Adding 5% dried fish powder to high-soybean fish diets restores gut health and immune function in rainbow trout, according to a 2026 study of 600 fish. Trout fed 35% soybean meal alone showed damaged intestines and weakened immune genes, but supplementing with fish powder reversed this damage and improved growth-related gene expression in both fish strains tested. This finding offers fish farmers a practical way to use cheaper plant-based feeds without harming fish health.
According to Gram Research analysis, scientists found that adding dried fish soluble (a powder made from fish) to rainbow trout diets high in soybean meal can protect the fish’s gut health and immune system. In a 66-day study with 600 trout, fish fed high amounts of soybean meal alone showed damaged intestines and weakened immune responses. However, when researchers added just 5% dried fish powder to the soybean-based diet, the trout’s gut health improved significantly, and important immune genes bounced back to normal levels. This discovery matters because fish farms worldwide are switching to plant-based feeds to reduce costs and environmental impact, but these diets can harm fish health. The findings suggest a simple, affordable solution that could help aquaculture become more sustainable.
Key Statistics
A 2026 research study of 600 rainbow trout found that high-soybean diets (35% soybean meal) caused significant intestinal damage and downregulated key immune genes including claudin 12, interleukin 8 and 10, and s100i2 in the fish’s intestines.
According to a 66-day feeding trial with 600 rainbow trout across two strains, supplementing 5% dried fish powder into high-soybean diets restored intestinal tissue structure, reduced inflammation markers, and improved insulin-like growth factor 1 (igf1) expression in the liver.
In a controlled aquaculture study of 600 trout, the Hagerman strain showed greater susceptibility to high-soybean diet damage than the commercial strain, with more severe intestinal crypt deepening and higher goblet cell density indicating strain-dependent sensitivity to plant-based feeds.
The Quick Take
- What they studied: Whether adding a small amount of dried fish powder to plant-based fish feed could protect rainbow trout from the negative effects of high soybean meal diets on their gut and immune health.
- Who participated: 600 young rainbow trout (average weight 22.1 grams) from two different strains: one bred at a research facility (ARS Hagerman strain) and one commercial strain commonly used in fish farming. The fish were divided into 30 tanks with 20 fish per tank.
- Key finding: Rainbow trout fed high soybean diets (35% soybean meal) showed damaged intestinal tissue and weakened immune responses. Adding 5% dried fish powder to these high-soybean diets restored gut health and improved immune gene expression in both fish strains, with particularly strong benefits in the Hagerman strain.
- What it means for you: If you eat farmed rainbow trout, this research suggests that fish farmers can now use more affordable plant-based feeds without sacrificing fish health or quality. This could make farmed fish more sustainable and environmentally friendly while keeping the product nutritious. However, this is early-stage research on fish, not humans, so more studies are needed before drawing conclusions about impacts on the fish we eat.
The Research Details
Researchers conducted a controlled feeding experiment over 66 days using 600 rainbow trout divided into 30 tanks. They tested five different diets: one based on traditional fish meal, two high-soybean diets (at 20% and 35% soybean meal), and two high-soybean diets supplemented with 5% dried fish powder. Two different trout strains were used to see if results varied between fish types. The fish were fed twice daily until they stopped eating (apparent satiation). At the end, scientists examined the fish’s intestinal tissue under a microscope and measured the activity of genes related to gut health, immune function, and liver health.
This research approach is important because it mimics real fish farming conditions using a recirculating aquaculture system (a closed-loop water system like commercial farms use). By testing two different trout strains, the researchers could determine whether the benefits apply broadly across different fish genetics. Measuring both physical tissue damage and gene expression provides a complete picture of how the diet affects fish health at multiple biological levels. This combination of methods makes the findings more reliable and applicable to actual farming operations.
The study used a factorial experimental design, which is a rigorous statistical approach that allows researchers to test multiple variables simultaneously. The large sample size (600 fish) and use of replicate tanks (30 tanks total) strengthen the reliability of results. Testing two different trout strains adds confidence that findings aren’t limited to one genetic type. The study measured both visible tissue damage and molecular gene expression, providing multiple lines of evidence. However, the study was conducted in a controlled laboratory setting, so results may differ slightly in commercial farm conditions. The research was published in a peer-reviewed journal, meaning other experts reviewed the methods and findings before publication.
What the Results Show
Rainbow trout fed the highest soybean diet (35% soybean meal) without fish powder showed significant intestinal damage. Their intestinal tissue had deeper crypts (small pockets in the intestinal lining) relative to villus height (finger-like projections that absorb nutrients), indicating tissue stress. These fish also had more goblet cells (mucus-producing cells), suggesting inflammation. The Hagerman strain showed more severe damage than the commercial strain. When 5% dried fish powder was added to the high-soybean diet, intestinal tissue damage was reduced substantially. The fish powder supplement restored normal intestinal structure and reduced signs of inflammation. Gene expression analysis revealed that high-soybean diets downregulated (reduced activity of) several important immune genes in the intestines, including claudin 12, interleukin 8 and 10, and s100i2. These genes are critical for maintaining intestinal barrier function and fighting infection. The fish powder supplement reversed this downregulation, restoring immune gene activity to near-normal levels. In the liver, high-soybean diets caused harmful changes: they increased nfkb (a stress-response gene) and catalase (an enzyme that responds to oxidative stress) while decreasing glutathione peroxidase and insulin-like growth factors (igf1 and igf2), which are important for growth and metabolism. Adding fish powder improved igf1 expression, suggesting better metabolic health.
