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A 2 × 2 factorial experiment was conducted to evaluate the effects of commercial diets (32% protein, 3% fat [32/3 diet] or 36% protein, 8% fat [36/8 diet]; Malick Aquafeed, Inc.) and the influence of strain (U.S. Department of Agriculture [USDA] 103 or Norris) on performance variables and mitochondrial respiratory chain enzyme activities in the liver, muscle, and intestine of channel catfish Ictalurus punctatus. Juvenile channel catfish (average weight = 29.71 and 38.79 g for USDA 103 and Norris strains, respectively) were stocked into sixteen 152‐L aquaria at a density of 10 fish/aquarium. Four aquaria were used for each dietary treatment. Fish were fed twice daily to apparent satiation for 16 weeks. Results indicated that diet and strain had significant (P < 0.05) main effects on weight gain, feed consumption, feed efficiency, and specific growth rate. Diet composition had a significant main effect on condition factor, regardless of strain. There was a significant (P < 0.0007) fish strain × diet composition interaction effect on feed efficiency. Regardless of strain, fish fed the 36/8 diet formula had higher activities for respiratory enzyme complexes I, II, III, and IV in the liver, intestine, and muscle than fish fed the 32/3 diet. The strain effect was significant but not consistent for complex I and IV activities in the liver and muscle and complex II activities in the liver and intestine. The fish strain × diet composition interaction effect was significant for complex I activity in the liver and for complex II and IV activities in the muscle. These data demonstrate that the USDA 103 strain of channel catfish appears to possess superior traits in growth performance compared with the Norris strain. Our data indicate that diet has an important effect on channel catfish production and that a commercial 36/8 diet produces enhanced growth performance and mitochondrial enzyme activities relative to a 32/3 diet.

A 2 × 3 factorial study was conducted to evaluate the effects of dietary lipid level on mitochondrial gene expression in mixed sex rainbow trout Oncorhynchus mykiss. Practical diets with a fixed crude protein content of 42%, formulated to contain 10% (42/10), 20% (42/20) and 30% (42/30) dietary lipid, were fed to triplicate groups of either low-feed efficient (F129; mean ± s.d. = 105.67 ± 3.04 g initial average mass) or high-feed efficient (F134; mean ± s.d. = 97.86 ± 4.02 g) families of fish, to apparent satiety, twice per day, for 108 days. At the end of the experiment, diets 42/20 and 42/30 led to similar fish condition factors, which were higher than that observed with diet 42/10 (P < 0.05). F134 fish fed diet 42/10 showed the highest hepato-somatic index, while there was no significant difference among all the other treatments (P < 0.05). When the group of F134 fish fed diet 42/10 was used as the calibrator for gene expression analysis, the five genes selected for their involvement in lipid metabolism (complex I-nd1, complex III-cytb, complex IV-cox1, complex IV-cox2 and complex V-atp6) were up-regulated in the muscle and down-regulated in both the liver and the intestine. There was a significant family × diet interaction regarding nd1, cox2 and atp6 in the liver; nd1, cytb, cox1, cox2 and atp6 in the intestine, and nd1, cytb, cox1, cox2 and atp6 in the muscle (P < 0.05). The overall results of this study constitute basic information for the understanding of molecular mechanisms of lipid metabolism at the mitochondrial level in fishes.