Date of Award
Master of Science
Seafood is the number one source of essential fatty acids, particularly, long-chain polyunsaturated fatty acids (LC-PUFA) in the human diet. As global population growth eventually surpasses what the world's wild commercial stocks can provide, reliance on the aquaculture industry to expand production will continue to increase in order to meet the demands of consumers worldwide. Currently, fluctuations in supply and cost coupled with environmental sustainability and contaminant concerns have motivated the aquaculture industry to research alternative lipid sources and feeding strategies in order to reduce the reliance on marine-derived resources. For most cultured species, replacing fish oil with terrestrial plant-based lipid sources is a minor dietary modification that has little consequence on production performance. However, fish raised on these plant-based lipid alternatives contain considerably higher medium chain polyunsaturated fatty acids (MC-PUFA) and n-6 fatty acids and less beneficial LC-PUFA and n-3 fatty acids within the fillets, thus negatively impacting the nutritional value of cultured seafood to the consumer. In order to alleviate this problem, producers can employ finishing strategies to restore fillet LC-PUFA content prior to harvest. As a complement to this approach, provision of dietary saturated fatty acids (SFA) and/or monounsaturated fatty acids (MUFA) in lieu of MC-PUFA appears to maximize the retention of LC-PUFA deposition during the grow-out period and may increase deposition during finishing. Accordingly, my objectives were to 1) assess whether the SFA, MUFA, and MC-PUFA content of the alternative lipid affected LC-PUFA levels in Nile Tilapia fed reduced fish oil feeds; and 2) using the optimal alternative lipid identified in the first objective, assess increasing fish oil replacement rates in conjunction with finishing to maximize product nutritional value and minimize fish oil usage in Nile Tilapia culture. To address the first objective, I assessed production performance and tissue composition of Nile Tilapia fed diets containing fish oil or blends of fish oil and various soybean-derived alternative lipids. Quadruplicate tanks of juvenile Nile Tilapia were fed diets containing fish oil (FISH, high in LC-PUFA) or a 50:50 blend of fish oil and standard (STD-SO, high in MC-PUFA), saturated fatty acid-enriched (SFA-SO, high in SFA), low α-linolenic (LO-ALA-SO, high in MC-PUFA), or hydrogenated (HYD-SO, high in MUFA) soybean oil for 16 weeks. Partial replacement of fish oil with soybean oils did not significantly affect production performance with the exception of the HYD-SO diet which yielded significantly reduced growth efficiency in comparison with some of the experimental diets, though not the FISH control. Despite distinctly different dietary fatty acid profiles, fillet fatty acid composition was similar among fish fed the FISH, SFA-SO, and HYD-SO diets. However, feeding the STD-SO and LO-ALA-SO diets resulted in significant enrichment of less desirable MC-PUFA and n-6 fatty acids within the fillet. Fillet LC-PUFA levels were equivalent among all groups despite the 50% reduction in dietary LC-PUFA intake among fish fed the soybean oil-based feeds. Based on these results, incorporation of STD-SO, SFA-SO, or LO-ALA-SO could be used as partial replacements for fish oil in Nile Tilapia feeds without impairing production performance, though SFA-rich soybean oils appeared to be the best alternative for maintaining a more "fish oil"-associated fillet fatty acid profile. Accordingly, the SFA-enriched soybean oil was selected for further study in the second objective trial that evaluated the effects of graded levels of fish oil replacement without or without implementation of finishing periods on production performance and fillet fatty acid composition. Nile Tilapia were fed feeds containing 100% fish oil (100-FO), the previously assessed SFA-enriched soybean oil (100-SFA-SO), or blends of fish oil and SFA-enriched soybean oil (50-SFA-SO, 75-SFA-SO). Triplicate groups of fish were fed the aforementioned diets exclusively throughout the feeding trial (100-SFA-SO unfinished, 75-SFA-SO unfinished, 50-SFA-SO unfinished) or in conjunction with 4 or 8 weeks of finishing with the 100-FO feed (100-SFA-SO + 4 wks, 100-SFA-SO + 8 wks, 75-SFA-SO + 4 wks, 75-SFA-SO + 8 wks, 50-SFA-SO + 4 wks, 50-SFA-SO + 8 wks) for a total of 20 weeks. Production performance was unaffected by dietary inclusion of SFA-enriched soybean oil when fed exclusively or in combination with fish oil, though growth performance was lower than observed in the previous trial and likely confounded by behavioral interactions and frequent spawning. After 12 weeks of consuming the SFA-enriched soybean oil grow-out diets, fillet levels of n-3 LC-PUFA were not statistically different from 100-FO control levels despite different levels of dietary inclusion. However, the high dietary levels of SFA in the experimental feeds did not translate into increased fillet SFA content, suggesting selective retention of LC-PUFA at the expense of fillet SFA. Finishing for 4 or 8 weeks increased fillet n-3 LC-PUFA content in all groups, though it appears that the 50- and 75-SFA-SO diets were more successful in maintaining acceptable health promoting n-3:n-6 ratios. Based on these results, SFA-enriched soybean oil-based feeds can be used as a cost-saving measure during grow-out, and the effects of these feeds on fillet fatty acid profile can be reversed to a considerable extent in as little as 4 weeks by implementing a finishing period prior to harvest. This approach is a promising strategy for minimizing fish oil usage while maximizing product value of cultured Nile Tilapia.
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