Pearl mullet (Alburnus tarichi Gilldenstalt, 1814) is an endemic fish species living in the Lake Van basin. Lakes Van and Ercek are soda lakes which have alkaline and brackish properties. Pearl mullet migrate to freshwater streams surrounding these lakes for spawning and return back to the lake environment after reproduction. Migration takes places from the lakes into freshwater streams and the fish are exposed to 2 different environmental conditions. The aim of this study was to assess erythrocyte osmotic fragility, a test providing information about the membrane stability of erythrocytes and the physiological strength of erythrocytes against physicochemical stress, during spawning migration. For this purpose, blood samples were collected into EDTA containing tubes from fish sampled from Lake Ercek and a freshwater environment pouring into the lake (Memedik Stream). Erythrocyte osmotic fragility was performed on the collected blood samples. The results indicated a significant increase in erythrocyte osmotic fragility at 0.4 and 0.5% NaCl concentrations for freshwater stream samples. It can be concluded that the increased erythrocyte osmotic fragility in the freshwater environment was causally related to physicochemical changes. Further research can be conducted on physiological mechanisms that cause this observed change in erythrocyte fragility which is an indicator of membrane stability.