Microalgae-blend tilapia feed eliminates fishmeal and fish oil, improves growth, and is cost viable

Aquaculture is expanding rapidly to meet global seafood demand, with feed inputs representing a major share of production costs. Conventional aquafeeds rely on ocean-derived fishmeal (FM) and fish oil (FO), raising sustainability and food security concerns as wild forage fish supplies are constrained and also diverted from human consumption. Even low FMFO inclusion rates in feeds for low-trophic species like tilapia contribute substantially to global forage fish demand. Terrestrial plant ingredients used to reduce FMFO have limitations, including anti-nutritional factors, lower digestibility, essential amino acid deficiencies, and lack of long-chain omega-3 fatty acids (EPA, DHA), altering the n-3/n-6 balance in farmed fish. Marine microalgae offer a promising alternative due to balanced amino acids, micronutrients, and long-chain n-3s, but evidence is limited for fully fish-free tilapia diets. This study aimed to test whether combining protein-rich defatted Nannochloropsis oculata biomass with DHA-rich whole-cell Schizochytrium sp. could fully replace FM and FO in Nile tilapia feed, while maintaining or improving growth, fillet quality (including DHA), digestibility, and economic viability.

Prior work highlights sustainability challenges of FMFO and the growing aquafeed market’s need for alternatives. While tilapia diets commonly include 3–10% FMFO to enhance performance, plant-based substitutes have drawbacks (low digestibility, anti-nutritional factors, essential amino acid imbalances, and lack of long-chain n-3 fatty acids), potentially diminishing human health benefits of consuming farmed tilapia due to altered fatty acid profiles. Marine microalgae have been investigated as FMFO replacements, providing essential amino acids, minerals, vitamins, and long-chain n-3 fatty acids. Earlier studies showed: (1) defatted Nannochloropsis oculata biomass can partially replace FM without compromising growth; and (2) Schizochytrium sp. can fully replace FO in tilapia diets with high digestibility and improved growth and fatty acid deposition. However, peer-reviewed evidence has lacked on fully fish-free microalgae diets for tilapia and on the market pricing and cost competitiveness of such feeds at scale, motivating the present integrated nutritional and economic evaluation.

