The main current source of LC omega-3 oils is seafood, including fish, crustaceans, such as lobsters, and molluscs, such as oysters and squid. Representative values for content of LC omega-3 in Australian seafood groups and individual species, together with data for other food groups, is shown in Table 1 and Table 2. Some land animals also contain LC omega-3 but generally in quite small quantities. Land plants, such as canola and linseed, contain some omega-3 although these are not the beneficial long-chain varieties. The short-chain omega-3 in land plants do not have the same health benefits as LC omega-3 [7,8].

In Australia, CSIRO has considerable interest in LC omega-3 and, in partnership with the Fisheries Research and Development Corporation, has profiled the LC omega-3 content of a wide range of seafood [16,17]. In terms of LC omega-3, different types of fish can have widely different amounts. For example, white-fleshed Australian wild fish, on average, contain approximately 350 mg per 150 g (representative of a typical serve size) (Table 1). However, the same quantity of slender tuna has 5,640 mg and swordfish has 1530 mg (Table 2). Different parts of the fish can contain different amounts of the LC omega-3. The season and location where the seafood was caught can also result in different contents, however, these effects are generally minor [17].

Over the past two decades the farming of fish has increased considerably. The amount of farmed fish on the market will soon surpass that of wild caught fish [37]. From an environmental point of view this seems very positive as the pressures on wild fish stocks can be quite severe as noted above. Beyond concerns for the environment it is also becoming increasingly clear that global fish stocks will not be able to meet human’s nutritional needs in the future [36]. Hence, farming fish appears to be a good opportunity to increase both global production and consumer needs.

As aquaculture expanded in the past decades, most carnivorous and many omnivorous farmed fish were initially all fed other fish (e.g., tuna ranching) or fish-derived products in pellet form generally comprising fish meal and fish oil as the major ingredients (e.g., salmon farming). Wild fish stocks were being harvested and used to feed the farmed fish. Due to increasing global demand for fish oil and environmental issues more and more suppliers are starting to use plant-based and other sources of ingredients and oils as feed for their fish [35,37,38]. However, the problem for consumers is that farmed fish do not naturally produce large quantities of LC omega-3, since in the wild fish gain their LC omega-3 from small aquatic plants, microalgae, that are consumed at the bottom of the marine food chain. The LC omega-3 oil is then passed up the food chain and stored in the fish bodies. As land plants produce the far less useful short chain (C₁₈) omega-3, feeding fish on land plant derived oils containing the short chain ALA results in far lower accumulation of LC omega-3 in the farmed fish and consumers do not gain the same health benefits.

In contrast, farmed fish can have even higher concentrations of LC omega-3 than fish caught in the wild when fed on fish oil based diets. For instance, the main farmed fish in Australia, such as Tasmanian Atlantic salmon from southern waters and barramundi from northern waters, provide up to 10 times more LC omega-3 (~3,000 mg/150g) than the average wild fish (Table 1). Until recent years Australian and New Zealand fish farms were largely using fish products in feed so the LC omega-3 content remained high. Further pressures on wild fish stocks will likely result in increased use of oils derived from land plants and or other ingredients in feeds, with: (i) lower relative levels (as % of total fatty acids), (ii) lower absolute content of the LC omega-3 oil and (iii) lower ω3 to ω6 ratio resulting than has previously occurred in farmed fish [34]. As the relative levels of these key LC omega-3 oils decrease, linoleic acid (LA, 18:2ω6) and oleic acid (OA, 18:1ω9c) increase; OA and LA are derived from the non-marine ingredients that are being increasingly substituted into aquafeeds [35]. However, farmed seafood species generally still remain an excellent source of the LC omega-3 oils. In Australia for example, farmed Tasmanian Atlantic salmon is widely available and remains an excellent source of LC omega-3 oil (1,610 mg/150 g serve, unpublished data 2010), clearly one of the best of all foods available for consumers.

A number of avenues can be considered for turning the current trend in LC omega-3 oils in farmed fish around. Practices at fish farms could be examined to make better use of the wild fish stocks, and also using ‘finishing diets’; the latter involves feeding fish on diets comprising plant or other ingredients during the grow-out phase, followed by the use of a fish oil diet during the final finishing period prior to harvest [37]. Making better use of current wild harvest ‘waste’ products, such as the liver and head, as sources of LC omega-3 oils, also could be expanded. The range of fish used could be widened, including species such as the underutilized Antarctic krill. Krill oil has attracted increasing interest in recent years and contains high (~30%) relative levels of EPA and DHA together with the carotenoid pigment astaxanthin, with much of the LC omega-3 present in phospholipids rather than the conventional triacylglycerol form that occurs in fish oil [34]. Considerable caution would be needed for increasing the harvest of this species as it underpins the Southern Ocean food web. The problem still remains of decreasing wild fish stocks and the need for farmed fish to also have high amounts of the LC omega-3.

