Introduction: Freshwater ecosystems are valuable ecosystems that support a significant biodiversity, from microscopic to macroscopic organisms, in these dynamic habitats. Streams and rivers are freshwater environments that are essential for maintaining a variety of aquatic organisms and ecological balance. Fishes of these ecosystem are important part of the diverse range of organisms that live in these environments. Fish, especially those found in tropical rivers, play an important role in the entire ecosystem. Aquatic organisms, including fish, require a number of physical and chemical factors for their survival and successful reproductive cycle in an aquatic ecosystem. The Sistan basin, located in southeastern Iran, is separated from all internal waters of Iran due to its geographical location and is shared with the Hirmand basin with Afghanistan. The Anjak fish is exclusively native to the Sistan basin, in the waters of the east of the country. The triple Hamun wetlands and the Chah Nimeh reservoirs of Sistan, including Chah Nimeh 1, 2, 3, and 4, with an area of approximately 15,000 hectares and a volume of approximately 1.5 billion cubic meters of fresh water, are one of the habitats of the Anjak fish, which has become rare in this habitat due to recent droughts.

Materials and Methods: In the present study, to investigate the habitat preference of Anjak in the Hamun Dam (Sistan Basin), sampling was conducted from 6 stations with 3 replicates each, for a total of 18 stations. A total of 9 physical and chemical variables of the water, including pH, water depth, river width, temperature, electrical conductivity (EC), Altitude, bed slope, and total dissolved solids, were measured and recorded simultaneously with sampling at each station. Also, the Sodium absorption rate (SAR), Sodium, Magnesium, and Calcium ions were recorded.

Results and Discussion: The results showed that the suitability of the studied species increased with an increase in all the factors studied except water depth, altitude, and pH. Increasing EC and TDS values led to a decrease in the suitability values of the studied species. In other words, the highest suitability was at a certain value and no significant fluctuation was observed. The results showed that the EC and TDS with values of 0.99 had the highest SI and Na absorbed in water with a value of 0.74 had the lowest SI index among the studied factors. The calculation of the total habitat suitability index for Anjak showed that the total value was 0.82. The results also showed that all the habitat variables studied had a significant relationship and high correlation with the suitability indices of habitat characteristics. According to the obtained results, it can be stated that the floodplain habitat of the Hamun (Adimi) wetland is a suitable habitat for the Anjak. Our finding showed with increasing depth, the suitability of the habitat for the studied species increases, given that Anjak has a large body height and depth, therefore such a mechanism is efficient and suitable for deeper parts of the habitat. Species with greater body depth prefer deeper parts of the habitat, which prevents body washing in aquatic environments and consumes less energy to cope with the water velocity.