Osmoregulatory changes accompanying smoltification in coho salmon

Abstract During the spring smoltification of the coho salmon ( Oncorhynchus kisutch ), alterations in osmoregulatory function occur in the urinary bladder, intestine and opercular membrane. Na and Cl reabsorption by the urinary bladder of seawater-adapted fish, initially at freshwater levels, is abolished during the period of smoltification. Intestinal fluid absorption of freshwater-adapted fish increases to seawater levels. The abundance of mitochondria-rich “chloride cells” in freshwater-adapted fish opercular membranes increases. All of these changes occur some weeks after the plasma thyroxine surge and coincide with increased brancial Na,K-ATPase activity. These data suggest that the appropriate time for seawater entry or placement of hatchery-reared coho salmon may be several weeks after the new moon-related thyroxine peak.