Changes in environmental salinity during the life of Pangasius krempfi in the Mekong Delta (Vietnam) estimated from otolith Sr : Ca ratios

Pangasius krempfi is a commercially important catfish in the Mekong River and is believed to migrate along the Mekong River basin. To verify this migration, elemental concentrations were measured in the water and in otoliths to infer the salinity of the water through the fish's lifetime. In 2017, eight element concentrations were measured along the Mekong Delta using solution-based inductively coupled plasma mass spectrometry (ICP-MS). Concentrations of Sr, Li and Rb were strongly and positively correlated with salinity. Otoliths were taken from P. krempfi caught in the brackish waters of the lower Mekong Delta and seven element : Ca ratios were measured from the core to the otolith edge using laser ablation ICP-MS. The Sr : Ca, Ba : Ca, P : Ca and Mn : Ca ratios varied through the lifetime of the fish, but only Sr : Ca was suitable for estimating ambient salinity. The Sr : Ca profiles in otoliths were analysed and significantly correlated between individuals, with all fish hatched in water with very low levels of salinity, indicating a single freshwater spawning ground, and then living in waters with higher salinity, with two types of migration behaviour. Some individuals may return to low-salinity waters when older. These conclusions were supported by the Ba : Ca and Mn : Ca ratios. These migration patterns may have implications for fishery management.