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 growth, metabolic stores, and endocrinology of coho salmon. Treatments were as follows: warm-fed, warm-not fed, cold-fed, and cold-not fed during the winter (January-February). The following parameters were measured: length, weight, whole body lipid, liver glycogen, hepatosomatic index, and plasma levels of insulin, insulin-like growth factor-I (IGF-I), and thyroxine (T4). Warm-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 in all other treatments grew from 21.5 +/- 0.3 to approximately 32 g and showed depressed growth during January and February. During the winter, liver glycogen, hepatosomatic index, plasma insulin, and IGF-I were highly influenced by manipulations in rearing conditions, whereas whole body lipid and plasma T4 were less affected. Plasma insulin levels fluctuated dramatically (from 2 to 7 ng/ml) in the two cold-acclimated groups shortly after the change in temperature. In general, the plasma insulin levels of the warm-fed fish were the highest (8-9 ng/ml), those of the warm-not fed fish were the lowest (2-5 ng/ml), and those of the two cold-acclimated groups were more variable 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 found 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 relatively unaffected by manipulations in feeding and temperature compared with either insulin or IGF-I. These data suggest that the insulin, IGF-I, and thyroid axes are differentially regulated under changing seasonal and/or environmental conditions in yearling salmon.