Study approvals: Experimental design and fish use protocol were approved by Dartmouth College IACUC; experiments followed relevant guidelines. Fish were euthanized by single cranial pithing. Diet formulation: Four isonitrogenous (37% crude protein) and iso-energetic (≈12 kJ/g) diets were formulated: a reference diet with FM (7%) and FO (3.2%) at commercial levels, and three microalgae diets replacing 33%, 66%, or 100% of FM with defatted N. oculata biomass (3%, 5.5%, 8% of diet by weight, respectively) and 100% of FO with whole-cell Schizochytrium sp. (6.2% of diet by weight) (33NS, 66NS, 100NS). Other base ingredients included corn gluten meal, soybean meal, wheat flour, and standard vitamin/mineral premixes; amino acids were balanced (L-lysine HCl, DL-methionine). Dried Schizochytrium sp. and menhaden FO sources are cited; N. oculata defatted biomass was donated by Qualitas Health. Proximate composition, amino acid profiles, and fatty acid profiles of ingredients and diets are documented in the paper’s tables and supplements. Feeding trial design: A completely randomized design with four diets and three replicate tanks each was used in recirculating aquaculture systems (RAS). A total of 480 Nile tilapia (mean initial weight 34.5±2.06 g) were stocked at 40 fish per 80-gallon tank (≤0.25 lbs/gal). Fish were acclimated to the reference diet for 7 days pre-trial. Water quality was monitored daily and maintained at 28.7±0.25 °C, pH 7.1±0.1, dissolved oxygen 6.1±0.15 mg/L, TAN 0.26±0.1 mg/L, and nitrite-N 0.3±0.01 mg/L. Feeding rates were 8% BW to day 60, 6% to day 121, and 4% to day 183, fed twice daily. Biomass sampling (subsamples and bulk) occurred at days 0, 60, 121, and 185; feed was withheld 24 h before weighing. Sampling and analyses: Ten fish at baseline, and six per tank at day 121 and day 185 (half filleted, half whole) were euthanized and stored at −20 °C, then freeze-dried and homogenized. Proximate, energy, amino acid, and fatty acid profiles were analyzed by a commercial lab. Macrominerals and trace elements in diets and fillets were quantified by ICP-MS (Agilent 7700x) following acid digestion and EPA method 6020a. Performance metrics: Final weight, weight gain (g), weight gain (%), feed conversion ratio (FCR = feed intake/weight gain), specific growth rate (SGR, %/day), protein efficiency ratio (PER), and survival (%) were calculated by standard formulas. In vitro digestibility: A pH-stat in vitro assay (per Yasumaru and Lemos with modifications) measured degree of protein hydrolysis using crude stomach and intestinal enzyme extracts from tilapia. Diet samples were ground and hydrolyzed at pH 2 (stomach phase) and pH 8 (intestinal phase) with automated titration to maintain setpoints. Degrees of hydrolysis (DH) for stomach and intestine were calculated from titrant volumes and constants; in vitro protein digestibility (IPD) was predicted from DH using IPD = 3.5093×DH + 70.248. Triplicate runs per diet. Economic analysis: Ingredient prices were compiled (2010–2019) and bootstrapped (10,000 replicates) to estimate median and 95% CIs. Hedonic pricing models (linear mixed-effects) estimated implied prices for defatted N. oculata meal (based on CP, ether extract, methionine, lysine) and whole-cell Schizochytrium sp. (based on selected fatty acid profiles of commodity oils). Transport/freight to Shanghai was incorporated. Diet costs ($/kg feed) were calculated from ingredient inclusion and prices. Economic conversion ratio (ECR, $/kg fish) was computed as FCR×diet price. Statistical comparisons used ANOVA and Tukey’s HSD (SPSS v21), with significance at p<0.05.

Growth and feed utilization:

• Fish fed the fish-free diet (100NS) showed significantly higher final weight, weight gain, percent weight gain, and SGR than the reference FMFO diet (p≤0.05). Examples: Final weight (g, mean±SE): Reference 139.9±4.5; 100NS 207.3±9.8. Weight gain (g): 106.6±13.1 vs 172.9±8.4. Weight gain (%): 318.8±28.0 vs 504.3±27.3. SGR (%/day): 0.62±0.05 vs 0.87±0.03.
• FCR tended to be better with the fish-free diet but was not statistically different: Reference 1.61±0.05 vs 100NS 1.40±0.03 (p=0.09). PER showed no significant differences (Reference 1.23±0.10; 100NS 1.30±0.02). Survival did not differ among diets (≈91–98%). Fillet composition and fatty acids:
• Fillet crude lipid was significantly higher with the fish-free diet (100NS 1.8%) compared to reference and partial replacement diets (0.8–0.9%) (p<0.05). Fillet crude protein and ash were highest in 100NS but not significantly different among diets.
• DHA deposition in fillet (mg/g) was significantly higher with microalgae diets; 100NS achieved 5.15 mg/g vs 2.47 mg/g for the reference (p<0.05). EPA in fillet was higher in the reference diet, reflecting its higher dietary EPA input.
• Total n-3 PUFA, n-6 PUFA, and LC-PUFA ratios did not differ significantly overall; several individual n-6 fatty acids (e.g., 18:3n-6, 20:3n-6, 22:4n-6, 22:5n-6) differed among diets. DHA (% TFA) was higher in microalgae-inclusion diets. Minerals and trace elements:
• No significant differences in fillet macrominerals among diets. Most trace elements were similar, with selenium lower in 33NS vs reference and not different between reference, 66NS, and 100NS. Arsenic in fillets was lowest in 100NS; boron, mercury, lead, and molybdenum were non-detectable. In vitro digestibility:
• The fish-free diet (100NS) had the highest degree of protein hydrolysis (DH) and in vitro protein digestibility (IPD), though differences vs reference were reported as not statistically significant in the text (Table 5 values: DH 5.64±0.54%; IPD 87.7±2.6% for 100NS; Reference DH 4.29±0.3%; IPD 85.3±0.9%). Economics:
• Hedonic analysis estimated median prices: defatted N. oculata biomass $0.44/kg [0.39, 0.49]; whole-cell Schizochytrium sp. $2.38/kg [1.93, 2.57]. Defatted N. oculata was much cheaper than FM (≈3.5× lower), while Schizochytrium was ≈1.4× higher than FO.
• Median formulated feed cost: Reference $0.64/kg [0.61, 0.68]; 100NS $0.68/kg [0.62, 0.73].
• ECR ($/kg tilapia): Reference 1.03 [0.95, 1.13]; 100NS 0.95 [0.82, 1.07], yielding lower production cost with the fish-free diet, attributable to its lower FCR despite slightly higher feed price.

