Fish density in NE Atlantic saltmarsh is three-fold higher than unvegetated shores

Atlantic saltmarshes are widely recognised as important and productive estuarine habitats, yet their role in supporting fish populations in northern Europe remains understudied. We used fyke and seine nets to assess fish assemblages at 14 sites across two estuaries in Carmarthen Bay, South Wales. Sampling took place in both saltmarshes and unvegetated estuarine shores monthly from October 2023 to September 2024. The age composition of the catch was predominantly juvenile (83%). Fish density was significantly higher in saltmarshes (5972 ind/ha) compared to unvegetated shores (1806 ind/ha; p < 0.001). Six species were present in saltmarshes across all seasons, including Atlantic herring ( Clupea herrangu s) and European flounder ( Platichthys flesus ) not previously documented year-round in UK saltmarshes. Several species, were significantly more likely to be caught within saltmarsh, including lesser sandeel ( Ammodytes tobianus ) and grey mullet species ( Chelon ramada, Chelon labrosus, Chelon aurata ), while no species were significantly more likely to be caught in unvegetated shores, demonstrating saltmarshes enhanced nursery function. In total, 19 fish species were recorded in saltmarshes, representing the highest species richness documented for UK saltmarshes. Clear seasonal shifts in community composition were observed, with peaks in European flounder ( Platichthys flesus ) in May, European bass ( Dicentrarchus labrax ) in June and Atlantic herring in July, likely reflecting staggered recruitment strategies that minimise competition during early life stages. These findings provide the first year-round assessment of saltmarsh fish assemblages on the west coast of the UK and highlight the ecological value of saltmarshes in supporting coastal fish communities. • The highest fish diversity in saltmarsh (n=19) was recorded in South Wales, UK • Fish density in saltmarsh in Wales, UK was three-fold higher than unvegetated habitat • Variation in seasonality: dominant fish species varied each month of spring-summer • Atlantic saltmarsh plays an important nursery role: 83% of fish were juvenile