Investigation of the nutritional requirements of Australian snapper Pagrus auratus (Bloch & Schneider, 1801): effects of digestible energy content on utilization of digestible protein.
Summary
The balance of dietary protein and energy in animal diets can affect a number of production characteristics including feed intake, weight gain and carcass composition. This balance is often referred to as the dietary protein to energy ratio and it is known to vary between different species and as animals grow or age. Protein in fish feeds can come from sources such as fishmeal, poultry meal, lupin meal and gluten meals. All feed ingredients can act as a source of metabolic energy, however some sources are better than others. Most carnivorous fish are better at using protein and fat as energy sources than they are at using carbohydrates. A knowledge of a fishes requirement for dietary protein and how this requirement is affected by the level and source of dietary energy means that diets can be designed specifically for different species. The aim of this experiment was to determine the appropriate protein to energy ratio of manufactured diets for juvenile Australian snapper.
Juvenile snapper were fed a series of test diets in which the protein content was varied but the energy content was held constant. The first series had a constant energy level of 15 MJ per kilogram of diet, the second series had a constant energy level of 18 MJ per kilogram of diet and the third series had a constant energy level of 21 MJ per kilogram. Within each energy series we incorporated seven diets containing increasing levels of protein. The level of protein in diets ranged from 210g protein per kilogram of diet to 560g protein per kilogram of diet. In total 21 diets were made and each diet was fed to three tanks of snapper for 57 days. Change in weight and carcass composition as well as feed intake and feed conversion ratio were calculated at the end of the experiment to determine the optimum protein to energy ratio of diets for juvenile snapper.
According to the results, optimal weight gain and protein deposition in juvenile snapper was reached when diets had a protein to energy ratio of about 23 g of protein per mega-joule of energy. Knowledge of this ratio will be invaluable to manufacturers of aqua-feeds for juvenile Australian snapper, allowing them to adjust or modify their feed formulations for this species. These benefits will also flow to snapper farmers as diets with optimal protein to energy ratio’s will improve production outcomes.