The effect of low temperature and fasting during the winter on metabolic stores and endocrine physiology (Insulin, insulin-like growth factor-I and thyroxine) of coho salmon, Oncorhynchus kisutch

The objective of this study was to examine the effect of winter feeding and fasting at both high (10 degrees) and low (2.5 degrees) temperatures on gr owth, metabolic stores, and endocrinology of coho salmon. Treatments were a s follows: warm-fed, warm-not fed, cold-fed, and cold-not fed during the wi nter (January-February). The following parameters were measured: length, we ight, whole body lipid, liver glycogen, hepatosomatic index, and plasma lev els of insulin, insulin-like growth factor-I (IGF-I), and thyroxine (T4). W arm-fed fish grew continuously throughout the experiment from 21.5 +/- 0.3 to 43.4 +/- 1.4 g and were larger than fish in the other treatments. Fish i n all other treatments grew from 21.5 +/- 0.3 to approximately 32 g and sho wed depressed growth during January and February. During the winter, liver glycogen, hepatosomatic index, plasma insulin, and IGF-I were highly influe nced by manipulations in rearing conditions, whereas whole body lipid and p lasma T4 were less affected. Plasma insulin levels fluctuated dramatically (from 2 to 7 ng/ml) in the two cold-acclimated groups shortly after the cha nge in temperature. In general, the plasma insulin levels of the warm-fed f ish were the highest (8-9 ng/ml), those of the warm-not fed fish were the l owest (2-5 ng/ml), and those of the two cold-acclimated groups were more va riable but intermediate. In contrast, plasma IGF-I levels showed a decline with temperature decrease (from 9 to 5 ng/ml) and more gradual changes than insulin with the change in feeding. The highest plasma IGF-I levels were f ound in the warm-fed fish (10-15 ng/ml), the lowest levels were in the cold -not fed fish (4-5 ng/ml), and those of the warm-not fed and cold-fed fish were intermediate. During the treatment period the T4 levels were relativel y unaffected by manipulations in feeding and temperature compared with eith er insulin or IGF-I. These data suggest that the insulin, IGF-I, and thyroi d axes are differentially regulated under changing seasonal and/or environm ental conditions in yearling salmon. (C) 2001 Academic Press.