Changes in Keys-Willmer Cell Numbers in the Gills of Steelhead Trout during Smoltification

Abstract
Steelhead trout (Salmo gairdneri) were monitored from February to November 1981 for Keys-Willmer cell (filament and lamellae) counts, plasma Na+ and K+, body coloring, and condition factor. The data indicated that the fish experienced a pre-smolt peak in filament cell count followed by changes in condition factor, body color, and Na+ level. This was followed by a more significant peak in filament cells with a corresponding decrease in lamellae cells. Because no increase in salinity was experienced, cell counts reflected a return to a freshwater physiology.