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By Rod Nickel

Feb 28 (Reuters) - The world's top agricultural traders and biotechnology firms are finding novel ways to make fish oil substitutes from grains and algae as they seek to cash in on consumer health fads that have led to a scarcity of the fatty acids commonly found in fish.

Fish are the fastest-growing protein source in a global food supply chain straining to feed a population of nearly 7.5 billion people.

To keep farm-raised fish healthy, they are fed Omega 3 fatty acids that are found in the oil of other fish. The same acids are increasingly popular in fish oil dietary supplements for humans.

The surging demand has pushed fish oil prices to a record high and presented the aquaculture industry with a problem: how to source more fish oil without putting depleted global fish stocks under even more pressure. About 90 percent of marine fish stocks worldwide are already fully or partially over-fished, according to the United Nations.

"We have finite fish oil, growing aquaculture and a world that needs more Omega-3s," said Mark Griffin, president of animal nutrition at Omega Protein Corp (OME.N) , the biggest U.S. fish oil producer. "They're going to have to come from somewhere else."

The short supply has attracted the world's largest grain traders, such as Cargill Inc (CARG.UK) , Bunge Ltd (BG.N) and Archer Daniels Midland Co (ADM.N) .

These agricultural giants are in the midst of transforming themselves into food processing and ingredient suppliers as they look to diversify away from bulk trading of grains and raw materials amid a four-year global supply glut.

The $2.4-billion fish oil sector is niche for major grain traders and represents a fraction of their income. But fish oil is the sort of high-return product they are targeting as they grapple with slim margins in their traditional business.

As demand outstripped supply, wholesale prices in top fish oil producer Peru soared to an average of $2,986 per metric tonne in 2016, the highest ever recorded.

Global annual production of fish oil has for years been limited to about 1 million tonnes, said Einar Wathne, president of Cargill's aqua nutrition business, in an interview from Norway.

"It could be a kind of showstopper for growth in aquaculture if we can't find other sources for these valuable Omega-3 fatty acids," Wathne said.

YELLOW FIELDS IN MONTANA

Cargill's plan to produce more fish oil could soon change the color of up to half a million acres of the landlocked Montana prairie, company executives told Reuters.

The firm plans to pay farmers there to grow a new variety of canola, distinctive for its bright yellow flowers. Half a million acres would be eight times as much farmland as is currently planted with canola in the state.

Vegetable oil made from canola is high in Omega-3s, and Cargill teamed up in November with chemical company BASF SE (BASFn.DE) to develop a canola type by 2020 that it will use to make oils for fish food. The new canola is genetically engineered to make long chain omega-3 fatty acids by introducing genes from algae in the ocean, another source of the fats.

A half million acres of canola could produce about 159,000 tonnes of oil - the equivalent of one-fifth of global fish oil supplies.

"It may be appealing, an opportunity to try new crops," said Tom Clark, one of Montana's few canola growers.

But he added that managing to change farmers' habits on such a large scale would be challenging.

In addition to Cargill, Dow Chemical (DOW.N) is developing its own variety of canola to make oil with similar Omega-3 acids as fish oil, and is counting on Canadian Prairie farmers to grow it.

U.S. seeds giant Monsanto is developing soybeans that can be processed into soy oil with the Omega-3 fatty acids, for food products such as baked goods and soups.

ADM launched an algae-based product DHA Natur for fish diets last year, and has "robust plans in 2017" for the product, said spokeswoman Jackie Anderson, who declined to give more details.

Bunge, working with TerraVia Holdings Ltd (TVIA.O) , started using algae to convert sugar into an Omega-3 ingredient for fish diets last year.

The company has capacity in Brazil to annually produce tens of thousands of tonnes of their product, AlgaPrime, said Walt Rakitsky, TerraVia's senior vice-president of emerging business.

Bunge and TerraVia are supplying the product to BioMar Group, the third-largest fish feed supplier.

The fish oil alternatives come with their own challenges. Algae oil is expensive to produce, and the canola and soybean varieties used to make oils rich in Omega-3s are genetically modified. That can be a sensitive issue, for example in Norway, which is the world's biggest salmon producer and has tough restrictions on genetically modified foods.

SWIMMING IN REVENUE

The $166 billion aquaculture industry accounts for half the world's fish, and sales are expected to expand up to 5 percent annually for at least the next three years, according to Rabobank analyst Gorjan Nikolik.

With high prices and concerns about sustaining fisheries, fish farms have for years reduced use of oil and protein-rich meal in diets, risking production of less-healthy fish, said Tom Frese, president of consultancy AquaSol.

That's why the development of fish oil substitutes is critical, said Vidar Gundersen, BioMar's global sustainability director.

"The timing now is of the essence," he said.

Booming fish farming drives up fish oil prices