Alternative trout feeds match traditional diets on growth

CURRENT FULL VERSION: A new rainbow trout feeding study suggests the aquafeed sector is getting closer to a practical answer to one of its longest-running questions: how far fishmeal and fish oil can be reduced without compromising performance. According to reporting on the newly published Animals paper, trout fed eco-efficient diets containing insect meal, microalgae, microbial proteins, selected plant proteins, and aquaculture by-products matched a conventional feed on growth, feed conversion, survival, and most flesh-quality metrics over a 97-day trial. (mispeces.com)

That finding fits into a broader shift in aquaculture nutrition. Earlier work from the same research network, including a 2023 paper tied to the EU-funded GAIN project, tested rainbow trout diets built from processed animal proteins, algae, microbial ingredients, and other alternatives as the industry looks beyond marine ingredients and soy-heavy formulations. That earlier study found alternative diets could support growth and, in some cases, consumer acceptance, while also highlighting tradeoffs in protein retention, digestibility, and sensory outcomes. (mdpi.com)

In the new study, fish reportedly grew from about 63 g to 335-353 g, with stable feed conversion near 0.78 and negligible mortality. Fillet composition stayed close across groups, at roughly 65.8% moisture, 16.3% protein, and 12.6% fat, and texture did not differ significantly. The biggest product-quality change was visual: diets without processed animal proteins produced a more yellow fillet, likely linked to xanthophyll pigments, while diets containing those proteins yielded paler flesh. That may not affect nutritional value, but it could matter in markets where pet parents and other seafood buyers expect a pinker salmonid fillet. (mispeces.com)

The sustainability story is also more nuanced than a simple “alternative ingredients are greener” narrative. Reporting on the paper says the experimental diets lowered carbon footprint versus the control, especially formulations built around processed animal proteins. But the environmental effect depended strongly on where ingredients came from and how they were produced. Microalgae and microbial proteins, for example, could account for as much as one-fifth of total impact in some formulations, while soybean protein concentrate remained a major contributor in the conventional diet. In other words, replacing fishmeal may shift environmental burdens rather than automatically reduce them. (mispeces.com)

Industry and academic context supports that caution. Recent trout studies have found encouraging performance with insect meals and microalgal ingredients, but outcomes vary with inclusion level, processing method, fatty acid balancing, and the specific endpoints being measured. A 2025 Animals study on insect meal mixtures in rainbow trout likewise reported no major penalties in growth or fillet composition, while other recent work has focused on how alternative ingredients affect omega-3 levels, phosphorus waste, digestibility, gut microbiota, and health markers. Similar tradeoffs are showing up in other farmed species: in a 6-week Animals trial in juvenile yellowtail, replacing 25% to 35% of fishmeal protein with shark by-product-based composite mixtures did not significantly change final weight, weight gain, specific growth rate, feeding rate, or survival, and some groups had better feed efficiency than controls. But EPA and DHA were significantly lower in two of the by-product formulations, reinforcing that acceptable growth does not always mean nutritional equivalence. (mdpi.com)

Why it matters: For veterinary professionals working in aquaculture, this is useful evidence that practical, lower-marine-ingredient feeds are moving from proof-of-concept toward operational relevance. But it also reinforces that feed evaluation can’t stop at weight gain and feed conversion. Flesh appearance, nutrient retention, fatty acid profile, gut and immune effects, and the true lifecycle footprint of novel ingredients all shape whether a formulation is viable in commercial settings. For practices advising fish producers, the message is that alternative feeds may be ready for broader use, but only with close attention to formulation details and farm-specific goals. (mispeces.com)

There’s also a wider supply-chain implication. Fishmeal and fish oil supply is widely viewed as constrained, and earlier background from the same authors notes that aquafeed research has increasingly turned toward processed animal proteins, insect meals, single-cell proteins, and algae because conventional marine inputs are unlikely to scale with aquaculture demand. Work in species such as yellowtail suggests by-product streams can also replace a meaningful share of fishmeal protein without obvious growth penalties, though not always without changes in fatty acid composition. If these newer formulations can preserve performance in commercially important species like rainbow trout, they could help diversify feed sourcing and reduce exposure to volatile marine ingredient markets. (mdpi.com)

What to watch: The next phase is likely to center on commercial validation — ingredient cost, availability, regulatory acceptance, lifecycle assessment, and whether processors and consumers accept color and sensory differences as these formulations move closer to routine use. Just as importantly, expect more attention to whether alternative formulations preserve EPA and DHA and other nutritional targets while maintaining growth. (mispeces.com)