Alternative trout feeds match conventional growth in new study

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A new rainbow trout nutrition study suggests the industry may be getting closer to commercially practical feeds that rely less on marine ingredients without sacrificing fish performance. Writing in Animals, researchers reported that eco-efficient aquafeeds containing insect meal, microalgae, microbial and yeast ingredients, plant proteins, and aquaculture by-products achieved growth, feed efficiency, nutrient retention, and flesh quality outcomes comparable to a conventional control diet in Oncorhynchus mykiss. The work is notable not just because it used alternative ingredients, but because it tested “practical” extruded feeds designed to reflect real-world formulation constraints. (mispeces.com)

The backdrop is a long-running pressure on aquaculture to reduce dependence on fishmeal and fish oil made from wild-caught forage fish. IFFO said its projection for 2025 global production was 5.6 million tons of fishmeal and 1.2 million to 1.3 million tons of fish oil, while noting aquaculture demand remains strong. At the same time, researchers and feed companies have been trying to preserve growth performance and long-chain omega-3 content as marine inputs are reduced. EU regulation has also helped open the door to some alternatives, with processed insect protein authorized for aquaculture feeds since 2017 under specified conditions. (iffo.com)

According to coverage of the new paper, the 97-day trout trial compared one conventional diet with three eco-efficient formulations: No-PAP, PAP, and Mix. Fish grew from about 63 g to 335 g to 353 g, feed conversion stayed close to 0.78, and mortality was close to zero across groups. Fillet composition remained broadly similar, at roughly 65.8% moisture, 16.3% protein, and 12.6% fat, and texture differences were limited. The main product-quality shift was color: diets without processed animal proteins produced more yellow fillets, while those with processed animal proteins yielded paler flesh. That may matter commercially in markets where trout appearance influences buying decisions. (mispeces.com)

The findings fit with a broader pattern in recent aquaculture nutrition research. A 2025 Animals paper found that mixtures of black soldier fly and mealworm meals could be used in low-fishmeal trout diets without harming growth performance, somatic indices, fillet chemistry, or texture, though pellet and some fillet color traits changed. A separate 2025 Foods study reported that a microalgal co-product could fully replace fishmeal in trout feed while maintaining growth, survival, flesh composition, and even favorable feed-cost metrics. And in another Animals study, juvenile yellowtail fed diets in which 25% or 35% of fishmeal protein was replaced with shark by-product-based composite mixtures showed no significant differences in final weight, weight gain, specific growth rate, feeding rate, or survival over 6 weeks; feed efficiency was significantly higher in two of the by-product diets, although EPA and DHA levels were lower in some formulations. Together, these studies suggest that the technical question of “can fish still grow well?” is increasingly being answered with “often yes,” at least under controlled trial conditions, while also reinforcing that fatty-acid outcomes can diverge even when growth does not. (mdpi.com)

Industry and academic commentary is now shifting toward second-order questions. The misPeces write-up on the new study framed the issue as less about whether fishmeal-free diets can work and more about what trade-offs they create around color and sustainability claims. That lines up with comments from UC Santa Cruz researchers in a 2025 microalgae feed announcement, which emphasized that fully replacing fish-derived inputs is only useful if nutritional value for humans and economic practicality are maintained as well. The yellowtail paper points in the same direction: replacing fishmeal with by-product proteins may preserve growth, but not necessarily the same n-3 fatty-acid profile. Inference: the next competitive frontier in aquafeed formulation is likely to be ingredient quality consistency, sensory outcomes, nutrient composition, and verified environmental performance, not simply crude replacement rates. (mispeces.com)

Why it matters: For veterinary professionals advising aquaculture operations, this is a meaningful signal that alternative-protein diets are moving from experimental concept toward applied nutrition strategy. If these feeds can reliably support growth and health, they may help producers buffer supply volatility in marine ingredients and diversify sourcing. But veterinary oversight will still matter, because formulation changes can affect digestibility, gut health, pigmentation, fatty acid profiles, and possibly disease resilience in ways that don’t always show up in topline growth data. Prior trout literature has noted that higher insect meal inclusion can reduce digestibility, often linked to chitin load, even when moderate inclusion levels perform well. The yellowtail results add another practical reminder: a diet can maintain growth and survival while still shifting EPA and DHA content, which matters for fish health goals and downstream product value. (iffo.com)

There’s also a practical communication issue for farms and veterinarians serving food-fish systems: “sustainable” is becoming a more complex claim. Some alternative ingredients may reduce reliance on forage fish, but their environmental value depends on how they’re produced, processed, and sourced at scale. That means nutrition decisions will increasingly sit at the intersection of fish performance, welfare, product quality, economics, and environmental accounting. For pet parents who ultimately buy trout-based food products, those upstream decisions may shape both price and perceived quality, even if they never hear terms like PAP, FIFO, or microalgal co-product. (mispeces.com)

What to watch: The next milestones are likely to be larger commercial validation trials, more detailed fatty-acid and sensory work, clearer life-cycle assessments, and evidence on whether these formulations remain cost-competitive as ingredient markets evolve. The yellowtail by-product study also highlights the need to track not just growth and feed efficiency, but whether fishmeal replacement changes EPA and DHA enough to matter commercially or nutritionally. (mispeces.com)