Abstract
A land-locked strain of chinook salmon was used to discriminate seawater preadaptive aspects of smoltification from development-associated aspects of smoltification. Liver total lipid and lipid class composition, as well as plasma concentrations of thyroxine (T 4), cortisol, and somatostatin-25 (sSS-25), the predominant form of somatostatin in the pancreas and plasma of salmonids, were measured in age 0 + and 1 + freshwater land-locked fall chinook salmon between March and May 1990. Liver total lipids in age 0 + fish displayed extensive seasonal fluctuation, cycling from high levels to low levels several times over the course of the sampling period. Springtime-associated changes in total lipids were primarily due to diminution of triacylglycerols. Plasma T 4 titers in the age 0 + fish also showed substantial seasonal variation with multiple peaks noted over the course of the sampling period. Changes in plasma T 4 were highly correlated with liver total lipid; seasonal high plasma T 4 levels were observed at the time of maximum lipid depletion. In age 1 + fish, plasma T 4 displayed a single peak in coincidence with elevated plasma cortisol levels. Plasma sSS-25 levels in age 1 + fish, measured during smoltification for the first time, also displayed seasonal variation; levels underwent changes generally opposite to those of T 4 in early spring. Dramatic increases in plasma T 4 were displayed by both age classes of fish when transferred from spring-fed hatchery water to lake water. Our results suggest that land-locked chinook salmon may experience multiple parr-smolt episodes during freshwater development and that springtime-associated and thyroid hormone-correlated lipid depletion occurs in the absence of the fish's exposure to seawater.
Published Version
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