Micro-algae biomass as an alternative resource for fishmeal and fish oil in the production of fish feed

PhD theses are an often neglected source of technical information that can have a direct bearing on the commercial production of microalgae and their products.

Hamed Safafar has published his PHD thesis on how microalgae biomass can be used as an alternative to fish meal and fish oil to make fish feed for commercial aquaculture. The work was carried out at the Technical University of Denmark.

It is increasingly recognised that using industrially caught fish as a source of fish feed for commercial aquaculture is not economically or environmentally sustainable. Microalgae contain almost all the nutrients required to feed commercially grown fish, and naturally play a major role in the nutrition of aquatic animals such as fish. They are therefore potentially a replacement for fish-based aquaculture feed. In order to be considered for this role the microalgae should contain all of the essential nutrients, be non-toxic, and be available at an economically viable price. It is essential that any microalgae being considered must be able to grow on low cost liquid media, and that they can be harvested cheaply and stored without significant loss of nutrient quality.

In this study, process water containing high concentrations of ammonia, and free from toxic compounds, representing the digestate from an anaerobic digestion plant, was used to grow the microalgae in flat panel photobioreactors. A number of species were studied, including
Nannochloropsis salina, Nannochloropsis limnetica, Chlorella sorokiniana, Chlorella vulgaris, Chlorella pyrenoidosa, Desmodesmus sp., and Arthrospira (Spirulina) platensis. Of these, Chlorella pyrenoidosa grew well in the process water and produced reasonable amounts of microalgal biomass (6.1g/L) that contained very high levels of protein (65% of dry matter) as well as useful amino acids and carotenoids, but no significant levels of long-chain polyunsaturated fatty acids. Nannochloropsis salina was grown on mixture of process water and standard growth medium and produced high levels of EPA , a long-chain polyunsaturated fatty acid and an important human nutrient obtained from oily fish or fish oil.

The microalgae were harvested by a process including cross flow micro-filtration, heat treatment, centrifugation, and flash drying. The drying technique had a significantly adverse effect on the nutritional quality of the biomass. It was found that the best results were obtained by using freeze-dried microalgae and storing the biomass at 5 C.

The thesis contains much information of direct value to commercial producers of microalgae-based aquaculture feed.