A number of options are currently being explored to assist with potential future problems of decreased populations of wild fish and the health and nutritional needs of consumers. Sources of LC omega-3 are currently being developed from microalgae by several companies [35,39,40], with considerable interest in single cell organisms (SCO) such as heterotrophic dinoflagellates and thraustochytrids and some species from other algal groups. Thraustochyrids were originally thought to be closely related to primitive fungi, although have more recently been assigned to the subclass Thraustochytridea (Chromista, Heterokonta), aligning them with heterokont algae such as diatoms and brown algae [41]. Whilst SCO sources are finding increasing use in infant formulas and other niche areas including nutrition of fish larvae, it is presently generally considered too expensive for use in fish farming, which is the current main user (~90% of global production) of fish oil containing LC omega-3 oils. As more research is conducted, including in the growing field of biofuels, it may be possible to reduce production costs for microalgae. The genetic modification of microorganisms such as yeast to produce LC omega-3 is also a future possibility. Yeast and similar bioproducts may be targeted as they can easily be fermented on a large scale, potentially making the production of LC omega-3 oil relatively efficient [42].

In an alternative approach to the use of microorganisms, a number of international teams are now using genetic engineering technology to allow land plant crops to produce the health-benefitting LC omega-3 oils (see Table 4); several reviews are available and provide summaries of the considerable progress to date towards this goal [43,44]. This alternative approach to obtaining LC omega-3 oils has resulted from concerns on over-fishing and about pollution of the marine environment [35,36,44], in addition to the increasing recognition of the enormous health and socio-economic benefits of these oils and the expanding global population and finally the clear need for new sustainable sources.

There are two distinct pathways leading to the biosynthesis of LC-PUFA in nature: 1. The conventional (aerobic) fatty acid desaturation/elongation pathway, and 2. The anaerobic polyketide synthase (PKS) pathway. The PKS route is similar to those involving the PKS complexes in antibiotic synthesis. No reports of successful expression of the PKS pathway in land plants have emerged to date. In contrast, insertion of the aerobic pathway leading to DHA production, into omega-3 C₁₈ PUFA accumulating land plants, has been reported in a number of studies. Transfer of genes from microorganisms to land plants led to accumulation of 2.4% EPA and 0.5% DHA in Arabidopsis (Mouseear cress, later improved to 4% EPA and 1% DHA) [45]–(the first reporting of a (model) land plant with DHA), 15% EPA and 0.2% DHA in Brassica juncea (Indian mustard) [46], 19.6% EPA and 3.3% DHA in soybean (Table 4) [47].

These initial findings from a number of research teams clearly indicate the feasibility of developing grain crops with significant amounts of LC omega-3 oil. Further research is required to develop commercial oilseed crops with LC omega-3 oil and increased levels of DHA in particular are still required. It is interesting to point out, that as little as one tablespoon (~10 mL) of soybean oil containing 20% of EPA could make a significant contribution to the recommended dietary intake of the beneficial omega-3 LC-PUFA. Considerable effort is now being focused on increasing the levels of DHA and it is not unrealistic to hope that DHA levels of 20% can be achieved in transgenic plants in the next few years.

Future novel land-based plants will likely provide the most economically viable source LC omega-3 oil for aquaculture and other applications. A land plant source of LC omega-3, if achieved and assuming their cultivation will be permitted, will be cheaper than using yeast or microalgae and could be used in fish feeds and other feed and food products to help deliver increased human health benefits [35,42]. Research involving the use of microbial genes in land plants has so far led to increases the production of LC omega-3 in a number of land plant species (Table 4). This future may be as little as five years away [48], although perhaps longer in the context of aquaculture feeds and for products for direct human consumption in particular to meet all the permitting requirements that are likely to be in place in 15 years [35,42].

Research in the area of land plant sources of LC omega-3 has the potential for significant health, social, environmental and commercial benefits. Consumer and industry acceptance of this biotechnology and new health and safety requirements set by regulatory bodies will be needed for oils from these novel land plants before they can be used by the aquaculture and other industries. Recentassessments of perceived consumer acceptance of novel land plant LC omega-3 technologies in Australia and the USA have reported that farmed fish were preferred as a delivery mechanism over capsules or functional foods [53,54]. It is likely that consumers will accept oil from a GM crop as an ingredient of aquafeeds as demand for fish oil intensifies, as knowledge about fishing impacts and benefits to human health of LC omega-3 increases, and as farmed fish prices decrease owing to reduced ingredient costs [35].

Considerable effort is now being focused on increasing the levels of LC omega-3 in land plants and it is not unrealistic to foresee that sufficiently high concentrations can be reached in the next few years. Similarly, research is needed to open up new avenues for delivery of LC omega-3 oils to consumers via grains including e.g. whole grains in foods, vegetative plant tissues, extracted oil in spreads, extracted oils as food ingredients and aquaculture and livestock feeds.

LC omega-3 oils are required for ongoing health and vitality and to provide consumers with benefits across a range of areas, including heart disease, stroke, rheumatoid arthritis, some forms of cancer and other disorders. It is recognized that an increase in LC omega-3 consumption is required, which, currently, means at least two serves of appropriately oily fish per week. Future supplies of the beneficial LC omega-3 oils may not be sufficient for the predicted increasing demands for their inclusion in both feeds and food and nutraceutical products. In addition, with the wild harvest fisheries from which the LC omega-3 are obtained under threat, we need to explore alternate means to ensure their future supply while still protecting the environment and ocean life. Potential alternate sources of LC omega-3 for use in aquaculture and other application areas include single cell microalgal oils and/or biomass, and under-utilized marine sources including krill, although expanding the fishing of these building blocks of the marine ecosystem should be carefully considered before a marked increase in exploitation of krill resources occurs. Finally, the exciting prospect is evolving for the further development and application of novel transgenic oilseeds with several research groups active in this area.