According to research reviewed by Gram, a 2026 study found that a protein made from the bacterium Methylococcus capsulatus can replace up to 25% of fish meal in Siberian sturgeon feed without reducing growth. Replacing 50% or 75% lowered nutrient digestion and altered immune markers, so the amount used matters.
Fish farms rely heavily on fish meal, a costly feed ingredient made from wild-caught fish. A 2026 study published in Aquaculture Nutrition tested whether a protein made from a bacterium called Methylococcus capsulatus could replace part of it in feed for young Siberian sturgeon. According to research reviewed by Gram, the researchers swapped 25%, 50%, or 75% of the fish meal with this bacterial protein over 70 days, using 540 fish. Growth stayed about the same across the groups, and the 25% swap performed about as well as the standard feed. Higher swaps, however, reduced how well the fish digested nutrients and changed some immune markers. The findings suggest the bacterial protein can partly replace fish meal, but the amount matters, and more research is needed before it is used widely.
Key Statistics
A 2026 study published in Aquaculture Nutrition found that Siberian sturgeon fed a diet with 25% of their fish meal replaced by bacterial protein grew about as well as fish on a standard diet over 70 days.
In this 70-day trial of 540 young Siberian sturgeon, replacing 75% of fish meal with bacterial protein reduced how well the fish digested nutrients and lowered levels of the immune enzyme lysozyme.
The Quick Take
- What they studied: Whether a protein made from a naturally occurring bacterium can replace some of the fish meal in farmed sturgeon feed without harming the fish.
- Who participated: 540 young Siberian sturgeon, a farmed fish species, starting at about 23 grams each. The fish were split into groups of 45 per tank, with three tanks per diet. Because this was an animal study, the results apply to fish, not people.
- Key finding: Fish fed a diet where 25% of the fish meal was replaced grew about as well as fish on the standard diet. At 50% and 75% replacement, the fish digested nutrients less efficiently, and some immune measures shifted. Overall growth did not differ significantly between the groups.
- What it means for you: This study does not change what you should eat today. It may matter over time for the cost and sustainability of farmed fish. The findings come from one fish species over 70 days, so they should be treated as early evidence.
The Research Details
Researchers created four feeds that were matched for protein, fat, and energy content. The control feed contained standard fish meal, while the other three replaced 25%, 50%, or 75% of that fish meal with the bacterial protein. Young sturgeon were randomly assigned to these feeds, which helps ensure that differences between groups come from the feed rather than from the fish themselves.
Over 70 days, the team tracked how much the fish grew and how well they digested the feed, including protein, dry matter, and key minerals such as phosphorus. They also measured blood chemistry, body fat and composition, and liver structure. Finally, they examined immune responses, activity of immune-related genes in the head kidney (an immune organ in fish), and the bacteria living in the gut.
This type of setup is a controlled experiment. Testing several replacement levels against a standard diet allowed the researchers to see where benefits stop and problems begin, which is useful for practical feed decisions.
A controlled feeding trial shows cause and effect more clearly than a survey or observation would. Because every diet was compared against the same control and measured with the same methods, the researchers could identify a threshold, roughly 25% replacement, where the bacterial protein still performed well. That threshold is the most practically useful result for feed producers.
The study was published in a peer-reviewed journal and used randomized assignment, replicate tanks, and a standard control group, all good signs of reliability. A total of 540 fish is a reasonable size for this kind of trial. However, the number of tanks per diet is small, so chance variation between tanks can influence results. The abstract also lists the 25% replacement level as 5.25% of the diet, but in one place it cites 5.75%, which appears to be a typographical error that readers should keep in mind. The trial lasted only 70 days, and it studied one species, so long-term and cross-species effects remain unknown.
What the Results Show
The main growth results were reassuring. Fish fed diets with bacterial protein did not show significant differences in growth performance, organ size measures, or key blood markers compared with fish on the standard diet. In practical terms, the fish grew about as well regardless of how much fish meal was swapped.
Digestion told a different story. The bacterial protein significantly affected how well the fish digested dry matter, protein, ash, phosphorus, and several amino acids. Digestibility, which is a measure of how much of a nutrient the body can actually absorb, appeared to decline as more fish meal was replaced. This is one reason the researchers concluded that higher replacement levels reduced nutrient use.
