Protein nutrition in the juvenile Australian short-finned eel (Anguilla australis australis Richardson)

Little is known about the nutrition of the Australian short-finned eel, Anguilla.australis.australis (Richardson) although it is considered as a prime candidate for inland aquaculture in Australia. This study provides information about the protein metabolism of the juvenile Australian short-finned eel. The efficiency of partitioning dietary protein into growth is closely related to the amount of nonprotein energy yielding substrates and the quality of protein (the availability and balance of amino acids) sources provided in the diet. The measurement of nitrogenous excretion can also give an insight into the nitrogen balance of fish and partly define the success of a particular nutritional regimen. Therefore, this study aimed at measuring the growth, growth efficiency and nitrogenous excretion of the juvenile Australian short-finned eel fed over a range of dietary protein:energy ratios and with selected Australian plant and animal proteins. Since the maximum protein growth will occur over a narrow range of dietary protein:energy ratios, the effects of increasing dietary crude protein contents at two different energy levels on the growth and growth efficiency were measured. A 10 % increase in dietary crude protein (from 25 to 35 %) in low protein:high non-protein energy diets positively affected weight gain whereas significantly (P <0.05) reduced weight gain was observed for a 10 % crude protein increase (from 45 to 55 %) in high protein:low non-protein energy diets. The whole body crude protein content was not affected by diet but the whole body crude lipid content decreased with the 10 % crude protein increase at each energy level. This study indicated a proteinsparing effect of non-protein energy sources in the diets of short-finned elvers. A lipid to carbohydrate ratio of 0.9 appeared to be needed for the maximum growth. The optimum dietary digestible crude protein (DCP):digestible energy (MJ DE) requirement of the short-finned elvers was investigated with 7.5 % crude protein increments (from 25 to 55 % of the diets) in iso-energetic diets. The optimum dietary digestible crude protein was estimated as dietary percentage (% CP DM) and as a dietary digestible crude protein:digestible energy ratio (g DCP/MJ DE) using total weight gain (g). Second order polynomial (quadratic) and 5-SKM (five parameter saturation kinetics) models were chosen for the estimation of the optimum dietary digestible crude protein. Two models gave similar results estimating the optimum dietary digestible crude protein as 43.0 % CP DM (±3.5) (r\\(^2\\) = 0.79; F=9.2157; P=0.021) and 41 % CP DM (r\\(^2\\)=0.77) or as 24.5 g.DCP/MJ DE (±1.7) (r\\(^2\\)=0.83; F=12.0573; P=0.012) and 23.5 g DCP/MJ DE (r\\(^2\\)=0.75) respectively. Whole body crude protein and lipid contents tended to increase with increasing dietary crude protein in this experiment. Nitrogen losses are primarily through faeces and metabolic excretion and largely influenced by dietary composition. Increasing protein content at two energy levels caused peak nitrogenous excretion rates 4-8 h following both the morning and afternoon feed. Daily ammonia-nitrogen excretion was significantly (P<0.05) higher on high protein:low non-protein energy diet (P55) compared to the P35 and P45 diets. Increasing dietary crude protein intake resulted in increasing ammonia- (y=0.022x+0.058; n=12; r\\(^2\\)=0.88; P<0.001) and urea-nitrogen (y=0.0044x+0.426; n=12; r\\(^2\\)=0.55; P<0.05) excretion in treatments. The proportional increase in ureanitrogen excretion to total nitrogenous excretion with increasing dietary non-protein energy sources also indicated that urea-nitrogen excretion in the Australian shortfirmed eel could be more responsive to nutritional variables. Fish meal is an expensive component of fish feeds and replacement of it with alternative protein sources without compromising the growth rate has been a priority in aquaculture nutrition research. Apparent digestibility coefficients were measured in order to assess the suitability of selected Australian plant and animal protein sources for fish meal replacement. Apparent crude protein digestibility was high for the selected Australian plant proteins and animal by-products. However, dry matter and energy digestibilities were found to be significantly (P<0.0001) higher for animal by-products than for plant proteins except for corn gluten meal. This was explained by the higher content of nitrogen free extract (NFE) in all the plant proteins except corn gluten meal. A final experiment was conducted to test the effects of fish meal replacement with corn gluten meal, meat meal, lupin meal and soy bean meal in diets using the optimum DCP/MJ DE ratio on growth, growth efficiencies and nitrogenous excretion by the Australian short-finned elvers by formulating the diets according to the optimum DCP/MJ DE ratio and ADC values determined in the present study. This study primarily showed better dietary protein retention efficiencies when juvenile Australian short-finned eel were fed low protein:high non-protein energy diets and established the optimum DCP/MJ DE requirement of this species for maximum growth. The establishment of the optimum protein:energy ratio and identification of highly digestible alternative protein sources provides a basis for decreasing nitrogenous waste production, maximising protein retention and better eel culture practises in Australia generally.