TOWARDS SUSTAINABLE FISH-FREE AQUAFEEDS: EVALUATING MICROALGAL CO-PRODUCT FOR REPLACEMENT OF FISH MEAL IN AQUACULTURE DIETS FOR NILE TILAPIA Oreochromis niloticus

Aquaculture, the world's fastest growing food sector, demands 70% of global supplies of fishmeal and fish oil for aqua feeds. Over-reliance of aquaculture feeds on fishmeal, fish oil, and plant-based protein (e.g. soy) is unsustainable financially, environmentally, and socially. Our overall goal is to combine different marine microalgae to achieve fish-free diets. We report here our recent study to develop aquaculture feed formulae using co-product left over after commercially raising microalgae to produce a nutraceutical.

Data from our current experiment showed excellent growth, feed conversion, and survival of tilapia fed a diet replacing 33% of fish meal with a promising marine microalga, Nannochloropsis oculata (Na.) co-product. We first determined digestibility of its nutrients for tilapia. We then conducted a feeding experiment with Na. co-product to determine the optimum level of fishmeal substitution for growth of tilapia and their fillet's fatty acid composition. We quantified digestibility by comparing nutrient composition of feed and fish feces using standard methods; and we conducted a three-month growth trial starting with juvenile tilapia using experimental diets (Table 1). Results show that co-product had significantly lower digestibility values for crude protein (73.5% vs. 81.1%) and methionine (64.1% vs. 88.1%) than the whole cells. However, we detected the highest lysine digestibility in the Na. co-product (81.1%). Results of the growth trial with Na. co-product showed: comparable and higher final weight, weight gain, specific growth rate (SGR), and protein efficiency ratio (PER) in Na.0 and Na.33 diets; and FCR, feed intake, and survival rate were not significantly different among diets (Table 2). Although Na. co-product contains higher amounts of protein compared to Na. whole cells (49.7% vs. 37.7%), high levels of anti-nutrients (fiber, non-starch polysaccharide-NSP, and trypsin inhibitor) may have depressed digestibility and growth at higher Na inclusion levels.

We are now testing combinations of other microalgae with Na. co-product to fully replace fishmeal and fish oil in tilapia feed. Adding NSP and protease enzymes could improve nutrient digestibility and growth. Our next experiment will explore adding enzymes to Na Co-Product diets in order to enhance nutrient digestibility and growth.