Impact of Environmental Variations in the Waters of the Shatt al-Arab on Fish Assemblages : The Northern Reach as a Case Study

Abstract

This study investigates the effects of environmental variations, particularly salinity, on the physicochemical properties of water and their implications for the composition and distribution of fish assemblages in the northern reach of the Shatt al-Arab during the period 2024–2025. Several key environmental variables with direct effects on fish were measured, including water temperature, salinity, pH, and dissolved oxygen. The current findings were also compared with those reported in previous studies conducted in the same area.

The results revealed clear seasonal variation, with elevated salinity levels recorded during summer and autumn and an overall increasing trend compared with studies from previous years. This increase was attributed to the water-management policies of neighboring countries, reduced freshwater discharges, and the increasing influence of tidal intrusion. pH values were broadly consistent with those reported in most previous studies, whereas a relative decline in dissolved oxygen concentrations was recorded, indicating a gradual deterioration in water quality.

These environmental changes were reflected in the fish assemblages, with an increase in marine and introduced species accompanied by a decline in some sensitive native species. A total of 27 fish species belonging to several major families were recorded within the study area, most notably Cyprinidae, Cichlidae, and Mugilidae, demonstrating a clear biological response to increasing environmental pressures.