CHANGES IN LIPID-METABOLISM AND PLASMA-CONCENTRATIONS OF THYROXINE, CORTISOL, AND SOMATOSTATIN IN LAND-LOCKED CHINOOK SALMON, ONCORHYNCHUS-TSHAWYTSCHA, DURING SMOLTIFICATION
Dj. Cowley et al., CHANGES IN LIPID-METABOLISM AND PLASMA-CONCENTRATIONS OF THYROXINE, CORTISOL, AND SOMATOSTATIN IN LAND-LOCKED CHINOOK SALMON, ONCORHYNCHUS-TSHAWYTSCHA, DURING SMOLTIFICATION, Aquaculture, 121(1-3), 1994, pp. 147-155
A land-locked strain of chinook salmon was used to discriminate seawat
er preadaptive aspects of smoltification from development-associated a
spects of smoltification. Liver total lipid and lipid class compositio
n, as well as plasma concentrations of thyroxine (T4), cortisol, and s
omatostatin-25 (sSS-25), the predominant form of somatostatin in the p
ancreas and plasma of salmonids, were measured in age 0+ and 1+ freshw
ater 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 T4 titers
in the age 0+ fish also showed substantial seasonal variation with mu
ltiple peaks noted over the course of the sampling period. Changes in
plasma T4 were highly correlated with liver total lipid; seasonal high
plasma T4 levels were observed at the time of maximum lipid depletion
. In age 1+ fish, plasma T4 displayed a single peak in coincidence wit
h elevated plasma cortisol levels. Plasma sSS-25 levels in age 1+ fish
, measured during smoltification for the first time, also displayed se
asonal variation; levels underwent changes generally opposite to those
of T4 in early spring. Dramatic increases in plasma T4 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 sal
mon may experience multiple parr-smolt episodes during freshwater deve
lopment and that springtime-associated and thyroid hormone-correlated
lipid depletion occurs in the absence of the fish's exposure to seawat
er.