| Ash is part of it, but there is also digestible material which is not fully utilized. Its a multi-step process where food has to be broken down into simpler molecules and then metabolized. Digestion is technically the process of breaking down complex molecules into simple molecules. After digestion, the material must be metabolized. This is either through catabolism where simple molecules are turned into energy, or anabolism where simple molecules are recombined into complex molecules in the form of koi tissue (growth). It is possible to digest the food, but not metabolize it. This is just as wasteful as not digesting it at all.
Where does it all go? The predominant elements in feed are carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). What are the primary waste products? The predominant wastes are carbon dioxide (CO2) and ammonia (NH3) - carbon, hydrogen, oxygen and nitrogen. The fecal material is ash plus other feed components which made it through the gut without being digested and metabolized. Digestibility, in this context, refers to the ease with which the koi can digest AND metabolize the feed.
That is about the extent of my understanding. I do not know why some feed components are easier to break down into simple molecules and metabolize. Temperature surely plays a role. Feed rate probably plays a role as well. The more a fish eats, the less efficient is the feed utilization.
I raise a few seahorses, but am not very good at it. Seahorses only eat live food and seem to keep eating as long as something is available. Given an over-abundance of copepods or brine shrimp nauplii, the food passes through the gut so fast that the pods and brine shrimp are still alive when they come out the other end! Its seems like you have to minimize food availability in order to give the gut time to do its job.
-steve hopkins |