Alternative sources of Omega-3 oils for Barramundi, Lates calcarifer, aquaculture

Fish oil (FO) is the major source of dietary lipid in carnivorous fish feeds including barramundi, Lates calcarifer, which is widely farmed in Asia and Australia. However, recent increases in FO prices, increased demand and the foreseen inability of wild fisheries to meet future requirements have created a need for cheaper and more sustainable alternatives. Vegetable oils (VO) can be produced in sufficient quantities to meet the growing aquaculture demand, although they lack the long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA) beneficial to human consumers. Some VO like rapeseed oil (RO), echium oil from Echium plantagineum (EO) and linseed oil (LO) have high levels of n-3 and n-6 short-chain (≤C18) PUFA that can accumulate or be converted into LC-PUFA by some fish species, although generally at low efficiency, and not to docosahexaenoic acid. In a series of comparative and factorial experiments, I investigated the growth and lipid changes of barramundi fed different dietary oils: FO, RO, LO and EO over conditions covering: a range of salinities and temperatures, subject to immunity stress or supplemented with plant-derived bioactive ingredients. In general, growth performance parameters were comparable for FO and VO treatments, and resulted in accumulation of VO-derived n-3 and n-6 PUFA. Salinity has no direct effect on growth or lipid metabolism regardless of the dietary lipid source. Endogenous conversion by barramundi of dietary PUFA into LC-PUFA is limited by more than one ratelimiting step and there is a preference for incorporation of LC-PUFA into the polar lipid fraction rather than neutral lipid. The growth of barramundi slowed at sub-optimal (20°C) temperature compared to optimal (30°C) temperature. PUFA from dietary VO deposits in muscle and are maintained under rapid temperature decreases. In contrast, excess LC-PUFA from FO depleted faster than occurs in VO fed fish. The production of pro-inflammatory eicosanoids in fish fed FO was lower than for fish fed VO following bacterial infection. EO significantly suppressed the production of the pro-inflammatory mediators compared to RO. Sesamin, a lignan in sesame seed, enhanced the conversion of dietary PUFA into LC-PUFA for the n-3 series rather than n-6 in early juvenile barramundi. However, sesamin had negative impact on fish growth at this early life-stage. Barramundi fed on VO are a rich source of LC-PUFA precursors, α-linolenic and stearidonic acid, and grow well under the different environmental conditions that are typical of outdoor barramundi farms. The use of terrestrial VO containing the LC-PUFA precursors and plant-derived bioactive compounds show promise for use in barramundi aquafeed in terms of fish growth and health as either partial or complete alternatives for FO. However, using currently available VO, high content of the n -3 LC-PUFA is not achieved