Study suggests insect- and algae-based trout feeds can match standard diets

CURRENT FULL VERSION: A new Animals study suggests rainbow trout can be raised on more sustainable, practical aquafeeds that incorporate insect meal, microalgae, single-cell ingredients, plant proteins, and aquaculture by-products without sacrificing the growth and quality benchmarks producers expect from conventional diets. According to the study abstract, the researchers compared a conventional control feed with three eco-efficient formulations and found comparable outcomes for growth, body composition, nutrient retention, and flesh quality in Oncorhynchus mykiss. (mdpi.com)

The finding lands in a feed market that has been trying to reduce dependence on fishmeal and fish oil for years. Earlier rainbow trout work has shown that insect meals, including black soldier fly and yellow mealworm, can often replace a meaningful share of fishmeal without hurting growth, digestibility, or fillet quality, though results vary by inclusion level, processing method, and the overall recipe. A 2023 trout study that included Filippo Faccenda among its authors framed the challenge clearly: plant ingredients have helped cut marine inputs, but newer options such as processed animal proteins, insect meals, single-cell proteins, and algae are increasingly being explored to push sustainability further. The same broader pattern is showing up beyond trout. In juvenile yellowtail, for example, a 6-week Animals trial found that replacing 25% or 35% of fishmeal protein with composite shark by-product-based mixtures did not significantly change final weight, weight gain, specific growth rate, feeding rate, or survival. Two of those by-product diets also produced significantly better feed efficiency than the control, although some formulations lowered the fish’s EPA and DHA levels. That is a useful reminder that reducing fishmeal dependence is increasingly feasible across species, but the nutritional trade-offs still need to be managed carefully. (mdpi.com)

What stands out in this new paper is the formulation strategy. Rather than testing one novel ingredient in isolation, the study evaluated “practical” extruded diets built from multiple alternatives, including insect and algae meals, with DHA and EPA supplied primarily from microalgae, according to the abstract provided in the source material. That approach mirrors where commercial feed development is heading: blending several alternative proteins and lipids to manage amino acid balance, fatty acid supply, palatability, cost, and ingredient availability at the same time. Related MDPI work in gilthead sea bream and European sea bass has pointed in the same direction, finding that mixed alternative-ingredient diets can maintain, and in some cases improve, nutrient efficiency and physiological markers relative to conventional formulations. The yellowtail by-product study adds another practical angle here: composite by-product mixtures can preserve growth and even improve feed efficiency, but not every mixture protects long-chain n-3 fatty acid status equally well. (mdpi.com)

The wider industry context helps explain why this matters now. The OECD-FAO Agricultural Outlook says fishmeal and fish oil are expected to remain major non-food uses of aquatic products through 2034, but it also notes that limited scope to increase fishmeal production is pushing aquaculture toward other feed ingredients. IFFO’s 2024 estimates, cited in 2025 conference coverage, indicate aquaculture still accounts for the overwhelming majority of fishmeal and fish oil use. In other words, even incremental reductions in marine ingredient dependence could have meaningful supply-chain effects if they can be achieved without compromising fish performance.

On regulation, this is no longer an early-stage policy conversation in Europe. EU rules permit processed animal protein derived from farmed insects in aquaculture feed, subject to production and segregation requirements, and the framework has expanded over time to include additional non-ruminant uses. That means the main barriers for insect- and algae-forward diets are increasingly practical ones, including price, raw material consistency, substrate rules, formulation know-how, and proof that performance holds up under commercial conditions. By-product-based strategies raise a similar practical question set: not just whether fish grow, but whether ingredient sourcing, nutrient consistency, and final fatty acid profiles remain acceptable at scale. (eur-lex.europa.eu)

Why it matters: For veterinarians and technical teams in aquaculture, the study supports a more realistic view of feed transition. The question isn’t simply whether one ingredient can replace fishmeal. It’s whether a full diet built around alternative proteins and oils can sustain growth, nutrient retention, flesh quality, and, ideally, health markers over time. That has implications for farm economics, environmental positioning, and fish health monitoring. It also suggests veterinary oversight may need to focus more closely on how these formulations affect gut health, oxidative status, liver condition, nutrient utilization, and product fatty acid quality as they move from research settings into routine use. The yellowtail results sharpen that point: growth can hold steady even when EPA and DHA shift, so “works” has to mean more than acceptable weight gain alone. (mdpi.com)

Expert reaction specific to this new study was limited in publicly indexed coverage, but the broader scientific literature is increasingly supportive, with caveats. Reviews and recent trout studies consistently describe insect meal and microalgae as promising alternatives, especially when used in balanced formulations rather than as blunt one-for-one swaps. Work with aquaculture by-products in other species points in a similar direction, showing that partial fishmeal replacement can preserve growth and survival and sometimes improve feed efficiency. At the same time, authors across the field note that outcomes still depend on species, processing, inclusion rates, and the need to preserve omega-3 content and overall fish health. That makes this paper less a final answer than another useful piece of evidence that practical reformulation is possible. (mdpi.com)

What to watch: The next milestone is commercial validation: longer trials, farm-scale economics, ingredient sourcing stability, health surveillance data, and product-quality measures that show whether these diets perform consistently across production cycles and regulatory markets. One recurring issue to watch, highlighted by the yellowtail by-product trial, is whether lower-fishmeal formulations maintain desirable EPA and DHA levels as well as growth. (eur-lex.europa.eu)