Alternative trout feeds match conventional performance in new study

CURRENT FULL VERSION: A new paper in Animals suggests rainbow trout can be raised on more diversified, eco-efficient feeds without giving up the growth and product-quality benchmarks producers expect from conventional formulations. According to the study, practical extruded diets incorporating insect meal, microalgae-derived long-chain omega-3s, single-cell ingredients, plant proteins, and aquaculture by-products achieved results comparable to a traditional control for growth, body composition, nutrient retention, and flesh quality. (mdpi.com)

The work lands in a feed market that has been under steady pressure to reduce reliance on fishmeal and fish oil. That pressure is structural, not temporary. FAO says aquaculture production has now surpassed capture fisheries for aquatic animals, and marine ingredients remain strategically important in feed. Industry data cited by IFFO indicate aquafeed consumed about 63% of fishmeal demand in 2024, with salmonids accounting for the largest share among fed aquaculture species. Separate market data compiled for the EU also show salmon and trout together account for a large share of fish oil use in aquaculture. (fao.org)

That backdrop helps explain why trout has become a major test case for alternative ingredients. Recent studies have found that insect meals can be used in low-fishmeal trout diets without harming growth or fillet quality, while microalgae-based approaches are also advancing. One 2025 study from UC Santa Cruz and collaborators reported that a trout feed based on recycled marine microalgae could fully replace conventional fishmeal ingredients while maintaining performance after earlier palatability issues were addressed. Another recent MDPI paper found that mixed insect meals performed comparably to conventional trout diets across growth, somatic indices, digestibility, gut histology, microbiota measures, and fillet quality. (news.ucsc.edu)

What appears to distinguish the new Animals paper is its emphasis on “practical” formulation. Rather than testing a single novel ingredient in isolation, the study combined multiple alternative inputs into complete feed strategies, including versions with and without processed animal proteins, and supplied DHA and EPA primarily through microalgae. That mirrors how commercial reformulation is likely to happen in reality: not through one-for-one swaps, but through blended ingredient baskets designed to balance amino acids, fatty acids, palatability, and cost. EU regulation has allowed certain insect processed animal proteins in aquaculture feed since 2017, which has helped create a clearer pathway for these kinds of formulations in at least some markets. (eur-lex.europa.eu)

The same broader trend is showing up beyond trout. In juvenile yellowtail, another recent Animals feeding trial tested composite by-product protein mixtures built around shark by-products and surimi or other processing by-products as partial fishmeal replacements. Over six weeks, replacing 25% or 35% of fishmeal protein did not significantly change final weight, weight gain, specific growth rate, feeding rate, or survival compared with a fishmeal-based control, and feed efficiency was significantly better in two of the by-product formulations. Whole-body composition was largely unchanged, although crude ash differed, and EPA and DHA levels were significantly lower in some diets. That is a useful reminder that reduced fishmeal dependence can be compatible with growth performance while still altering nutrient profiles that matter for fish physiology and final product quality.

Industry and academic commentary around adjacent studies points in the same direction. UC Santa Cruz researcher Pallab Sarker said the sector has been working to find feed solutions that reduce stress on ocean ecosystems, while the university noted that future partnerships between microalgae producers and aquaculture could help both industries scale. In parallel, recent feed research in other species, including European sea bass, has also concluded that combining insect, algae, and by-product ingredients can sustain performance and improve formulation flexibility. Taken together, the signal is that alternative aquafeeds are maturing from niche sustainability projects into a broader formulation strategy. (news.ucsc.edu)

Why it matters: For veterinary professionals, especially those supporting aquaculture health and production, feed changes like these aren’t just a procurement story. They affect nutrient delivery, gut health, flesh composition, feed intake, and potentially welfare outcomes if palatability or digestibility slips. The encouraging message is that well-formulated alternative diets may avoid obvious penalties in trout performance. The caution is that “sustainable” ingredients can still carry trade-offs. A life-cycle assessment in rainbow trout aquaponics found that adding black soldier fly meal did not change the overall production footprint materially in that system, but did increase some feed-related energy use and greenhouse gas metrics, underscoring that ingredient origin and processing matter as much as ingredient label. The yellowtail by-product study adds a second practical caution: maintaining growth does not automatically preserve fatty acid composition, since some replacement diets lowered EPA and DHA. (sciencedirect.com)

For practices and fish health teams, that means feed evaluation is likely to become more multidimensional. Growth and feed conversion remain central, but so do fillet quality, nutrient retention, omega-3 delivery, ingredient consistency, and any downstream effects on intestinal health or product acceptance. It also means veterinarians may increasingly be asked to weigh in on feed transitions as part of herd- or stock-level health planning, especially as producers respond to cost pressure, sustainability targets, and retailer scrutiny around marine ingredient dependence. (mdpi.com)

What to watch: The next phase will be commercial validation: larger production trials, clearer economics for insect and algae ingredients, more use of by-product protein blends, and more scrutiny of life-cycle impacts, especially energy intensity and emissions from ingredient manufacturing. Expect future studies to focus less on whether trout can grow on these diets and more on which ingredient blends deliver the best balance of health, performance, cost, regulatory acceptance, omega-3 retention, and environmental return. (sciencedirect.com)