Combining defatted Nannochloropsis oculata (protein source) with DHA-rich whole-cell Schizochytrium sp. (lipid source) enabled complete replacement of fishmeal and fish oil in a tilapia diet while improving growth performance and enhancing fillet DHA content. The improved SGR, weight gain, and trend toward better FCR indicate high nutrient digestibility and utilization, consistent with prior findings that Schizochytrium is highly digestible for tilapia. The highest in vitro protein digestibility with the fish-free feed suggests that the microalgae blend may be particularly compatible with tilapia digestive physiology, potentially via enzyme stimulation or more accessible protein matrices, though mechanistic details require further study. From a human nutrition perspective, the fish-free feed substantially increased fillet DHA, nearly doubling DHA content compared with the conventional FMFO feed, thereby improving the health value of farmed tilapia without relying on wild-caught resources. Mineral and trace element profiles in fillets were broadly comparable across diets, with low arsenic levels and non-detectable heavy metals, indicating no adverse mineral accumulation from the microalgae ingredients. Economically, although the fish-free feed had a slightly higher price per kg due to the current cost of Schizochytrium, overall production costs per kg of fish (ECR) were lower than the reference diet because of improved feed efficiency. As production of microalgal biomasses scales (e.g., via biorefineries producing high-value algal oils alongside lower-cost defatted biomass), costs are expected to decrease further, improving competitiveness relative to FMFO. These findings support the commercial viability of microalgae-based, fish-free tilapia feeds that can reduce pressure on forage fisheries while enhancing product nutritional quality.

This study demonstrates, for the first time, that a tilapia feed combining two commercially available marine microalgae—defatted Nannochloropsis oculata biomass and whole-cell Schizochytrium sp.—can fully replace fishmeal and fish oil, improve growth performance and specific growth rate, and markedly increase fillet DHA content. Despite a slightly higher feed price, the fish-free diet reduced economic conversion ratio due to improved feed efficiency, indicating cost viability. The approach leverages under-utilized defatted microalgal biomass generated by expanding algal oil industries, offering a sustainable, scalable path to fish-free aquafeeds that reduce reliance on wild fisheries and support better human nutrition. Future work should: (1) elucidate digestive enzyme responses and in vivo digestibility mechanisms to microalgae blends; (2) optimize processing (e.g., extrusion, enzymatic treatments) to further improve digestibility and FCR; (3) evaluate performance across life stages, production systems, and strains; (4) expand microalgae combinations and inclusion levels; and (5) track market developments to improve cost competitiveness of Schizochytrium and related algal ingredients.

The study was conducted under controlled RAS conditions with juvenile Nile tilapia and may not capture performance variability across different production systems, life stages, or genetic lines. Protein digestibility was assessed using in vitro assays, which, while informative, do not fully substitute for in vivo measurements. Economic results rely on hedonic price estimates and historical commodity prices (2010–2019) with assumed freight logistics; actual market prices and costs may vary. The current cost of Schizochytrium remains higher than fish oil, and broader cost competitiveness depends on further scaling and technological advancements. The authors note that further research is needed to elucidate digestive enzyme profiles under different dietary regimes.