Alternative trout feeds match traditional growth in new study

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Rainbow trout fed aquafeeds built around insect meal, algae, microbial ingredients, plant proteins, and by-products matched the growth and quality benchmarks of fish raised on a more conventional diet in a new Animals paper. According to reporting on the study, the 97-day feeding trial found that fishmeal- and fish oil-free formulations maintained productive performance, with trout growing from about 63 g to roughly 335 g to 353 g, feed conversion near 0.78, and almost no mortality. (mispeces.com)

That result matters because rainbow trout remains one of the species where replacing marine ingredients is nutritionally and commercially harder than it is in lower-trophic fish. Earlier trout studies have shown partial or even full replacement can work under some conditions, but performance often depends on the exact blend, processing quality, palatability, amino acid balance, and omega-3 strategy. A 2023 study in Journal of Marine Science and Engineering from the same broader research context tested practical alternative trout diets rich in processed animal proteins, algae, microbial meals, insect meal, and by-products, framing the challenge as finding formulations the feed industry could realistically adopt. More recent work has also shown that microalgal co-products can fully replace fishmeal in trout diets under experimental conditions, reinforcing the direction of travel toward diversified ingredient baskets rather than one-for-one substitutions. Similar signals are emerging in other carnivorous marine fish: in a separate Animals study, juvenile yellowtail maintained growth and survival when 25% to 35% of fishmeal protein was replaced with composite shark by-product-based mixtures, suggesting by-product proteins can meaningfully reduce fishmeal dependency beyond trout as well. (mdpi.com)

The new paper appears to push that practical formulation approach further. Reporting on the article says researchers compared a conventional control with three eco-efficient diets labeled No-PAP, PAP, and Mix, using combinations of single-cell ingredients, insect meal, selected plant proteins, aquaculture by-products, and microalgae as the main source of DHA and EPA. Growth, body composition, nutrient retention, and flesh quality remained broadly comparable across diets. But the study didn’t present the alternatives as a simple win across every metric: fish fillet color changed with ingredient choice, with xanthophyll-rich diets producing more yellow flesh, while diets containing processed animal proteins produced paler fillets. (mispeces.com)

That color finding is especially relevant for production systems serving markets with fixed expectations around trout appearance. Even if nutritional performance holds, visible changes in flesh color can affect grading, marketability, and pet parent or consumer acceptance downstream. The earlier 2023 trout paper from this research line explicitly included sensory and consumer-acceptance analysis, underscoring that practical feed reformulation is not only a growth question, but also a product-quality question. The yellowtail by-product study adds another reminder that “no growth penalty” does not automatically mean nutritional equivalence in every respect: while whole-body composition was largely unchanged there, crude ash differed and EPA and DHA were significantly lower in two of the shark by-product mixture diets. (mdpi.com)

The sustainability angle is also more nuanced than a headline about “alternative proteins” might suggest. Reporting on the new study says some alternative diets reduced carbon footprint versus the control, particularly those that included processed animal proteins, but the gains were not uniform. Ingredient origin mattered, and microalgae and microbial proteins increased emissions in some formulations, while soy protein concentrate remained a major contributor in the conventional diet. That aligns with broader industry discussion: reducing reliance on fishmeal and fish oil is strategically important as aquaculture output grows, but replacement ingredients bring their own cost, supply, and environmental profiles. By-product-based proteins may improve circularity and reduce pressure on marine ingredients, but their nutritional profile still has to be managed carefully, especially where long-chain n-3 fatty acids are concerned. FAO has noted that aquaculture now exceeds capture fisheries in aquatic animal production, while fishmeal and fish oil remain important indirect-use products in the sector. (mispeces.com)

For veterinary professionals working with aquaculture operations, this study supports a more pragmatic message than a revolutionary one. Alternative formulations can preserve growth and baseline fillet quality in trout, but ration design still has to account for fatty acid delivery, palatability, pigmentation, gut health, and ingredient consistency lot to lot. The yellowtail findings fit that same practical frame: partial fishmeal replacement with composite by-product mixtures supported growth over 6 weeks and even improved feed efficiency in some groups, but not all mixtures preserved EPA and DHA to the same extent. There are also regulatory and procurement realities. In the U.S., AAFCO documentation shows dried black soldier fly larvae are defined for use in salmonids, not broadly across all species, and USDA APHIS sets import rules for certain insects and other invertebrate materials used in feed. In other words, the science is advancing, but commercialization still depends on what can be sourced, approved, and priced competitively. (aafco.org)

Industry reaction around adjacent work suggests cautious optimism. Coverage in aquaculture trade media has increasingly framed insect meal and microalgae as promising tools for improving resilience and reducing dependence on marine ingredients, while also emphasizing affordability and feed attractiveness as barriers to broad uptake. That’s consistent with the takeaway here: the most commercially relevant innovation may not be a single novel ingredient, but formulation strategies that combine several alternatives to balance nutrition, cost, sustainability, and product quality. The yellowtail trial, which used composite by-product mixtures rather than a single substitute ingredient, points in the same direction. (thefishsite.com)

What to watch: The next questions are whether these diets hold up at commercial scale, how feed costs compare with conventional programs, whether pigment management can be refined without losing sustainability gains, how consistently EPA and DHA can be maintained as fishmeal is displaced, and whether regulators and feed ingredient bodies expand the pathways for wider use of insect-, algae-, and by-product-based inputs in aquaculture feeds. (mispeces.com)