The lipid composition of five species of marine finfish from temperate Australian waters was determined. Claims that the lipids of Australian fish contain high levels of omega-6 fatty acids and arachidonic acid (AA) were investigated. Individual fish were analysed from samples collected at three locations and two seasons, and they were found to have fatty acid compositions of similar nutritional value to those from northern hemisphere temperate waters. Levels of AA, eicosapentaenoic acid and docosahexaenoic acid were also found to be comparable. The variability in fatty acid compositions was greater between samples taken from different locations than from different seasons. Lipid contents and compositions were found to exhibit some species-characteristic nature, indicating a need for accurate species identification. A high-performance liquid chromatographic (HPLC) method for fish identification was developed, which was successful. The application of HPLC to processed fish was investigated and it was found to be suitable for gamma-irradiated and infrared dried fish, but not for those that had been smoked or microwave cooked. / Doctor of Philosophy (PhD)

Fish oil sparing and replacement is a major focus in the fields of aquaculture and aquaculture nutrition. Most of the commercial fish oil production is consumed by the aquafeed industry due to its highly digestible energy and elevated content of long-chain polyunsaturated fatty acids (LC-PUFAs; ARA – 20:4n-6, EPA – 20:5n-3, and DHA – 22:6n-3), being a valuable ingredient. Given the finite supply and the growing demand for fish oil its price has increased quite drastically, leading to the search for alternative lipid sources. Generally, vegetable- and terrestrial animal-origin alternatives lack LC-PUFAs, which are physiologically important nutrients for all fish, and considered essential fatty acids for carnivorous species. When fish oil is spared or replaced by alternative lipids fish survival, growth performance, and fish health are commonly impaired if adequate levels of essential fatty acids are not provided within feeds. Additionally, fish oil sparing typically distorts fillet fatty acid profile and associated nutritional value compared to a fish oil-based diet reflecting the composition of the alternative lipid used. It is clear that to address the fish oil bottleneck in aquafeed manufacturing, researchers must understand the essential fatty acid requirements of the key commercial fish species. Fatty acid essentiality in fish has been investigated, and there is preliminary evidence that not all LC-PUFAs may be equally required, with DHA being more important, and EPA being more expendable. Whereas ARA has not been investigated in the same extent as n-3 LC-PUFAs. Additionally, certain fatty acids groupings such as saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) may influence LC-PUFAs bioavailability, and in some cases maintain or enhance LC-PUFAs deposition. The current dissertation sought to provide new knowledge regarding LC-PUFA requirements of marine carnivorous fish (White Seabass Atractoscion nobilis, California Yellowtail Seriola lalandi and Florida Pompano Trachinotus carolinus) in the context of C18 PUFA-rich (i.e. polyunsaturated fatty acid with chain length of 18 carbon atoms) and SFA- and MUFA-rich alternative lipids. Determine if all LC-PUFAs (ARA, EPA, DHA) are equally important in meeting fatty acids requirements and also determine the effects of dietary SFA, MUFA, and C18 PUFA content in fish oil sparing and tissue deposition of LC-PUFAs. The overall findings highlighted that DHA and ARA appear to be the primary drivers of fatty acid essentiality, whereas EPA is likely required in minor amounts. It was also demonstrated that DHA/EPA ratio had little-to-no effect on fish performance. Additionally, LC-PUFA requirements seem to be more flexible than previously assumed being influenced by dietary fatty acid profile. LC-PUFAs in marine finfish are more bioavailable in the context of SFA-/MUFA-rich alternative lipids, thus, reducing the requirements for these nutrients and allowing the fish’s physiological demand to be met with dietary levels below the minimum levels recommended. Finally, these findings suggest that although marine fish accept a variety of alternative lipids, those rich in SFAs and/or MUFAs seem advantageous in terms of limiting the effects of fish oil sparing on tissue fatty acid profiles.