Picochlorum as an alternative to Nannochloropsis for grouper larval rearing

Abstract Nannochloropsis oculata is widely used in grouper larvae hatcheries for the production of green water to stabilize water quality, provide shielding for light-sensitive fish larvae, and breed super-small rotifers (SS-rotifers), the first feed of grouper larvae. However, the practical use of N. oculata is hindered by an insufficient yield of this alga. This study evaluated Picochlorum strain S1b, an indigenous marine microalga capable of cultivation at high growth rates, to determine its suitability as an alternative to N. oculata in grouper larvae hatcheries. Feasibility was evaluated mainly according to two parameters: 1) the nutritive value of the microalga for SS-rotifers; and 2) the nutritive value of SS-rotifers for grouper ( Epinephelus coioides ) larvae. The nutritive value of algae was evaluated by comparing the proliferation rate and egg ratio of SS-rotifers. The nutritive value of SS-rotifers was evaluated by comparing the size and amino acid composition of SS-rotifers feeding on different microalgae. We also examined the “green water effect”, i.e., changes in the vibrio community in the rearing tank following the introduction of microalgae, as well as its influence on the survival and growth rates of grouper larvae. In these experiments, S1b provided the highest nutritive value for SS-rotifers, particularly when grown under mixotrophic conditions (S1b [mixo]). The cultivation of SS-rotifers using S1b [mixo] increased the percentage of small-size organisms, which are highly suitable as first feed for grouper larvae; and cultivation with S1b [mixo] altered the amino acid composition of SS-rotifers to more closely match that of grouper larvae, thereby enhancing their nutritive benefits. Green water produced with either Picochlorum S1b or N. oculata was shown to inhibit the blooming of vibrio, an opportunistic pathogen of fish. Grouper larvae reared in green water produced with Picochlorum S1b had the same survival rate but a higher growth rate than those reared in green water produced with N. oculata . These results demonstrate the potential benefits of replacing N. oculata with Picochlorum S1b for the rearing of grouper larvae.