Long-distance migrations of catfishes in two large tropical rivers: The Ayeyarwady and Mekong rivers

Understanding fish migration ecology is crucial for fisheries management. In this study, trace element and strontium isotope ratios were analysed in otoliths (ear-stones) of four economically important catfish species (Pangasiidae): Pangasius krempfi and P. mekongensis in the Mekong River, and P. pangasius and Pangasius sp. in the Ayeyarwady River to understand their migratory behaviour. Our findings showed that all four catfish species exhibited long-distance migrations: specifically, growth occurred in marine and brackish environments but migrations to fresh water were undertaken for spawning (anadromy). We identified cyclical variation in otolith elemental concentrations, suggesting these catfishes either migrated repeatedly between brackish and marine environments, or remained in estuarine areas where water chemistry varied seasonally. Additionally, evidence of multiple migration strategies was found among these catfishes. Regardless of species or river system there was consistent evidence supporting long-distance migrations that extended up to 3000 km on a return trip between feeding (marine) and spawning grounds (freshwater). To maintain viable populations of these long-distance migratory catfishes, ensuring river-to-sea connectivity is essential for the completion of their life cycles. Local and regional management plans should be developed to regulate anthropogenic pressures (e.g. dams and fishing) along their migration routes and current or planned water infrastructure should address the requirement for unimpeded bi-directional movement.