Lipids, fatty acids, and their derivatives play a role in virtually every physiological process that occurs and for this reason dietary lipid composition and content represent a massive sector of overall nutrition. Nowhere is this more true than in finfish nutrition where lipid can exceed protein in the body composition of finfish, a testament to the physiological and energetic importance of this nutrient class (Tocher2003). Aside from physiological importance, lipids are indispensable energy sources, especially for finfish, which are not well-adapted to carbohydrate utilization.
Dietary protein and energy must be kept in proper balance because a deficiency or excess of dietary energy can reduce growth rates. Fish fed diets deficient in energy will metabolize more expensive dietary protein to meet energy requirements. Excess dietary energy can decrease protein intake and suppress growth.
finfish do not require carbohydrates in their diet, … complex carbohydrates cannot be digested and utilized efficiently by most finfish species. A general dichotomy exists in the carbohydrate digestive ability of warmwater omnivores and herbivores versus the inability of coolwater and coldwater carnivores, which lack the appropriate function necessary for digestion of carbohydrates.
For this reason, diets fed to these fish rarely contain more than 20% complex carbohydrate
What seems a standard 40% protein in carnivorous diets is adequate in my opinion. Its important to limit lipids (fat) if longevity is a goal. For a long time I agreed with you that carnivores couldn't use starch as an energy source. I used egg whites as a binder for this very reason. After some spirited discussion on the subject I had to test it. I fed a diet containing 20% starch certain it would fail. Using iodine as a reagent I tested the water and nothing. I was shocked at these findings. In fact warm water carnivorous species can use starch as energy.
Any of you that keep fish in tanks can test this. Take some water from the tank that you have been feeding a carnivorous fish food that contains starch, add iodine and see for yourself. If you see purple specs or the water turns purple starch is present.