Immune responses also shifted. Levels of complement component 3 and lysozyme activity, both part of the fish’s natural defense system, decreased as the bacterial protein increased. Certain immune-related genes in the head kidney and some liver fat measures, specifically cholesterol and cholesteryl ester, were also altered. The study does not establish whether these changes would affect disease resistance in real farm conditions.
The gut microbial community stayed largely the same across diets. The main bacterial groups, Fusobacteriota, Firmicutes, and Proteobacteria, were present in all groups, and the overall community structure did not differ significantly.
The researchers concluded that a 25% replacement level supported biological responses comparable to the control diet, which makes it the most promising option tested. The study also found that the gut microbiome was not meaningfully disrupted, which is reassuring for farmers concerned about feed side effects. The changes in immune markers and liver fat chemistry are important signals for future research, even though the study does not show they harm the fish.
Earlier research on bacterial and other single-cell proteins in fish feed has generally found that partial replacement of fish meal can work, though results vary by species, protein source, and inclusion level. This study fits that pattern: moderate replacement appears workable, while higher levels bring trade-offs. Its contribution is adding detailed data on Siberian sturgeon, a species that has received less attention than more common farmed fish.
The study looked only at juvenile Siberian sturgeon over 70 days, so it is unclear how the fish would perform over a full growing cycle or to market size. Results may not carry over to other fish species, to different water conditions, or to commercial farm settings. The number of tanks per diet was limited, and the abstract contains a numerical inconsistency about the 25% replacement level. The study also measured immune markers but did not test how well the fish resist actual infections. Finally, nothing in this research speaks to the safety or nutritional value of the bacterial protein for people who eat the fish.
The Bottom Line
Based on this single study, there is moderate confidence that replacing about 25% of fish meal with this bacterial protein can work in Siberian sturgeon feed without hurting growth. There is low confidence about replacing 50% or more, since digestion and immune markers appeared to decline at those levels. Feed producers should confirm results in longer, larger trials before changing commercial formulas.
Fish farmers, aquaculture feed companies, and researchers working on sustainable seafood production should pay close attention. Everyday consumers do not need to change their diets based on this study. People interested in the long-term sustainability of farmed fish may find the research relevant, but it does not offer practical guidance for personal nutrition.
Feed changes in aquaculture usually require several years of testing, including larger trials, different fish sizes, and cost analyses before wide adoption. Even if the approach succeeds, consumers would likely see any effect only gradually, through changes in farmed fish supply or pricing, rather than as a direct benefit in the short term.
Frequently Asked Questions
Can bacteria be used as a substitute for fish meal in fish feed?
A 2026 study suggests they can, at least partly. Siberian sturgeon fed diets in which 25% of fish meal was replaced with protein from the bacterium Methylococcus capsulatus grew similarly to fish on a standard diet over 70 days. Higher replacement levels reduced nutrient digestion.
What is methanotrophic bacterial protein?
It is protein produced by bacteria that feed on methane, a natural gas. Methylococcus capsulatus is one such microbe, and when grown and dried, its cells provide a protein-rich ingredient. In a 2026 sturgeon study, it was tested as a partial substitute for fish meal in farmed fish feed.
Does replacing fish meal with bacterial protein harm farmed fish?
At a 25% replacement level, the study found no significant drop in growth or body composition in Siberian sturgeon. At 50% and 75%, fish digested nutrients less well, and immune markers such as lysozyme activity declined. Researchers say the inclusion level should be chosen carefully.
Why do fish farms look for alternatives to fish meal?
Fish meal, made from wild-caught fish, is a major protein source in farmed fish feed. Its supply is limited and its price fluctuates, which pushes farms to seek alternatives. Bacterial protein may offer a more stable supply, although this 70-day study covered only one fish species.
Want to Apply This Research?
- If you eat farmed fish, log the species, origin, and whether the label mentions sustainable or responsibly farmed production. Tracking this over a month can help you see how often you choose farmed seafood and how that changes as you learn more.
- Use the app to set a simple goal, such as including fish or seafood twice a week from sources with clear sustainability information. Pair this with a reminder to check labels or ask about farming practices when buying fish.
- Review your seafood choices every three months and note any changes in product labeling or availability. Watch for new research on fish feed and sustainability, and update your logs or preferences as credible evidence develops. Treat single studies like this one as early signals rather than final answers.
This article summarizes a single animal study on farmed Siberian sturgeon and does not provide dietary or medical advice for people. Findings may not apply to other fish species, to different farming conditions, or to human nutrition. Consult a qualified healthcare professional or registered dietitian before making decisions about your diet or health.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.