Even though widely-consumed fish such as Nile tilapia may frequently be raised on farms, the food that they eat still contains ecologically-important fish that are caught in the ocean, depleting wild stocks. That's why Dartmouth College scientists are now looking at replacing the fishmeal in that food with existing algae meal.

Nannochloropsis oculata is a marine microalga that's already being grown commercially – oils extracted from it are used in products such as nutraceuticals and biofuels. After the extraction process has been completed, what's left over is large quantities of algae meal that's still rich in protein and omega-3 fatty acids. One of those omega-3's, eicosapentaenoic acid, is said to be essential to fish growth and quality.

In lab tests, the researchers used the algae meal to replace varying percentages of the fishmeal in regular tilapia feed. They found that when the algae meal replaced 33 percent of the fishmeal (which itself made up 7 percent of the feed), the result was "fish growth, feed conversion, and survival similar to the reference diet" – "feed conversion" refers to the rate at which feed is converted into a desired output, which is flesh in the case of tilapia.

Although the algae meal (pictured above) contains more protein by weight than the whole-cell microalga does before oil-extraction, it isn't as easily digestible. To that end, the scientists have suggested adding enzymes to the feed, both to compensate for the lower digestibility and to maximize nutrient availability. The hope is that once the technology is developed further, it could make the fishmeal completely unnecessary.

"The possibilities for developing a sustainable approach to aquaculture are exciting," says lead scientist, assistant professor Pallab Sarker. "Our society has an opportunity to shift aquafeed's reliance on fish-based ingredients to a fish-free product that is based on marine microalgae, and our findings provide new insight into how we can get there."

A paper on the research was recently published in the journal PLOS ONE. Members of the same team previously showed that the fish oil in tilapia feed could be replaced with the marine microalga Schizochytrium.

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