Tidal dynamics shape early-stage fish assemblages in the mouth region of a temperate estuary, South Africa

Estuarine nursery habitats underpin the productivity of coastal fisheries worldwide, providing refuge, feeding grounds and conditions critical for survival during the most vulnerable stages of the fish life cycle. Yet the tidal mechanisms regulating larval fish access to these habitats remain poorly understood, particularly the relative contribution of spring and neap tidal phases to recruitment into warm temperate South African estuaries. Using replicated nocturnal plankton tows across spring and neap tidal cycles at the mouth of the Swartkops Estuary, results show that tidal phase exerts a significant influence on the density, diversity and community composition of ingressing larval fishes. Spring tides yielded twice the larval density (172 vs. 65.60 per 100 m 3 ) and nearly double the species richness (52 vs. 28 species) of neap tides, with Soleidae, Gobiidae and Blenniidae dominating across both tide states. These differences are consistent with greater tidal forcing enhancing current velocities, promoting water column mixing, and strengthening salinity gradients that act as olfactory cues for larval recruitment. Developmental stages also differed between tidal phases, with postflexion larvae dominating spring tide catches and preflexion larvae more prevalent during neap tides. These findings demonstrate that tidal phase is a critical structuring force in estuarine recruitment dynamics, with direct implications for larval fish monitoring and the management of estuarine nursery habitats critical to the sustainability of coastal fisheries.