Researchers at UC Santa Cruz have developed a fish feed that contains no fish at all. In a 12-week laboratory trial, young rainbow trout grew as well on a recipe built from two microalgae and canola oil as they did on a standard diet made with fishmeal and fish oil.

The trout also survived and turned feed into body weight just as efficiently, according to a new study published in October 2026 in the journal Antioxidants. The result matters because trout, like salmon, are carnivores and among the biggest users of fishmeal and fish oil in aquaculture.

“This is really groundbreaking,” said Pallab Sarker, a sustainable aquaculture scientist at UC Santa Cruz who worked on the research. “The future of feeding the world should not come at the expense of our ocean, and finding sustainable alternatives is essential to producing more food without depleting the marine resources we depend on.”

Why feed makers are looking beyond wild fish

Fish farms now produce more than half of the world’s aquatic animals, according to FAO. The UN agency expects that share to keep growing, and every one of those fish needs feeding.

Fishmeal and fish oil have traditionally supplied the protein, fats and omega-3 fatty acids that farmed fish need. Producers make them from small wild species such as sardines and anchovies. The study cites estimates that fish farming uses about 87 percent of the world’s fishmeal and 74 percent of its fish oil.

Supply is also tightening. Fishmeal output in the countries, tracked by marine ingredients body IFFO, fell 28 percent in the first quarter of 2026.

IFFO says El Niño has hit Peru’s anchovy fishery, which normally supplies about one-fifth of the world’s fishmeal.

Turning small fish into feed also leaves fewer for larger ocean fish, seabirds and marine mammals. People in countries that rely on these species lose out too. Climate change adds another layer of risk by warming oceans and disrupting marine ecosystems.

Replacing fishmeal and fish oil has proved difficult. Researchers and industry have tried land-based plant and animal products, yeasts and even insects. Some of these ingredients cost too much or lack the right nutrient balance for fish. Others change the nutritional quality of the fillet. Trout, which naturally eat other fish, may also notice the change and eat less.

Years of trial and error

The UC Santa Cruz team has spent years on this problem. Its first success came with tilapia. In 2020, after six years of research, the group reported a fully fish-free feed for the species, built from two microalgae. In a six-month test with 480 Nile Tilapia, the fish grew so efficiently that production cost per kilogram came out lower than on conventional feed. This was despite some ingredients costing slightly more.

Trout proved harder. Anne Kapuscinski, one of the lead researchers, said in 2021 that a cost-viable fish-free diet would be tougher to achieve for trout than it had been for tilapia, according to a report at the time. An early microalgae feed tested in 2020 fell short because the fish ate less of it.

The team’s 2025 study removed only the fishmeal. It used the leftover meal from a microscopic marine alga called Nannochloropsis oculata. It also added taurine, a nutrient found in fishmeal that encourages feeding, and lecithin, a fat-based additive. Those additions helped the fish accept the diet, and growth was similar to that on fishmeal. The remaining job was to replace the fish oil.

What went into the feed

The new study kept that fishmeal-free formula and added the missing piece, replacing the fish oil. The Nannochloropsis meal came from Qualitas Health, a US producer that extracts EPA-rich oil from the alga for human supplements. That protein-rich leftover has little commercial use, so the team treated it as a by-product. In the fish-free recipe it made up 8 percent of the diet.

Fish oil was replaced with Schizochytrium, a second microalga. It is rich in DHA, another omega-3 fatty acid that fish need and that people also get from eating fish. Producers grow it in industrial tanks, feeding it sugar-based waste from corn, beet or cane processing. Feed and ingredient companies including BioMar, Corbion, ADM and Veramaris have expanded work on Schizochytrium-based alternatives to fish oil, the study notes. It is already used in salmon diets.

The team tested Schizochytrium two ways, as whole dried cells and as extracted oil. Whole cells made up 5 percent of the fish-free recipe. Canola oil supplied extra energy and a plant-based omega-3, making up 12 percent. Taurine went into every diet. The feed was shaped into pellets at about 90°C, a relatively low temperature chosen to protect the microalgae’s nutrients.

How the trial was run

The trial ran at the university’s Center for Agroecology, using 16 tanks that clean and reuse their own water. Each tank held 40 juvenile rainbow trout weighing less than a gram, 640 fish in all. Four diets were each tested in four tanks:

• the standard fishmeal and fish oil diet;
• a diet replacing 75 percent of both;
• a fully fish-free diet with whole Schizochytrium cells;
• a fully fish-free diet with the extracted oil.

