Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 143

Energy is obtained from the metabolism of protein, lipid, and carbohydrates. Fish are more efficient at using protein as an energy source than birds or mammals, since ammonia synthesis does not require energy, unlike uric acid and urea (Wright and Land 1998). However, protein is very costly and the resulting ammonia excretion can be problematic for water quality. Carbohydrates may serve as an energy source either via endogenous metabolism (e.g. absorbed glucose) or via microbial fermentation producing volatile fatty acids, but the ability to use carbohydrates varies greatly by species. Carbohydrates are generally less preferred as an energy source for carnivores compared to omnivores and herbivores, and in marine fish compared to freshwater fish. Lipids are often a preferred energy source in commercial fish feeds, as they can spare the cost of protein and are used efficiently by fish. Efforts in commercial aquaculture to use lipids as an energy source and spare protein for muscle accretion have resulted in significant improvements in growth efficiency of aquaculture species (Hixson 2014). However, there is a limit to this effect; excessive energy relative to protein can reduce total feed intake (and so limit intake of essential nutrients) and can increase body lipid deposition, which may not be optimal for fish health.

The energy of a food item may be defined based on different types of measurement. Gross energy (GE) is the total energy released upon combustion of a food item and does not necessarily represent the amount of energy available to the animal. However, this is the easiest measure of energy to determine. On average, GE values for carbohydrate, protein, and lipids for fish are 4.11, 5.64, and 9.44 kcal/g, respectively (NRC 2011). Digestible energy (DE) is based on the amount of energy in a food minus the amount of energy excreted in feces after that food is consumed and is generally 10–30% less than GE (NRC 2011). Metabolizable energy (ME) is based on the digestible energy of a food minus the energy to excrete any waste products via urinary and gill losses and generally reflects another 3–6% loss of energy (NRC 2011). When evaluating the energy needs or status of an animal, it is critical that the same units are considered. For example, if the requirements are predicted in units of ME, then the diet items should be considered in similar units or adjusted based on the assumptions described above.