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Can plant-based feeds make aquaculture sustainable?

Some scientists are replacing sardines and anchovies with soybeans and corn as food for farmed fish.


In a capacious warehouse northeast of downtown Bozeman, Montana, a lanky scientist named Rick Barrows leans over a blue tank teeming with rainbow trout. The fish thrash expectantly at the arrival of human visitors, and Barrows smiles behind his scrub-brush mustache. “You go to the poultry nutrition conference, and there are 4,000 people there,” he says wryly. “You go to the U.S. fish nutrition conference, and there are 30.”

Rick Barrows is a U.S. Department of Agriculture fish nutritionist  an esoteric occupation, true, but a vital one. Here at the Bozeman Fish Technology Center, a century-old federal facility tucked into a patchwork of ranchlands, Barrows is changing how the world produces its seafood.

Aquaculture is the fastest-growing form of food production in the world, at around 8 percent per year; the World Bank projects that two-thirds of our fish will come from farms by 2030. From a sustainability standpoint, that might be good news: Because they’re cold-blooded and water-dwelling, fish don’t have to heat their bodies or support their own weight. That means they can devote calories to packing on muscle, making them a more efficient source of protein than four-legged livestock. “Aquaculture is most likely to meet the growing demand for animal products with the least demand on ecosystems,” one Conservation International official told The Guardian in 2011.

Rick Barrows uses a vacuum coater to infuse flax oil into pellets of rainbow trout feed containing a vitamin formula that he and colleagues developed.
Steve Ausmus, Agriculture Research Service

But aquaculture carries plenty of ecological baggage. Perhaps its greatest irony is that growing fish on farms has traditionally required extracting other fish from the sea. Well over half the global harvest of so-called forage fish — the small silver creatures, like sardines, menhaden and anchovies, that form the bedrock of marine ecosystems — gets ground into fishmeal and fish oil to feed bigger farmed species, like salmon. The West is hardly immune to the repercussions. Pacific sardines have experienced a nearly decade-long collapse, starving brown pelicans and California sea lions and prompting federal managers to close the fishery in July 2015. While sardine stocks are notoriously prone to natural boom-bust cycles, fishing pressure appears to have exacerbated this crash.

To avert ecological harm — and defray rising costs — fish farmers have lately begun reducing the proportion of fishmeal in aquaculture feed, replacing ground-up sardines and anchovies with soybeans and corn. The challenge is that many delectable fish, such as salmon and trout, are carnivores, ill-adapted to subsisting on vegetable matter. Thanks in large part to Rick Barrows, however, some companies are already raising carnivorous fish on all-plant diets. For over two decades, Barrows has been developing vegetarian feeds for species such as salmon, cobia, walleye and, most of all, trout. 

“We consider rainbow trout the white rat of the aquaculture world: They grow fast and they’re inexpensive to obtain,” Barrows says as his fish churn the surface. “We use trout to develop basic dietary knowledge that we apply to other species.” 

The future of seafood, in other words, is being developed with the help of freshwater fish, 600 miles from the nearest ocean.


From one perspective, farming carnivorous fish doesn’t make much sense. The planet is swimming with herbivores and omnivores that require less protein-dense meals. Tilapia, which flourishes on a chickenfeed-like corn diet, is the country’s fourth most-eaten form of seafood, though its popularity hasn’t come without environmental consequences, such as pollution and ecosystem invasions.

But carnivores have a crucial physiological advantage over their plant-eating counterparts. "It’s hard to catch your meal if you’re a carnivore — you have to chase after it,” says Mike Rust, aquaculture science coordinator at the National Oceanic and Atmospheric Administration. As an evolutionary consequence, carnivores “have machinery designed to get every last nutrient out of what they eat.”

From a sustainability standpoint, that digestive efficiency is a major advantage. The trick, then, isn’t to turn carnivores into herbivores; it’s to make plant foods behave like fishmeal.

Barrows leads me to another vast room in the USDA complex, this one packed with an obstacle course of humming equipment, a maze of ovens and chutes and glowing green lights. A rich vegetative aroma, reminiscent of a brewery, wafts in the close air; food pellets crunch underfoot. This is where the alchemy happens, where Barrows and his team transform vegetation into something with a nutritional profile that approximates meat.

“This is a smaller version of what the food production industry uses,” says Jason Frost, a technician with the drooping mustache of a 19th century saloonkeeper. “Cat food, dog food, all kinds of cereals.” Indeed, the fragrant brown nuggets that fill one row of garbage cans would look perfectly at home floating in a bowl of milk.

Converting vegetation into viable aquaculture feed, however, is not easy. Fishmeal is rich in minerals, like iron and selenium, and amino acids, like lysine, that plants lack. Excluding just one crucial constituent can be disastrous: Without zinc, for instance, trout develop cataracts. Moreover, soy and other crops are packed with so-called anti-nutrients, defense compounds that cause inflammation in fish’s small intestines. And even if you assemble all the right components, it’s worthless if the fish can’t actually digest it. Carnivorous fish lack the enzymes to break down the cellular walls of most microalgae, for instance, though Barrows can overcome that by supplementing feeds with the proper enzymes.

To create a well-balanced feed, Barrows’ team tests as many as 50 ingredients each year, blending and tweaking like a mixologist. Most concoctions begin with soy, though not all; some manufacturers prefer to keep their feeds free of genetically modified organisms, and it’s hard to find non-GMO soy. Corn, algae, even black-fly larvae find their way into Barrows’ creations. He also supplements some feeds with trimmings from an Oregon fish-processing plant, waste that would otherwise be discarded. Barrows has developed vegetarian diets for nine species altogether, including tricky marine carnivores like white seabass and yellowtail.

