Homogeneous fish communities use intertidal salt marshes at high tide, despite environmental diversity: insights from a large-scale monitoring program on French coasts.

Salt marshes have dramatically declined in area throughout history and are still subject to numerous anthropogenic pressures. These habitats appear essential for certain fish species, which use them in high abundances. Some studies suggested that fish habitat use of salt marshes could greatly depend on the condition of the vegetation and notably its management. However, most of these studies were restricted to limited numbers of sites or samples, making it difficult to confirm these assumptions. To meet this challenge, a large-scale, standardized and temporally coordinated monitoring program has been set up on a network of 18 salt marshes distributed over the French coasts of the English Channel and the Atlantic, with the aim of describing fish communities and identifying potential driving environmental factors of fish diversity and abundances. Creeks were environmentally characterized and sampled for fish in three seasons in 2015, 2017, 2019 and 2021. Fish captures were greatly variable in abundances among space and time, poorly diversified (≤10 species per sample), and largely dominated by mullets, gobies and seabasses. The abundances of the main fish taxa in the captures were poorly explained by the environmental variables considered. Overall, only seafront and salinity might have conditioned the occurrence and abundance of some species across the network. Our results emphasize the crucial importance of long-term, large-scale monitoring initiatives to deal with the inherent variability of fish community composition in salt marshes, and the need for further research to better understand the utilization of this habitat by fish.