A novel pattern of smoltification in the most anadromous salmonid: pink salmon (Oncorhynchus gorbuscha)

Pink salmon (Oncorhynchus gorbuscha) enter seawater earlier and smaller (0.2 g) than any other salmonid following a two-stage salinity tolerance process initiated around yolk-sac absorption and completed with seawater entry. For the first time, this two-stage ontogeny of salinity tolerance was characterized by either holding posthatch pink salmon in fresh water or transferring them to seawater every 2 weeks. A window of salinity tolerance around yolk-sac absorption was evidenced by a period of zero morbidity in seawater compared with 100% morbidity for newly hatched alevins and 25% morbidity for fry (∼0.2–0.3 g). Increased hypo-osmoregulatory ability at the time of yolk-sac absorption was indicated in fish held in fresh water under constant photoperiod (12 h light : 12 h dark) and temperature (5 °C) by a switch from catabolic to anabolic growth, increased gill Na + K + -ATPase activity and α-1b/α-1a isoform expression, and a plateau in whole-body water content, implying that pink salmon go through a form of smoltification. A large increase in whole-body [Na + ] observed in fresh water at yolk-sac absorption may represent a unique strategy for maintaining water balance once fish enter seawater.