Relationship between Hydromorphological and Ecohydraulic Characteristics Flow Regime to Habitat Suitability Capoeta gracilis (Keyserling, 1861): Understanding Habitat–Population Dynamics Fishes and Ecological Flow Scenarios of Zarrin-Gol River

 Investigation of ecological, hydrological status and habitat Suitability of river is one of the key elements in river ecosystem management. In this regard, in the present study, in order to evaluate the ecological sustainable conditions in Zarrin-Gol River of Golestan Province, during stepwise calculations, echo-hydraulic and hydromorphocological analysis of the habitat of Capoeta gracilis (Keyserling, 1861) and ecological flow were studied estimation using hydrological and flow methods (Tennant and Aquatic base Flow) during the 10-year statistical period (2009-2019), hydraulic method (wetted perimeter) and habitat simulation model in different seasons of 2017 and 2018. For this purpose, physical simulation of habitat based on habitat suitability criteria curves was performed by field studies and sampling of hydraulic (depth, velocity and bed index) and ecological (aquatic communities) parameters. First, the geometry of the river model and cross sections were created using the topographic map of Zarrin-Gol River and ARC-GIS software. Then hydraulic simulation of river was performed with HEC-RAS software. In the next step, hydraulic and habitat simulations were performed in PHABSIM model. In this ecological-hydraulic model, the relationship between hydraulic flow and habitat needs of the target species was established using the Weighted Usable Area and the flow- suitability curve of the target species was extracted in different discharges in both adult and juvenile tow age group and new method of classifying the ecological flow regime was considered according to the indices of habitat suitability (water depth, flow velocity and bed). By elicitation of flow-habitat suitability curve, suitability river in terms of quantity and location was investigated. Based on the results, the optimal ecological flow regime to provide the ecological potential of habitat of index species is different by combining different methods for the studied periods during the year and accordingly, to maintain optimal conditions and protect the ecosystem and habitat components of biological communities, The required flow between 1.43 (in July-February) to 3.76 m3/s (in April), with an mean annual bio-flow of 1.74 m3/s (equivalent to 74.3% of  natural stream of the river) should be inside of Zarrin-Gol River. Also, the susceptibility of the Capoeta gracilis species to changes in velocity and bed materials is high and the weighted usable area increases with the depth remaining in optimal conditions. According to the results of this study, the distribution of habitat suitability along the Zarrin-Gol River, under the influence of geometric, hydraulic parameters and flow rate, indicates the favorable habitat conditions of the Capoeta gracilis species in the downstream area and poor conditions in the upstream area.