Alternative trout feeds match traditional diets in growth trial

CURRENT FULL VERSION: A new Animals study suggests rainbow trout can maintain commercial growth and product standards on practical aquafeeds that rely far less on traditional marine ingredients. In a 97-day feeding trial, researchers from Fondazione Edmund Mach, the University of Florence, and SPAROS evaluated four extruded diets: a conventional control and three eco-efficient alternatives built with insect meal, microalgae, microbial proteins, yeasts, selected plant proteins, aquaculture by-products, and, in one formulation, processed animal proteins. The headline finding was straightforward: removing fishmeal and fish oil did not appear to reduce productive performance under the conditions tested. (mispeces.com)

The study lands in a sector that’s under sustained pressure to decouple aquaculture growth from finite marine feed inputs. FAO reported in June 2024 that aquaculture surpassed capture fisheries as the main producer of aquatic animals for the first time, with 94.4 million tonnes of aquatic animals produced in aquaculture in 2022, representing 51% of total aquatic animal production. That growth has intensified interest in alternatives to fishmeal and fish oil, including insect meals, microalgae, single-cell proteins, and terrestrial by-products. Earlier rainbow trout work by overlapping author teams had already shown that alternative feed formulations could preserve growth while raising questions about flesh quality and consumer acceptance, so this latest paper builds on an active research line rather than arriving in isolation. (fao.org)

According to reporting on the new paper, trout in all treatment groups grew from about 63 g at baseline to 335–353 g by the end of the trial, with feed conversion around 0.78 and almost no mortality. The tested diets were designed to be practical, not just experimental proofs of concept, and combined several replacement strategies at once: insect meals for protein, microalgae as the main source of DHA and EPA, and additional support from microbial proteins, yeasts, plant ingredients, and aquaculture by-products. That multi-ingredient approach matters because it reflects how commercial formulators actually work when no single alternative ingredient can fully replace the nutritional and functional roles of fishmeal and fish oil. (mispeces.com)

The findings also align with adjacent trout research. A 2025 Animals paper found that partially defatted black soldier fly meal, full-fat yellow mealworm meal, and mixtures of the two performed comparably to low-fishmeal controls on growth, somatic indices, fillet composition, and several gut and tissue health measures in rainbow trout. Earlier studies have likewise reported that microalgal ingredients can support trout growth and omega-3 delivery, though color and sensory traits may shift depending on the formulation. Similar substitution work in other species points in the same direction, with caveats: in juvenile yellowtail, a 6-week Animals trial found that replacing 25% or 35% of fishmeal protein with composite shark by-product-based mixtures maintained final weight, weight gain, specific growth rate, feeding rate, and survival, and some formulations significantly improved feed efficiency versus control. But the yellowtail study also found significantly lower EPA and DHA in some by-product diets, underscoring that growth equivalence does not automatically preserve fatty acid quality. Taken together, the literature suggests the technical question is increasingly becoming one of optimization rather than simple feasibility. (mdpi.com)

Industry and specialist coverage of the new paper has already framed the result in those terms. Mis Peces summarized the study as evidence that fishmeal- and fish oil-free diets are now technically viable in trout, but argued that the trade-offs may shift toward fillet color, perceived product quality, and the true scale of sustainability gains. That interpretation is consistent with the authors’ earlier 2023 work, which explicitly connected alternative formulations with consumer acceptance questions in rainbow trout. While I didn’t find a separate institutional press release or named outside expert quote tied directly to this March 2026 paper, the broader expert conversation in the literature has been fairly consistent: alternative ingredients are promising, but formulation details still shape digestibility, gut health, nutrient retention, and marketability. (mispeces.com)

Why it matters: For veterinary professionals working in aquaculture, the study reinforces that feed strategy is becoming a central health, welfare, and business issue, not just a nutrition line item. If alternative diets can maintain growth and major quality metrics, farms may gain more flexibility against volatile marine ingredient markets and sustainability demands from retailers, regulators, and investors. In Europe, that flexibility is supported by regulation: Commission Regulation (EU) 2017/893 sets conditions for the use of processed insect protein and certain non-ruminant processed animal proteins in aquaculture feeds. But veterinary teams will still need to watch the secondary effects that determine whether a formulation succeeds in practice, including pigmentation, texture, nutrient digestibility, intestinal health, and consistency across life stages and production systems. The yellowtail by-product study sharpens that point: unchanged growth and survival did not guarantee unchanged nutrient composition, with crude ash and some long-chain n-3 fatty acids shifting under certain replacement strategies. (eur-lex.europa.eu)

There’s also a practical lesson in the study design itself. The diets did not rely on a single “silver bullet” ingredient. Instead, they blended insects, algae, microbial inputs, and by-products to spread nutritional risk and reduce dependence on any one raw material stream. For feed manufacturers and fish health teams, that likely means future progress will come from integrated formulation strategies, supported by better data on ingredient variability, long-term fish performance, and downstream product quality. That’s especially relevant in salmonids, where small changes in flesh appearance can have outsized commercial consequences. The yellowtail work offers a complementary example from another production species: by-product mixtures can help reduce fishmeal dependence, but the exact blend still matters for feed efficiency and fatty acid outcomes. (mispeces.com)

What to watch: The next phase will likely center on validation beyond controlled trials: larger farm-scale studies, clearer life-cycle assessments, economics of ingredient supply, and sensory or retail acceptance work around fillet color and quality. It should also include closer tracking of nutrient retention, especially EPA and DHA, as more feeds lean on insects, algae, and terrestrial or processing by-products. If those pieces hold up, practical low-marine or marine-free trout diets could move from promising research result to a more routine part of aquaculture nutrition programs. (mispeces.com)