Intra-annual dynamics of larval fish assemblages and their environmental associations in the central Southern Black Sea

Abstract

Larval fish assemblages are crucial indicators for monitoring marine ecosystem dynamics and the recruitment potential of fishery stocks. This study investigates the spatiotemporal dynamics of larval fish assemblages in the central Southern Black Sea (Sinop) and the environmental parameters shaping these patterns. An intra-annual monthly sampling survey identified 35 species across 20 families; the assemblage structure was dominated by Engraulis encrasicolus, Gobius niger, Sprattus sprattus, Mullus barbatus and Trachurus mediterraneus. Multivariate analyses revealed a clear temporal structuring of larval fish assemblages due to intra-annual variation. However, no significant spatial separation was observed among the stations. Distance-based Linear Models (DistLM) indicated that assemblage variability was primarily associated with salinity, chlorophyll a and temperature, as well as light transmission and current velocity. Species-specific responses of dominant taxa to environmental variables were examined using Generalized Additive Models (GAMs), revealing distinct environmental partitioning among species. Model results indicated that E. encrasicolus abundance was primarily associated with chlorophyll a and low salinity gradients, whereas G. niger exhibited a response to multiple variables, including low salinity, low light transmission, and a wide thermal range. Conversely, the distribution of S. sprattus was strictly limited by low temperatures and specific ranges of light transmission. These findings suggest that larval fish assemblages in the Southern Black Sea are structured by a complex combination of physical forcing (such as mesoscale hydrological features) and biological interactions, in which differentiated environmental responses facilitate species coexistence.