Study finds insect- and algae-based trout feeds match control diets

A newly published Animals study suggests rainbow trout can be raised on practical aquafeeds that incorporate insect meal, microalgae, plant proteins, and aquaculture by-products without sacrificing the core metrics producers care about: growth, body composition, nutrient retention, and flesh quality. In the trial, researchers compared a conventional control diet with three eco-efficient formulations — No-PAP, PAP, and Mix — and found the alternative feeds achieved outcomes comparable to the traditional formulation. (mdpi.com)

That result lands in the middle of a long-running push to reduce aquaculture’s dependence on fishmeal and fish oil from wild-caught forage fish. In rainbow trout especially, researchers have been testing insect proteins, rendered animal proteins, single-cell ingredients, and algae-derived lipids as replacements or supplements for years. Prior studies cited by the trout nutrition literature show that partially defatted black soldier fly meal, yellow mealworm meal, and mixed alternative-protein strategies can often preserve growth and digestibility, at least at defined inclusion levels. (mdpi.com)

The broader evidence base helps explain why this new paper is notable. A 2025 Foods study from University of California researchers found that a microalgal co-product, Nannochloropsis sp. QH25, fully replaced fishmeal in rainbow trout diets while maintaining growth, feed conversion, survival, fillet composition, and cost-effectiveness, with the 100% replacement diet posting the lowest feed cost per kilogram of fish produced. (mdpi.com) Earlier work in Aquaculture Journal also found that several alternative ingredients, including insect meal and mixed formulations, produced no negative effects on trout growth, muscle composition, fish health, or final nutritional value overall, although the authors noted that microalgae inclusion rates still needed optimization. (mdpi.com)

That pattern is not limited to trout. In juvenile yellowtail, a 6-week Animals feeding trial tested composite by-product protein mixtures as partial fishmeal replacements and found that replacing 25% to 35% of fishmeal protein with shark by-product-based blends did not significantly change final weight, weight gain, specific growth rate, feeding rate, or survival. Two of the surimi-linked by-product diets also showed significantly better feed efficiency than the control. Whole-body composition was largely unchanged, but EPA and DHA levels were lower in some formulations, highlighting a recurring tradeoff in alternative-feed development: growth may hold steady even when fatty-acid outcomes shift. That species difference matters, but so does the broader signal — by-product mixtures are increasingly joining insects and microalgae as realistic tools for reducing marine-ingredient dependence.

That nuance matters. The mixed-diet literature in trout is encouraging, but it’s not uniformly simple. In the 2022 Aquaculture Journal paper, the DMIX formulation that combined microalgae, insect meal, and tuna-canning by-products delivered intermediate growth and conversion results, yet also improved some fillet fatty-acid quality indicators, including a stronger omega-3 to omega-6 profile. In other words, some alternative feeds may trade a bit of zootechnical simplicity for sustainability or nutritional advantages, depending on how they’re formulated. (mdpi.com)

Industry and research interest in insect-derived aquafeeds also continues to build. A recent MDPI review assessing insect-derived feed technologies described scientific publication growth as strong, but noted that commercialization still depends on translating research into scalable technologies, patents, and market-ready supply chains. That aligns with what trout studies have shown so far: the biology is increasingly supportive, but ingredient consistency, processing methods, price, and regulatory fit still determine whether a formulation becomes commercially useful. (mdpi.com)

Why it matters: For veterinary professionals working in aquaculture, feed changes aren’t just a nutrition story. They affect fish welfare, gut health, liver health, nutrient digestibility, production efficiency, and ultimately the health profile of the final food product. Earlier trout work involving fishmeal-free diets with insect and poultry by-product meals specifically evaluated welfare-related hematology, biochemistry, organ indices, and histopathology, underscoring how alternative feed adoption increasingly intersects with veterinary oversight rather than sitting solely with feed formulators. (mdpi.com) The yellowtail findings reinforce another practical point for clinicians and production teams: acceptable growth alone is not the whole story if changes in ingredient mix alter EPA and DHA deposition. If these newer eco-efficient diets continue to perform well, they could give producers more flexibility to reduce marine ingredient dependence without creating new health-management tradeoffs. That said, formulation details still matter, especially where microalgae inclusion, lipid balance, palatability, long-term health effects, and final fatty-acid profiles are concerned. (mdpi.com)

What to watch: The next phase will likely focus on commercial-scale validation, longer-duration feeding studies, and closer comparisons of economics, welfare markers, gut microbiota, and fillet nutrient profiles across ingredient blends. Watch, too, for whether suppliers can provide consistent insect, algae, and by-product inputs at scale, and whether future papers move from “comparable in trials” to “adopted in practice.” The yellowtail data also suggest one specific watchpoint for future work: how far fishmeal can be reduced before omega-3 quality markers begin to slip, even when growth and feed efficiency remain acceptable. (mdpi.com)