Aquaculture, the fastest growing food sector, is expected to expand to produce an additional 30 million metric tons of fish by 2030, thus filling the gap in supplies of seafood for humans. Salmonids aquaculture exploits the vast majority of fishmeal and fish oil rendered from ocean-dwelling forage fish. Most forage fish diverted to these commodities are human-food grade, and all are primary prey for marine predators. Rising costs, price volatility, and environmental sustainability concerns of using these commodities for aquaculture feed are driving the global search for alternatives, including marine microalgae originating from the base of marine food webs but produced in culture. We report the first evaluation of two marine microalgae, Nannochloropsis sp. and Isochrysis sp., for their potential to fully replace fishmeal and fish oil in diets of rainbow trout (Oncorhynchus mykiss), an important model for all salmonid aquaculture. We conducted a digestibility experiment with dried whole cells of Nannochloropsis sp. and Isochrysis sp., followed by a growth experiment using feeds with different combinations of Nannochloropsis sp., Isochrysis sp., and Schizochytrium sp. We found that digestibilities of crude protein, crude lipid, amino acids, fatty acids, omega 3 polyunsaturated fatty acids (n3 PUFA), docosahexaenoic acid (DHA), n6 (omega 6) PUFA in Isochrysis sp. were significantly higher than those in Nannochloropsis sp. Digestibility results suggest that for rainbow trout diets Isochrysis sp. is a better substitute for fishmeal and fish oil than Nannochloropsis sp. The lower feed intake by fish fed diets combining multiple microalgae, compared to fish fed the reference diet, was a primary cause of the growth retardation. In trout fillets, we detected an equal amount of DHA in fish fed fish-free diet and reference diet. This study suggests that Isochrysis sp. and Schizochytrium sp. are good candidates for DHA supplementation in trout diet formulations.

Aquaculture, one of the world’s most rapidly growing food sectors, faces several concerns about its sustainability. A major concern is using fishmeal and fish oil from ocean-derived small pelagic fish (sardine, anchovy, etc.) in aquaculture feed. The aquafeed industry is seeking new sustainable ingredients to replace fish meal. This study focused on microalgal co-product, Nannochloropsis sp. QH25 co-product (leftover after oil extraction for nutraceuticals) is a novel aquafeed ingredient that can replace fishmeal in rainbow trout diets. A nutritional feeding experiment was conducted and compared fishmeal-containing rainbow trout diets with microalgal co-products that replaced fishmeal as follows: 0% replacement in reference diet (fishmeal, no microalgal co-product) and test diets with 33%, 66%, and 100% replacement of fishmeal using microalgal-product. Results showed the complete replacement diet yielded fish growth, feed conversion, and survival similar to the reference diet. Depositions of macronutrients, amino acids, fatty acids, macro minerals, and several trace elements in the filet were not significantly different across diets. Economic conversion ratio (ECR) analysis showed that the rainbow trout fed the 100% replacement diet had the lowest feed cost per kg of fish produced. Microalgal co-products can fully replace fishmeal in trout feed while maintaining fish performance, flesh composition, and cost-effectiveness.

Global food production, both aquatic and terrestrial, is rife with sustainability issues. Aquaculture is a rapidly growing industry that is on track to surpass wild-caught fishery production by 2030. The integration of aquaculture and agriculture systems could help to reduce some of their respective and shared environmental issues. Integrated Aquaculture–Agriculture (IAA) has been shown to provide a variety of benefits to both aquaculture and agriculture including increased system water use efficiency (WUE), a reduction in fertilizer use, and increases in total farm yield and resilience. This study was performed to examine the potential for rainbow trout aquaculture effluent to improve the yield, nutrient composition, and WUE of lettuce, as well as act as a partial fertilizer replacement. We used four irrigation treatments throughout the experiment: city water, aquaculture effluent, city water with added organic fertilizer, and aquaculture effluent with added organic fertilizer. Throughout the trial, there were some issues of herbivory in the growing plot, but overall, the lettuce receiving city water with organic fertilizer (30. ± 3.4 g/plant) showed significantly higher fresh biomass than effluent treatments (18 ± 1.4 g/plant and 17 ± 2.9 g/plant) at the end of the trial. We did not detect any significant differences among treatments in the macro- and micronutrients of the lettuce shoots and roots. However, the WUE of effluent treatments (4.4 ± 0.030 kg/m³ and 1.0 ± 0.070 kg/m³) was greater than that of organic fertilizer (0.28 ± 0.050 kg/m³) and control treatments (0.23 ± 0.040 kg/m³). The results indicate that adopting IAA in California could improve statewide WUE and conserve water for sustainable agricultural production.

As global demand for affordable, high-quality protein continues to rise, commercial aquaculture has emerged as a critical solution. The industry’s reliance on ocean-derived fishmeal (FM) and fish oil (FO) for aquafeed poses sustainability concerns. Marine microalgae offer a promising alternative due to their comparable nutrient profiles and potential for large-scale, sustainable production. We conducted a feeding trial with juvenile rainbow trout to evaluate fish-free, microalgae-based diets as alternatives to FM and FO. Diets were formulated using Nannochloropsis oculata (N. oculata) by-product as a protein source and Schizochytrium sp. (whole-cell or oil) as a source of DHA and key antioxidants. Diets included a reference formulation containing FMFO and three experimental diets: 75% replacement of FMFO using N. oculata and Schizochytrium whole-cell (NSW75), 100% replacement using both (NSW100), and 100% replacement using N. oculata and Schizochytrium oil (NSO100). The NSW100 diet supported growth and feed conversion comparable to the FMFO control; the NSO100 had lower growth than the FMFO control. Cost analysis revealed that fish-free diets had the numerically lowest, though not significantly different, values for formulated feed costs and economic conversion ratios, highlighting their commercial potential. These findings demonstrate that microalgae-based aquafeeds combining N. oculata and Schizochytrium sp. can viably replace FMFO.

The sustainability limitations of fishmeal and fish oil have increased interest in nutrient- rich microalgal co-products for aquafeeds. This study evaluated the effects of processing on the nutritional composition and apparent digestibility of a defatted Nannochloropsis sp. QH25 co-product in rainbow trout (Oncorhynchus mykiss). Experiment 1 assessed the effects of extrusion at 90 °C or 127 °C, with or without precooking, and enzymatic treatment with protease, xylanase, and β-glucanase. Based on nutrient composition, the 90 °C extrusion treatment without precooking was selected for Experiment 2, which compared a reference diet with three diets containing 30% raw, extruded, or enzyme-treated co-product, using four replicate tanks per diet. Extrusion at 127 °C reduced crude protein, gross energy, and methionine concentrations, whereas extrusion at 90 °C largely preserved nutrient composition. Precooking reduced several essential amino acids but increased methionine and did not affect tryptophan. Crude protein digestibility was numerically higher in the extruded co-product than in the raw co-product, although the difference was not statistically significant. Most essential amino acids showed similar digestibility in the raw and extruded treatments, whereas enzymatic processing resulted in lower crude protein and amino acid digestibility. Omega-3 fatty acid digestibility remained high across diets. Overall, the raw co-product showed high nutrient digestibility, while extrusion resulted in only small, generally non-significant numerical differences and enzymatic processing generally resulted in lower nutrient digestibility under the conditions evaluated.