Trout get a vegetarian feed at the Bozeman Fish Technology Center.
Rick Barrows

Still, obstacles remain. Vegetarian feeds generally cost more, though the gap has narrowed in recent years as fishmeal’s price has climbed. And plant-based feeds aren’t always ecologically pure. “The question I would ask is, are you cutting down rainforest in Brazil to grow the soybeans?” says Geoff Shester, California program director for the marine conservation group Oceana. While Oceana supports the aquaculture industry’s move toward plant-based feed, it argues that sustainably harvested forage fish, fed to humans instead of animals, is the best solution to the piscivore’s dilemma. As Shester puts it: “We prefer foodwebs to feedlots.” 

The Technology Center’s most exciting feeds, therefore, may be the ones that repurpose agricultural waste. One product incorporates pistachios discarded by farmers — “nuts that are too big, too small, too green, too light,” Barrows explains. Working with a company called Adaptive Bioresources, Barrows has figured out how to process the pistachios into meal that’s high in nutritious omega-3s and contains around 55 percent protein, allowing it to supplant fishmeal. According to John Hamilton, the company’s owner, pistachio-raised salmon and trout raised are already entering the market. Pistachio trees are wildly profitable, and California’s farmers have ramped up their planting, despite drought. “As the price for waste almonds comes down, we’ll be looking at those, too,” Hamilton says. 

Though these discarded pistachios could someday impact U.S. salmon farming, they’ll never be abundant enough to affect, say, Chile’s farms. Still, they’re a reminder that creativity offers big dividends. Barrows and another company, Montana Microbial, have made similar use of unwanted malting barley, a key ingredient in beer. Brewers generally desire barley with a low protein content, in the 11 to 13 percent range; any higher and the beer turns cloudy. In dry years, however, the crop often exceeds that threshold, creating a product that’s substandard for beer, but suitable for fish. The brewer’s loss is the fish farmer’s gain.  


While Barrows fiddles with their food, his collaborators in Idaho are refining the other side of the equation: the fish themselves.

Back in 2000, Ron Hardy, director of the University of Idaho’s Aquaculture Research Institute, began breeding a trout that’s better at processing plant-based diets. It was a long process: Rainbow trout take two years to reach sexual maturity, and the first three generations that Hardy raised couldn’t grow as quickly on Barrows’ soy-based meal as their fishmeal-eating cousins could.

But by around 2008 — generation four — the soy-munching trout families began catching up. These days, Hardy is studying his seventh generation, and some of the soy-eaters have taken a slight lead. Hardy’s fish reach two pounds in just nine months, twice as fast as when the research began. Just as important, they retain the nutritional benefits of conventional fish. At least for rainbow trout, then, the grand dream of efficiently raising a plant-fed piscine crop has already been achieved.

Fish nutritionist Rick Barrows (right) captures trout from 6-foot-diameter tanks for technician Jason Frost to weigh and measure. These trout were fed fishmeal-free, plant-based feed.
Steve Ausmus, Agriculture Research Service

And these fish of the future are already infiltrating the supply chain. At some Idaho trout farms, a quarter of the crop is composed of Hardy’s trout. An added bonus, for finicky American consumers, is that veggie-fed trout taste less fishy. (Hardy, who developed a taste for potent flavors while teaching in Thailand, finds this mystifying. “People in the U.S. would rather eat bland fish that they can gussy up with sauce and seasoning,” he says, incredulous.)

A critical question remains unanswered: Why are these seventh-generation trout so proficient at turning plants into muscle? Are they more metabolically efficient? More tolerant of anti-nutrients? One intriguing clue is that plant-selected trout avoid gut inflammation, a condition that can impede growth, perhaps because trout bred for plant consumption have different microbial communities living in their guts. Just as fecal transplants have been used to help humans suffering from intestinal disorders like Crohn’s disease, Hardy speculates that researchers might eventually develop probiotics from the microbiomes of soy-fed trout that could help other fish better digest plant-based feed.

Back in Bozeman, Barrows guides me down a narrow hallway, one wall checkered with plastic bags bursting with feed — black feeds made of soldier fly larvae, green algae feeds, yellow corn feeds, brown soy feeds, and so on. To be sure, these myriad stocks aren’t panaceas: Poultry and pigs gobble down a third of the world’s fishmeal and fish oil, meaning that improvements in aquaculture can only achieve so much. And as Oceana’s Shester points out, the aquaculture industry’s recent efficiency gains haven’t kept up with the world’s growing demand. We’re getting better at raising fish, but we’re also raising more of them. 

But while Barrows’ feeds may not save the seas by themselves, they could help alleviate the pressure on the world’s precious forage fish stocks. The Bozeman Fish Technology Center’s colorful food wall is proof of possibility, a mosaic ode to dietary diversity and experimentation. The old truism that fish farming requires wild fish is, well, no longer true — or at least it doesn’t have to be.

“Aquaculture’s biggest strength is its biggest weakness,” Barrows muses. “With poultry, you’ve only got chickens and turkeys. But there are over 200 species of fish being cultured around the world — you’ve got guys working on trout, catfish, cobia, and so on. That dilutes the research somewhat. But it also makes for a very exciting environment.”

Ben Goldfarb is a High Country News correspondent.