The commercial trout strain showed greater resilience to high-soybean diets compared to the Hagerman strain, suggesting that genetic background influences susceptibility to plant-based feed stress. The 20% soybean diet caused less damage than the 35% soybean diet, indicating a dose-dependent effect where higher soybean levels cause more problems. The s100v2 gene, which relates to intestinal health and immune function, improved with fish powder supplementation. These secondary findings suggest that the benefits of fish powder supplementation are consistent across different biological markers and that even moderate soybean inclusion can cause measurable stress in sensitive fish strains.
Previous research has shown that high soybean meal diets can damage fish intestines and suppress immune function, a problem called soybean meal-induced enteritis. This study confirms those findings and provides a practical solution. Earlier work suggested that fish-derived ingredients could help mitigate soybean-related problems, but this is one of the first studies to systematically test dried fish soluble as a specific supplement and measure its effects on both tissue structure and gene expression. The findings align with the growing body of evidence that sustainable aquaculture requires balancing cost-effective plant-based feeds with protective supplements to maintain fish health.
The study was conducted in a controlled laboratory setting with a recirculating aquaculture system, so results may not perfectly match commercial farm conditions with different water quality, stocking densities, or environmental stressors. The research focused only on rainbow trout, so findings may not apply to other fish species used in aquaculture. The study measured gene expression at a single time point (end of the 66-day period), so researchers couldn’t track how gene activity changed over time. The exact composition and processing method of the dried fish soluble product wasn’t fully detailed, which could affect reproducibility. The study didn’t measure growth performance, feed efficiency, or final fish size, so the practical farming benefits beyond health markers remain unclear. Long-term effects beyond 66 days weren’t evaluated.
The Bottom Line
Fish farmers should consider supplementing high-soybean diets with 5% dried fish powder to protect fish gut health and immune function. This recommendation has moderate to strong evidence from this controlled study. The supplement appears safe and effective across different trout strains. However, farmers should start with small-scale trials before converting entire operations, as real-world conditions may differ from laboratory settings. Confidence level: Moderate (based on one well-designed study; additional research would strengthen confidence).
Fish farmers and aquaculture companies should care about these findings because they offer a practical way to use cheaper plant-based feeds without sacrificing fish health. Feed manufacturers could incorporate this finding into product development. Consumers interested in sustainable seafood should care because this research supports more environmentally friendly aquaculture practices. Regulatory agencies overseeing aquaculture feed standards should consider these results. People with fish allergies should not be affected, as the supplement is added to fish feed, not to human food directly. This research is most relevant to commercial rainbow trout farming operations.
Based on this 66-day study, improvements in intestinal health and immune gene expression appeared within 2 months of dietary supplementation. Fish farmers could expect to see measurable health benefits within 8 to 10 weeks of switching to supplemented diets. However, long-term effects beyond 66 days haven’t been studied, so it’s unclear whether benefits persist indefinitely or if adaptation occurs over time.
Frequently Asked Questions
Does adding fish powder to plant-based fish feed actually improve fish health?
Research shows that 5% dried fish powder supplementation restores intestinal tissue structure and immune gene expression in rainbow trout fed high-soybean diets. A 2026 study of 600 fish found the supplement reversed intestinal damage and improved growth-related genes within 66 days.
How much fish powder do farmers need to add to soybean-based fish feed?
The research tested 5% dried fish powder supplementation in high-soybean diets and found this level effectively mitigated negative effects. This means adding 5 pounds of fish powder per 100 pounds of feed, making it a relatively small and cost-effective addition.
Does this work for all fish species or just rainbow trout?
This study specifically tested rainbow trout, so results may not apply to other fish species. Different species may respond differently to soybean-based diets and fish powder supplements. Additional research on other species would be needed to confirm broader applicability.
Will fish farms actually use this supplement if it costs extra money?
Dried fish powder is relatively inexpensive compared to traditional fish meal, and the 5% supplementation level is a small addition. The cost savings from using high-soybean diets likely outweigh the modest supplement expense while maintaining fish health and product quality.
How long does it take to see health improvements after adding the supplement?
The 66-day study showed measurable improvements in intestinal tissue structure and immune gene expression within this timeframe. Fish farmers could expect to observe health benefits within 8 to 10 weeks of dietary supplementation based on current evidence.
Want to Apply This Research?
- If you’re a fish farmer using this research, track daily feed consumption rates and monitor fish behavior (activity level, feeding response) weekly. Measure intestinal health markers through periodic tissue sampling every 2 to 3 weeks. Record water quality parameters (pH, ammonia, nitrate) daily to ensure consistent conditions. Document any visible signs of disease or stress.
- Implement a gradual dietary transition: replace 5% of your current high-soybean feed with dried fish powder supplement over a 1-week period. Monitor fish response during this transition. Maintain consistent feeding schedules (twice daily at apparent satiation) and track feed conversion ratios. If using an aquaculture management app, log diet changes and correlate with health observations.
- Establish a baseline of current fish health metrics before implementing the supplement. After 4 weeks, compare intestinal health indicators and immune markers to baseline. Continue monitoring every 4 weeks for at least 12 weeks to confirm sustained benefits. Track growth rates, survival rates, and feed efficiency alongside health markers. Document any changes in fish behavior, appearance, or disease incidence. Use this data to calculate return on investment from the supplement cost versus improved fish health outcomes.
This research describes findings from a controlled laboratory study on rainbow trout and does not directly apply to human nutrition or health. The study was conducted in a recirculating aquaculture system, which may differ from commercial farming conditions. Results are specific to rainbow trout and may not apply to other fish species. Fish farmers should consult with aquaculture nutritionists and conduct small-scale trials before implementing dietary changes at commercial scale. This summary is for informational purposes and should not replace professional aquaculture advice. Always verify findings with current research and industry standards before making operational changes.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.