Study supports insect- and algae-based trout feeds as practical options

Version 2 — Full analysis

Rainbow trout may be one step closer to more practical low-marine-ingredient diets. In a new Animals paper, researchers reported that eco-efficient aquafeeds incorporating insect meal, microalgae-based DHA and EPA, selected plant proteins, single-cell ingredients, and aquaculture by-products delivered growth, nutrient retention, body composition, and flesh-quality results comparable to a conventional trout feed. The study tested a control diet against three alternative formulations — No-PAP, PAP, and Mix — in Oncorhynchus mykiss, with the central message that sustainability-driven reformulation did not appear to compromise core production outcomes. (mdpi.com)

That result lands in a research area that has been building for several years. Trout nutrition studies have repeatedly shown that insect ingredients, especially black soldier fly and yellow mealworm meals, can replace part of fishmeal without major hits to growth or feed efficiency when diets are carefully balanced. Separate work on DHA-rich microalgae, including Schizochytrium, has also found good digestibility and supports its use as a fish-oil substitute in rainbow trout feeds. More recent formulation studies in trout and other species suggest the strongest results may come not from any single replacement ingredient, but from blended strategies that combine insect, algal, microbial, plant, and by-product inputs. That broader by-product theme is showing up outside trout as well: a 6-week Animals trial in juvenile yellowtail found that replacing 25% to 35% of fishmeal protein with composite shark by-product-based mixtures did not significantly change final weight, weight gain, specific growth rate, feeding rate, or survival, and some diets even improved feed efficiency versus control. (mdpi.com)

That broader context matters because fishmeal and fish oil remain under supply and cost pressure. IFFO’s latest market update projected 2025 global production at about 5.6 million tons of fishmeal and 1.2 million to 1.3 million tons of fish oil. Even when supply rebounds, the industry expectation is not a return to heavy dependence on marine ingredients, but more diversified formulations. IFFO and other industry sources have pointed to growing demand from aquaculture and continued interest in alternative ingredients as feed makers manage price volatility, sustainability targets, and raw-material access. (iffo.com)

The trout paper’s framing is especially notable because it emphasizes “practical aquafeeds,” not just experimental substitutions. That echoes earlier rainbow trout work from the EU-funded GAIN project, which found that alternative formulations using processed animal proteins, insect meal, microalgae, and by-products could preserve growth and, in some cases, consumer acceptance. Other trout studies have reported that insect meal inclusion did not negatively affect nutrient digestibility, gut histology, microbiota, or fillet composition, although ingredient choice and inclusion rate still matter. The yellowtail study adds a useful nuance here: growth can be maintained with by-product protein blends, but nutritional quality markers do not always move in parallel. In that trial, whole-body composition was largely unchanged except for crude ash, yet EPA and DHA levels were significantly lower in two of the by-product-based diets than in the control. In other words, the field is moving from “can this ingredient work?” to “what blend works best under commercial constraints?” — and that includes preserving fatty-acid profiles, not just growth. (mdpi.com)

On regulation, the path is more open than it once was. The European Commission authorized the use of processed animal protein derived from insects in aquaculture feed under Regulation (EU) 2017/893, removing a key barrier for feed developers in the region. That doesn’t settle questions around cost, sourcing, or formulation consistency, but it does mean evidence like this can translate more readily into commercial feed development than in earlier years, at least in Europe. (eur-lex.europa.eu)

Why it matters: For veterinarians and aquaculture health teams, feed changes are never just a nutrition story. Reformulation affects growth, nutrient utilization, flesh quality, gut health, welfare, and potentially disease resilience, all while shaping environmental outputs and farm economics. The significance of this study is that it adds support for multi-ingredient, lower-footprint feeds that appear to preserve the performance markers clinicians and production teams care about. At the same time, the yellowtail findings reinforce an important practical point: acceptable growth and survival do not automatically guarantee equivalent lipid quality, so veterinary and production teams may need to watch tissue fatty-acid outcomes alongside routine performance data when marine ingredients are reduced. If those results are reproducible at scale, veterinary professionals may increasingly be asked to evaluate not whether insect or algae ingredients are acceptable in principle, but how specific formulations affect health monitoring, flesh traits, waste outputs, and consistency across life stages and farming systems. (mdpi.com)

There’s still reason for caution. Much of the strongest evidence comes from controlled trials, and some studies show that outcomes vary by insect species, processing method, inclusion level, amino acid balancing, and lipid strategy. The yellowtail work is a reminder that by-product mixtures can perform well on growth while still changing EPA and DHA deposition, depending on the blend used. Industry adoption will depend on whether feed manufacturers can secure reliable ingredient streams at workable prices and whether performance holds in commercial settings over longer production cycles. That is likely where the next wave of research and market attention will go. (mdpi.com)

What to watch: Expect follow-up work on farm-scale economics, digestibility and health metrics under commercial conditions, and whether blended insect-algae-by-product diets can deliver consistent results across trout production stages and other high-value species while maintaining desired fatty-acid profiles. (mdpi.com)