Dried Fish Solubles Show Potential to Improve High-Soy Diets for Rainbow Trout

Adding 5% dried fish solubles to soybean meal-based diets improved growth and selected fillet-quality traits in rainbow trout (Oncorhynchus mykiss), according to a peer-reviewed study published in the journal Aquaculture on 22 September 2026.

Researchers from the University of Idaho, Washington State University, USDA Agricultural Research Service, and Bowling Green State University conducted a 66-day feeding trial involving 600 juvenile rainbow trout with an initial weight of approximately 22 g. The study tested five diets: a fishmeal-based control; diets containing 20% or 35% soybean meal; and those same two soybean meal diets supplemented with 5% dried fish solubles. Two trout strains were compared — a commercial strain and the ARS Hagerman strain, the latter previously developed through long-term selection for growth on a fishmeal-free, all-plant-protein diet containing substantial soybean meal.

Ingredient profile and origin

The dried fish solubles were produced through enzymatic hydrolysis and drying of stickwater generated during food-grade processing of Pacific whiting (Merluccius productus), described by the researchers as an MSC-certified species. The raw material originated from non-edible material remaining after filleting, headed-and-gutted processing, or surimi production.

The ingredient contained around 63% crude protein and 19% crude lipid and supplied indispensable amino acids, EPA, DHA, and minerals. Around one-third of its protein fraction consisted of low-molecular-weight peptides and free amino acids below 3 kilodaltons. The authors described the product as a traceable, upcycled ingredient aligned with circular-economy principles, noting that its dried form offers greater stability, longer shelf life, and easier handling and incorporation into feeds compared with raw stickwater or condensed fish solubles.

Growth and feed intake

The clearest growth response occurred in the 35% soybean meal diet supplemented with dried fish solubles. Fish receiving this diet reached an average final weight of 177.42 g, compared with 135.69 g for the corresponding unsupplemented diet. Daily feed intake was also significantly higher in the supplemented high-soy group, while diet had no significant effect on feed conversion ratio (FCR). The researchers therefore attributed the growth response primarily to increased voluntary feed intake rather than improved feed efficiency.

Energy retention was lower in the unsupplemented soybean meal groups than in the fishmeal treatment, while the supplemented diets did not differ significantly from the fishmeal group. Survival was unaffected by diet or strain.

Strain comparisons

The commercial strain generally showed better growth and nutrient utilization, including lower FCR and higher protein efficiency ratio (PER), apparent net protein utilization, and energy retention. Both strains generally responded similarly to dried fish-soluble supplementation. The researchers noted that differences in diet formulation, trial duration, fish size, and comparison strains could have contributed to the Hagerman strain's performance not entirely following previous observations. They also suggested that the presence of other animal protein ingredients in the soybean meal-based diets may have reduced some of the nutritional and intestinal challenges associated with high soybean meal inclusion.

Body composition and fillet quality

Fish fed the unsupplemented 35% soybean meal diet had higher body moisture and lower lipid levels. Adding dried fish solubles brought these levels closer to those observed in fish fed the fishmeal diet and increased whole-body energy. Supplementation also increased taurine levels, while diet influenced the overall amino acid composition of the fish.

Effects on fillet quality varied with soybean meal inclusion level. At 20% soybean meal, dried fish-soluble supplementation improved fillet yield and resilience. At 35% soybean meal, supplementation reduced liquid loss, particularly during storage. No significant effects of diet or strain were detected for hydroxyproline or total collagen content.

Implications and limitations

The authors describe the study as the first to examine strain-specific effects of dried fish solubles in aquaculture nutrition, and suggest the ingredient could be used as a functional supplement in high-soybean meal diets rather than as a direct fishmeal replacement. However, the study notes that the dried fish solubles tested are not yet commercially available.

The study is published as: Singha, K. P., Small, B. C., Sealey, W. M., Overturf, K., & Kumar, V. (2026). Functional inclusion of dried fish soluble in diets containing high soybean meal and low fishmeal improves growth, nutrient utilization, and fillet quality of two rainbow trout (Oncorhynchus mykiss) strains. Aquaculture, 744532. https://doi.org/10.1016/j.aquaculture.2026.744532