THE USE OF MEALS DERIVED FROM HETEROTROPHIC AND AUTOTROPHIC MICROALGAE IN DIETS FOR JUVENILE RED DRUM, Sciaenops ocellatus

Continued availability of compound feeds has contributed to the expansion of the world food fish aquaculture production. Fishmeal and fish oil, derived from small pelagic fishes, have been widely used as components of such feeds because they possess desirable nutritional characteristics. Fishmeal has a high protein content, with an ideal amino acid balance. In turn, fish oil has an outstanding fatty acid profile that includes good provisions of polyunsaturated and highly unsaturated fatty acids (PUFA and HUFA, respectively). Unfortunately, progressive depletion of global fish stocks, coupled with increasing demands for fishmeal from the aquaculture and animal feed industries, has induced a dramatic rise in fishmeal and fish oil prices, prompting efforts to limit their inclusion in aquafeeds. As a result, alternatives to fishmeal and fish oil have actively been sought in the past years. Efforts are being made to find PUFA and/or HUFA-rich feedstuffs that can be produced in a predictable and technologically controlled manner.

One such group of feedstuffs is represented by microalgae. Used for the production of biodiesel due to their high lipid content, microalgal oils, along with the resulting algal lipid-extracted by-products, have become available in recent years. However, whole microalgal products may be more attractive for inclusion in aquafeeds due to cost and contribution of both protein and lipid. Interestingly, various microalgal species are characterized by their high content of PUFA and HUFA and the large volumes at which they can be produced. To date, lipid concentrates or meals derived from autotrophic microalgae genera, such as Spirulina sp., Nannochloropsis sp., Isochrysis sp., Tetraselmis sp., Navicula sp., Chlorella sp., and Desmochloris sp., have experimentally been incorporated into balanced feeds for marine fish species, with variable, but promising results. The heterotroph Schizochytrium limacinum, probably less well studied but equally interesting, has been employed as a partial substitute of fishmeal and fish oil in diets of marine fish.

For the red drum, Sciaenops ocellatus, recent studies have shown successful replacement of at least 10% fishmeal and soy protein concentrate by lipid-extracted algal meals from the autotrophs Navicula sp., Nannochloropsis sp., and Chlorella sp. However, the use of microalgal meals derived from heterotrophic and other autotrophic species has not been investigated.

In the present study, the use of the heterotroph S. limacinum and some autotrophic microalgae in diets for juvenile red drum was evaluated. A reference diet was formulated to contain 40% crude protein (CP) and 10% lipid using menhaden fishmeal and soy protein concentrate (SPC) as the protein sources. Then, experimental diets were formulated to replace from 5 to 25% CP of the reference diet by meals derived from S. limacinum and from autotrophic microalgae. Preliminary data indicate that CP from fishmeal and SPC can be replaced, to some extent, by the algal meals without causing significant reductions in fish performance.