Building upon many prior experiments, researchers from the University of California Santa Cruz developed new aquafeed formulations for farmed rainbow trout that replaced traditional fishmeal ingredients with varying levels of the microalgae Nannochloropsis sp. QH25. Researchers tested their feeds and found that they could fully replace fishmeal with microalgae while maintaining the same levels of fish growth, nutritional value for humans, potential cost-effectiveness, and other key metrics.
Assistant Professor Pallab Sarker has been working for years, alongside Professor Anne Kapuscinski and the rest of the UC Santa Cruz team, to create new feed formulations by recycling leftovers from marine microalgae that are already being grown commercially for use in human dietary supplements. Several years ago, the team combined a few different types of microalgae to formulate a potentially cost-competitive feed for Nile tilapia that was totally fish-free and performed better than conventional feed in several key metrics.
Tilapia have the advantage of being natural vegetarians. It’s more difficult to remove fishmeal and fish oil from diets for fish that are natural predators of other fish, such as salmon and trout and the UCSC team wanted to demonstrate their specific set of techniques on rainbow trout.
Some of their early attempts to replace wild-caught fish ingredients in trout feed with Nannochloropsis sp. microalgae formulations failed. Trout didn’t grow nearly as well on early experimental diets. But in the process of that research, the team identified the problem: rainbow trout were picky eaters. They didn’t like the taste of the microalgae as much as fish-based feed, so they ended up eating less of it, and their growth was stunted as a result.
“Trout and salmon eat other fish, so they really like fish smell and fish flavor,” Sarker explained. “After the disheartening results from our prior study, we learned that we could try adding taurine and lecithin as feeding stimulants, and that ended up being a breakthrough for the current study. Taurine is a chemical that fishmeal contains naturally, so when you exclude fishmeal, you also exclude taurine, and that’s part of what we believe made the original feed unappetizing to the fish.”
In their latest research, the team tried several new techniques that seem to have paid off. They added taurine and lecithin to their feed as flavor and smell enhancers, and they used new processing techniques. They extensively sieved feed ingredients, to improve mixing and texture, then formed pellets using an extrusion process with high temperature and pressure, instead of the “cold pellet” process they had used previously.
Trout gave the new menu rave reviews
During a roughly two-month “growth study” with more than 500 trout, researchers assigned specific groups of trout to be fed twice a day with either a conventional reference diet or new experimental feed formulations that replaced fishmeal with microalgae at levels of 33%, 66%, or 100%. The results showed no significant difference in the level of fish growth between the conventional feed and the experimental feeds, even when fishmeal was fully replaced with microalgae. The estimated potential cost-effectiveness of the feeds and measurements of the nutritional value for humans from the resulting fish fillets were also comparable.
Following this success, the team next aims to develop a formula that replaces fish oil too, in order to achieve a new fully fish-free trout feed. They’ll also see if they can further increase the amount of microalgae used in their feeds, in order to replace some other feed ingredients typically sourced from land-based agriculture. They hope this could improve the nutritional value of the resulting fish, lower the carbon footprint of their feeds, and reduce aquaculture’s competition for land-based food resources with people and livestock. Their ultimate goal is to contribute to the variety and quality of fish-free feed options available to fish farmers.
Overall, Sarker says marine microalgae have huge potential for helping the aquaculture industry grow sustainably, but he cautions that, in order to realize that potential, the microalgae production industry will itself need to continue to grow and improve. Currently, the only cost-effective way to use Nannochloropsis sp. microalgae in fish feed is by recycling leftovers from the production of human supplements, as the UCSC researchers did for their study. To grow this same microalgae from scratch specifically for use in aquafeed would be too expensive at the moment.
Reference:
Sarker PK, Schoffstall BV, Kapuscinski AR, McKuin B, Fitzgerald D, Greenwood C, O’Shelski K, Pasion EN, Gwynne D, Gonzalez Orcajo D, et al. Towards Sustainable Aquafeeds: Microalgal (Nannochloropsis sp. QH25) Co-Product Biomass Can Fully Replace Fishmeal in the Feeds for Rainbow Trout (Oncorhynchus mykiss). Foods. 2025; 14(5):781. https://doi.org/10.3390/foods14050781
