Hyperosmolality tolerance of freshwater largemouth bass Micropterus salmoides to brackish in early ontogenetic stages of eggs, embryos, and yolk-sac larvae

Salinity is recognized as a pivotal factor limiting the migration of freshwater fish to brackish environments. The largemouth bass (LMB, Micropterus salmoides), a globally translocated freshwater fish, exhibits estuarine distribution, yet its hyperosmoregulatory capacity during early ontogenetic stages remains inadequately understood. To investigate the impact of freshwater salinization, a series of experiments were conducted in Guangzhou and Foshan, China. Results reveal that freshwater-activated mature eggs and naturally fertilized oocytes maintained nearly identical osmotic homeostasis, with diameters of 1.38±0.07 mm and 1.37±0.05 mm, respectively. Furthermore, both exhibited peak water excretion at a salinity of 15.0. Remarkably, a reduction in water permeability was observed in hyperosmotic environments. Spontaneous hatching rates increased from 27.5%±14.4% in the 1.0 group to 75.1%±12.0% in the 6.0 group under fluctuating temperature conditions. Yolk-sac LMB larvae consistently reduced survival time from 12.5 d at 1.0 to 50.7±2.1 min at salinity of 35.0. Similarly, more developed larvae also experienced a decrease in survival time. Logistic regression models fitting lethal time with salinity indicated a sharp decrease between 10.0 and 20.0. These findings offer practical insights for predicting distribution patterns and enhancing aquaculture technology for LMB. Moreover, they may contribute theoretically to the broader understanding of the osmoregulatory mechanisms of freshwater fish.