The fish were fed twice a day, six days a week, for 84 days.

How the trout performed

By the end, the fish weighed between about 33 and 35 grams. They looked healthy, with no signs of illness or deformities, and survival did not differ between diets. Feed conversion, the amount of feed needed to add a kilogram of weight, was statistically similar across all four diets.

The fully fish-free diet with whole Schizochytrium cells matched the standard diet for weight gain and final weight. So did the 75 percent replacement. Fish on both fully fish-free diets also absorbed more of the protein in their feed than fish on the standard diet.

The diet using extracted Schizochytrium oil was the exception, because those fish grew less than fish on the standard diet. The authors suggest it supplied less EPA and fewer natural antioxidants, such as carotenoids, than whole cells. They did not measure antioxidant activity in the fish.

Fillet quality and omega-3 levels

Fillet protein, fat and moisture did not differ significantly between diets, and amino acid content was similar. DHA and total omega-3 levels in the fish’s bodies also held steady. EPA, another omega-3, was lower on the fish-free diets. The authors link this to the Nannochloropsis meal, because much of the alga’s EPA had already been extracted for supplements.

Can microalgae feed compete on cost?

Cost has been the main obstacle for microalgae feeds. The team estimated it using UC Santa Cruz’s Cruz Aquafeed Sustainability Tool. On those figures, the fish-free diets had the lowest feed cost and the lowest cost per kilogram of fish produced. The differences were not statistically significant, so the study treats the diets as cost-viable rather than clearly cheaper.

Ingredient prices explain the result. Fishmeal cost US$1.54 per kilogram and fish oil US$1.68. Nannochloropsis meal cost only US$0.54 and canola oil US$0.88. Schizochytrium was dearer, at US$2.38 for whole cells and US$4.41 for the oil. The cheap leftover meal and the smaller quantity of oil needed offset that.

These are database prices from 2020 to 2022, not today’s market prices. The calculation also left out manufacturing, labour, transport and minor additives such as taurine. The study says plant-based diets remain cheaper, though they carry environmental costs such as land use and fertiliser run-off.

Sourcing microalgae can still be costly, Sarker said, because reusing by-products from current commercial production has not been set up on a wide scale. Finding a use for the leftovers would give that waste a value of its own and help microalgae feeds compete.

Kapuscinski, director of the Coastal Science and Policy Program at UC Santa Cruz and one of the study’s authors, sees a business case beyond price. “If we can reduce the use of ocean-derived nutrients, and if we can actually replace it with something like marine microalgae, where you can have more control over the stability of your yield, you’re going to end up with more stable economic outlook for the producers of the feed ingredient, compared to increased volatility of fishmeal and fish oil supplies and prices as our oceans are warming,” she said. “You’ll also have less potential for harm to marine biodiversity.”

Testing beyond the laboratory

The feed has also been tried outside the laboratory. The team ran a three-month trial at a commercial trout farm at Mount Lassen in California. A larger three-month trial followed at the San Joaquin State Trout Hatchery in Friant, run by the California Department of Fish and Wildlife. Neither trial showed a statistically significant growth difference from commercial feed, the university says.

At the San Joaquin hatchery, the question was whether it could switch diets without changing how it produces fish.

For a commercial operation trying something new, “being able to match performance is definitely a win”, said Devin Fitzgerald, a research associate at California Sea Grant who worked on the trial. The feed was made at the Bozeman Fish Technology Center in Montana, on a warehouse-size extruder, and shipped to California.

Getting the feed into general use

Adoption is the next hurdle. Fitzgerald said fish-free feeds often rely on expensive, unusual ingredients, and manufacturers need large orders to reach economies of scale. Without demand from farmers, costs stay high. If the state’s largest trout producer adopted the diet, it could set an example for others, and the Department of Fish and Wildlife is open to further trials.

What comes next

The researchers say the right balance of EPA and DHA still needs to be pinned down. “The next step is to move from promising experimental results towards practical applications,” said Sarker. “We need to evaluate these feeds at larger scales, plus other microalgal byproducts.”

California Sea Grant and the US Department of Agriculture funded the work, with partial support from the Cynthia and George Mitchell Foundation. The trial data are publicly available.