A groundbreaking study led by researchers from the University of California, Santa Cruz, has demonstrated that co-products of the microalga Nannochloropsis sp. can completely replace fishmeal in rainbow trout diets without compromising growth, nutritional quality, or profitability. This discovery could lay the foundation for more sustainable aquaculture practices, reducing the industry’s reliance on finite marine resources.
The promise of microalgae
Microalgae, particularly species like Nannochloropsis sp., have emerged as a promising alternative to fishmeal. These microscopic organisms are rich in proteins, essential amino acids, and long-chain polyunsaturated fatty acids (LC-PUFAs), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are vital for fish health and human nutrition. Additionally, microalgae can be cultivated using non-arable land and saline or wastewater, reducing competition with terrestrial agriculture.
Building on numerous previous experiments, the research team developed new aquaculture feed formulations for farmed rainbow trout, replacing traditional fishmeal ingredients—produced by grinding wild fish—with different levels of a novel ingredient: a marine microalga species called Nannochloropsis sp. QH25. The researchers tested these feeds and found that they could fully replace fishmeal with microalgae while maintaining the same levels of fish growth, human nutritional value, potential profitability, and other key metrics.
Testing a microalgae co-product in rainbow trout diets
In this new study, the researchers tested a defatted microalgae co-product (a byproduct of oil extraction for nutraceuticals) as a substitute for fishmeal in rainbow trout diets. The co-product, derived from Nannochloropsis sp. QH25, was incorporated into the feed at different levels: 33%, 66%, and 100% replacement of fishmeal. The results were compared with a reference diet containing fishmeal but no microalgae co-product.
Key findings: growth, nutrition, and cost
In their latest research, the team tested several new techniques that yielded promising results. They added taurine and lecithin as flavor and aroma enhancers and used advanced processing techniques. The feed ingredients were thoroughly sieved to improve mixture uniformity and texture, and pellets were formed using a high-temperature, high-pressure extrusion process instead of the “cold pelletizing” method used previously.
As a result, the study revealed that rainbow trout fed the 100% microalgae co-product diet exhibited growth, feed conversion ratios (FCR), and survival rates comparable to those fed the reference fishmeal-based diet. Notably, the composition of trout fillets (including proteins, amino acids, fatty acids, and minerals) remained consistent across all diets, ensuring that the nutritional quality of the fish was not compromised.
One of the most significant findings was the economic conversion ratio (ECR), which measures the cost of feed per kilogram of fish produced. The 100% replacement diet had the lowest ECR, making it the most cost-effective option. This suggests that microalgae co-products not only have the potential to replace fishmeal but can also reduce feed costs, a critical factor in the aquaculture industry.
Environmental and economic benefits
Using microalgae co-products offers several environmental advantages. Unlike fishmeal, microalgae cultivation does not rely on wild fish populations, reducing pressure on marine ecosystems. Additionally, microalgae can be grown in non-arable areas and do not require freshwater, making them a more sustainable option compared to terrestrial crops.
From an economic perspective, the study highlights the potential of microalgae to become a cost-competitive alternative to fishmeal. As large-scale microalgae production facilities continue to expand, the cost of microalgae biomass is expected to decrease further, making it an even more attractive option for aquafeed manufacturers.
Challenges and future directions
Following this success, the research team aims to develop a formula that also replaces fish oil, achieving a fully fish-free trout feed. They will also explore increasing the amount of microalgae used in their feed to replace other ingredients typically sourced from terrestrial agriculture. This could enhance the nutritional value of the resulting fish, lower the carbon footprint of the feed, and reduce competition between aquaculture and terrestrial food production. Their ultimate goal is to contribute to the diversity and quality of fish-free feed options available to fish farmers.
While the results are promising, challenges remain. The cost of producing microalgae biomass is still higher than that of fishmeal, and scaling up production to meet global demand will require significant investment in infrastructure and technology. Additionally, further research is needed to optimize the inclusion levels of microalgae co-products in feed for different fish species and life stages.
The study also highlights the importance of taurine and lecithin as feed additives to enhance palatability and growth performance in microalgae-based diets. These additives were crucial in ensuring that trout consumed the microalgae feed at the same rate as the fishmeal-based diet.
Conclusion
A significant breakthrough in the quest for sustainable aquafeeds. By demonstrating that microalgae co-products can completely replace fishmeal in rainbow trout diets, this research offers a viable solution to one of the industry’s most pressing challenges. As global seafood demand continues to rise, innovations like this will be essential to ensure that aquaculture can meet this demand without depleting marine resources.
The findings also underscore the importance of continued research and development in microalgae cultivation and feed formulation. With further advancements, microalgae could play a crucial role in creating a more sustainable and economically viable aquaculture industry, benefiting both the environment and consumers worldwide.
Reference (open access)
Sarker, P. K., Schoffstall, B. V., Kapuscinski, A. R., McKuin, B., Fitzgerald, D., Greenwood, C., Pasion, E. N., Gwynne, D., Gonzalez Orcajo, D., Andrade, S., Nocera, P., & San Pablo, A. M. (2025). Towards Sustainable Aquafeeds: Microalgal (Nannochloropsis sp. QH25) Co-Product Biomass Can Fully Replace Fishmeal in the Feeds for Rainbow Trout (Oncorhynchus mykiss). Foods, 14(5), 781. https://doi.org/10.3390/foods14050781
