterranean transitional ecosystems, offering baseline information for conservation and management.