ROLE OF ADIPOSITY IN FOOD-INTAKE CONTROL OF JUVENILE CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA)

Experiments were conducted re, determine if adiposity affected feed in take in juvenile chinook salmon (Oncorhynchus tshawytscha) with differ ent nutritional histories. Fry were fed high-fat (23%) or low-fat (3%) diets at high (satiation) and low (one-half satiation) ration levels for 7 months before the start of the intake experiment. This pre-treat ment produced fish averaging 22 g with 11.3% (high-fat diet) and 5.4%, (low-fat diet) body fat when fed to satiation or 11 g with 7.0% (high -fat diet) and 3.3% (low-fat diet) body fat when fed at one-half satia tion. Experiment 1 had a 2 x 2 factorial design where duplicate groups of 20 fish from the high-ration groups (22 g) were fed high- (16%) or low- (4%) fat diets twice daily to satiation 6 days/wk for 3 weeks. D aily feed intake was recorded. The same protocol was used in experimen t 2 on fish (40 fish per tank) from the smaller low-ration groups (11 g). Feed intakes on day 1, cumulative feed intakes after 21 days and p lasma levels of insulin and insulin-like growth factor 1 (IGF-1) were compared using two-way ANOVA with initial whole body fat and dietary f at as the independent variables. In both experiments, high body fat le d to significantly lower feed intake on day 1 and after 21 days of fee ding. High dietary fat levels caused greater intake on day 1, but by t he end of the experiments, this effect was not significant, suggesting some adaptation to the diets. Insulin in plasma showed greater respon se to dietary fat (high-fat diets causing higher insulin levels), wher eas IGF-1 responded more to body fat level (high body fat led to highe r IGF-1 levels). Our results show that in both fast-and slow-growing j uvenile chinook salmon, adiposity plays a role in regulation of feed i ntake and that adiposity appears to interact with IGF-1. Dietary fat l evels had transient effects on intake, but plasma insulin levels consi stently reflected the dietary fat levels. An important implication of our findings is that food intake, and possibly growth, could be retard ed if a feeding regime results in high body fat levels. (C) 1997 Elsev ier Science Inc.