Residents and Transients—Fish Community Dynamics in a Highly Anthropised Tidal North Sea Estuary Across the Annual Cycle

Tidal estuaries provide important spawning, nursery and feeding habitats for a wide diversity of species and serve as transit areas for diadromous fishes. However, they are globally among the most impacted aquatic ecosystems, pressured by anthropogenic alterations and global change. Assessing local fish communities and their temporal dynamics is crucial to determine the ecological baseline of these ecosystems and to serve as an indicator of their ecological status. Here, we studied the fish community in a typical estuary of the Wadden Sea in central Europe, the strongly tidal and highly anthropised River Ems. Weekly bycatch samples from a commercial stow net fishery were analysed regarding fish community composition, individual length and biomass covering fourteen consecutive months. Thirty‐nine diadromous, estuarine, marine and freshwater species were recorded, amending previously reported numbers upward. Total fish abundance peaked in November 2020 and was lowest in the first half of the year 2021, corroborating previous observations of a cyclical intra‐annual community pattern in estuaries. The community structure changed significantly throughout the year, coinciding with variation in water temperature, dissolved oxygen concentrations, salinity and atmospheric pressure. During most months, European smelt ( Osmerus eperlanus ) and flounder ( Platichthys flesus ) dominated in abundance, and sand gobies ( Pomatoschistus spp.), European eel ( Anguilla anguilla ), river lamprey ( Lampetra fluviatilis ), three‐spined stickleback ( Gasterosteus aculeatus ) and European perch ( Perca fluviatilis ) occurred in substantial shares during single months. Some historically common species such as twaite shad ( Alosa fallax ), houting ( Coregonus cf. oxyrinchus ) and Atlantic salmon ( Salmo salar ) showed very low abundances. The non‐native Western tubenose goby ( Proterorhinus semilunaris ) was first recorded in this area. Intra‐annual changes in abundance and length composition of species of commercial or conservational interest, e.g., smelt and river lamprey, are discussed in detail. We show that high sampling resolution and complete annual coverage are crucial for a full picture of the fish community and conclude that the ecological status of the River Ems remains poor.