Abstract
Lake Van is the largest lake in Türkiye (3.775 km 2 ) and the world’s largest soda lake. Pearl mullet ( Alburnus tarichi ), also known as the Van fish, is endemic to Lake Van and is accepted as an endangered species. Pearl mullet migrate from Lake Van to the freshwater streams in spring for spawning. This study aimed to investigate the responses of the antioxidant, osmoregulation, and energy metabolism and also the histological changes in pearl mullets before and after spawning. Fish were sampled in the mouth of the River Karasu for the pre-spawning and the upper part of the river for post-spawning. The size of fish (5 male and 5 female) did not differ significantly ( p > 0.05) between samplings. Na⁺, K⁺-ATPase, Mg-ATPase, and Ca-ATPase activities in post-spawning samples were significantly ( p < 0.05) higher in the osmoregulatory organs (gill or kidney). Likewise, activities of antioxidant enzymes in the liver increased ( p < 0.05) after spawning. Oppositely, Fe levels in the kidney were much higher ( p < 0.05) in pre-spawning compared to post-spawning. However, acetylcholinesterase activity in the muscle and brain did not change significantly ( p > 0.05) during the migration. Heat shock protein (HSP70 +) cells in tissues increased in fish after spawning. The cost of metabolic shift was reflected in the immediate energy reserves of the muscle and liver, as the levels declined significantly ( p < 0.05) after spawning. Additionally, histological changes were observed in the gills and liver during spawning migration, but not in the digestive tract. This study presented comprehensive data on the biological adaptation mechanisms of pearl mullet during anadromous migration, emphasising its vulnerability. Graphical Abstract