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With the increasing use of plant protein meals to replace fishmeal in aquafeeds, there is a concomitant need to properly characterise the requirements for individual amino acids in the diet of carnivorous fish species such as barramundi (Lates calcarifer). This may be especially important for methionine (Met) and taurine (Tau) which are known to be limiting in these ingredients. This project aimed to define the requirements for these nutrients for barramundi, as well as to elucidate some of the mechanisms underpinning any observed stimulatory effect on growth.The aim of the first experiment was to evaluate the methionine (Met) and total sulphur amino acid (TSAA) requirements of juvenile barramundi to act as a baseline for further study. A requirement for Met of between 10.5 (95% of maximum response) and 13.6g kg-1 (99% of maximum response) in a diet with 592g kg-1 CP and 6.6g kg-1 Cys (17.1-20.2g kg-1 TSAA; 1.8-2.3% CP Met + 1.1% CP Cys) was established. Additionally, it was concluded that at least 40% of dietary Met can be replaced by Cys. The impact of nutrient response model choice and the mode of expression of requirements on interpretation of these figures is discussed in Chapter Two. The experiment presented in Chapter Three was designed to elucidate some of the metabolic roles of Met. Results suggested that expression of genes of the sulphur amino acid turnover pathways may be chiefly regulated by feed ingestion and not, as was expected, by the amino acid profile of the feed. Taurine biosynthesis was apparent, as was the conservation of previously described markers of proteolytic pathways. Significantly, two forms of Methionine Adenosyltransferase (MAT) appeared to be active. A number of important genes were investigated for the first time in this species and shown to be nutritionally regulated. Whether Tau is required in the diet of juvenile barramundi, and what effect dietary Met has on this requirement, was investigated and is presented in Chapter Four. The best-fit response model predicted a Tau requirement of 5.5g kg-1 DM (0.96% CP), similar to reported values for several other species. The fit of this model, however was relatively weak (R2 = 0.183). The response to variable dietary Met was more pronounced, highlighting its importance to this species. It was concluded that taurine appears to be conditionally essential to barramundi.An experiment investigating the role of dietary Tau in affecting protein and SAA turnover in juvenile barramundi was undertaken, the results of which are presented in Chapter Five. Most significantly, two pathways of Tau biosynthesis were observed to be active in this species with the sulphinoalanine pathway being more responsive to feeding. Changes in the utilisation of Cys when dietary Tau was in excess were also suggested, along with an apparent link between adequate supply of taurine and both somatotropic index and TOR pathway-mediated growth stimulation. Unexpectedly, the expression profile of several genes suggested that excessive Tau may have a negative impact on protein accretion and growth.Calculation of protein and amino acid loss exponents, and their application to predicting requirements for Met for maintenance and growth in barramundi, was the focus of Chapter 6. Contrary to previous assumptions used in nutritional models, the exponents derived for the proteinaceous amino acids were significantly different to that of protein. An influence of fish size on the utilisation of non-protein nitrogen was also suggested. Similarly, predicted partitioning of ingested Met was proposed to change with increasing fish size, with weight gain suggested to drive the requirement for this amino acid in smaller fish and maintenance requirements representing a greater proportion of the total requirement in larger fish. Consequently, it was concluded that the use of the Met-loss-adjusted metabolic body weight exponent derived in this study may provide a more accurate representation over the protein weight exponent in modelling Met utilisation during growth in this species. The main findings of this thesis were that juvenile barramundi do have a defined requirement for Met, a proportion of which can be spared by dietary Cys, and that the mode of expression and choice of nutrient response model can have a significant impact on the interpretation and application of this requirement. As well, several genes associated with SAA and protein metabolism in this species were investigated, and shown to be nutritionally regulated, for the first time in this species. Significantly, two forms of MAT appear to be active and it was suggested that barramundi possess the capacity to synthesise Tau from precursor SAA through two separate pathways. Thus, Tau was concluded to be conditionally essential in this species. Finally, the utilisation of individual amino acids was shown to differ significantly from that of protein as a whole but, in the case of Met, consideration of this in a published nutrient utilisation model appeared not to significantly improve the accuracy of utilisation efficiency predictions. |
