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by Marlene Fritz
For Ron Hardy, sustaining the world’s fisheries resources—like charity—begins at home. If Idaho’s farm-raised rainbow trout can eat more plant-based feeds and less ocean fishmeal, more bottom-of-the-food-chain fish can remain in the seas to nourish other fish.
Since 1996, University of Idaho animal scientists have been developing plant-based feeds for rainbow trout—and rainbow trout for plant-based feeds—at the UI Hagerman Fish Culture Experiment Station. They were joined in 2000 by a research team from the USDA Agricultural Research Service (ARS). In a few more years and depending on economics, Hardy says Idaho’s farmed trout could be eating primarily barley, wheat, corn, or soybean products and only minimal supplements of fishmeal, fish oil, and other marine proteins.
“We can already produce trout on an all-plant diet, and they’ll grow only 8 to
10 percent slower and about 10 percent less efficiently than fish on a fishmeal
diet,” says Hardy, Hagerman-based director of the university’s Aquaculture Research
Institute. “That’s not bad. Our challenge now is to figure out why we’re still
getting that 10 percent penalty and what we can do to close that gap.”
A related challenge: developing affordable alternatives to fishmeal
ARS lead scientist Rick Barrows says blends of corn gluten meal, soy protein concentrate, and wheat gluten meal are currently too costly for most trout producers. “We need less expensive ingredients—either by developing new strains of plants or new methods of processing them”—or even by deriving from ethanol production a byproduct like barley protein concentrates.
Gibson Gaylord, an ARS animal physiologist at Hagerman, is evaluating the digestibility of six different barley cultivars in rainbow trout diets and the ability of meals made of barley, wheat, corn, soybeans, and canola—plus amino-acid supplements and other products—to meet the dietary needs of rainbow trout.
Meanwhile, ARS geneticist Ken Overturf is working the other end of the equation: identifying the fish families most likely to thrive on plant-based feeds despite their carnivorous preferences for insects and smaller fish. That means painstakingly teasing apart the contributions of hundreds of trout genes in hopes of finding the handful that are most responsible for utilizing carbohydrates, which reach levels of 75 percent in plants.
Rainbow trout raised on plant food already grows 10 percent less
efficiently than fish raised on fishmeal diets.
Now researchers seek to make up that 10 percent.
photo by Brad Beckman
What effect does each feed have on a fish family’s energy use, muscle structure, and growth, Overturf and Gaylord ask. Does a particular family react to a grain-based feed with intestinal disorders or autoimmune responses? How well does it convert plant oils to the heart-healthy omega-3 fatty acids for which consumers value fish
“We’ve hardly touched the growth rates we can get from fish,” Overturf says. “If we’re able to provide them with the energy they need, there’s no reason they can’t grow a lot better.”
“The fact is that we are just going to have to do this,” says Matt Powell, associate director of the Hagerman center. “The amount of fishmeal in the world is finite, and if we’re going to grow more fish we’re going to have to rely on other sources of protein.”
Barley may boost trout immunity to viruses. Also, fish and diabetes
Some feeds even appear to boost disease resistance over other feeds. University of Idaho fish nutritionist Wendy Sealey has found that 50 percent of the rainbow trout fed barley high in betaglucans—an immune-stimulating nutraceutical—survived exposure to infectious hematopoietic necrosis virus, compared with 22 percent fed a wheat-based diet. The high betaglucans barley lines were developed by ARS scientists An Hang, Phil Bregitzer, and Don Obert at Aberdeen.
Because feeding barley didn’t take a toll on growth, Sealey says fish feed formulators “can utilize it when it’s cheaper than wheat without fear of detrimental effects—and probably it will have positive effects on disease resistance.” The next step in her research: evaluating the impacts of high betaglucans barleys on resistance to coldwater disease, a common ailment in Idaho fry.
Recently, Powell has documented rainbow trout’s satisfactory performance when fed 15 percent carbohydrate diets: they tapped carbohydrates for energy, sparing more proteins for muscle growth. However, if carbohydrates comprise 35 percent of their diet, the fish become overloaded with them—Powell calls them “swimming diabetics”—and their muscle growth suffers.
“Over time, the genes and the animal itself begin to adapt to the new diet, but we don’t know for how long before their growth falls off due to glucose toxicity,” Powell says. “We’re trying to figure out the window during which we can utilize alternative ingredients in fish diets and still get healthy fish to market.”
Maintaining omega-3 levels
Rainbow trout aren’t fussy about their dietary oils. That’s a good thing, because the world’s fish oil—from “forage” fish like herring, anchovies, and sardines—is already fully allocated, most of it to aquaculture. Unfortunately, fish that get their dietary oils from, say, corn, reflect the fatty acid profile—and flavor—of corn oil.
Hardy is leading a research effort to make better use of plant oils in fish diets without sacrificing the desirable omega-3 fatty acid content of the finished product. “The challenge we face is very simple: We feed a plant oil blend for part of the growth cycle and then finish off the fish with a high fish-oil diet. But when should we start the shift and which oils should we use?”
Perhaps the fish’s natural life cycle can be used—or fooled—to deposit fat at the most beneficial time, or perhaps its gene expression can be harnessed to convert a shorter-chain fatty acid, like linseed oil, to longer-chain omega-3s. Hardy is exploring both angles. He notes that researchers elsewhere are successfully extracting omega-3s directly from their original source: algae. “It can be done. It’s a question of economics now.”
Contact Ron Hardy at rhardy@uidaho.edu.
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