he Fish Site reports that often the algae chosen for fish feeding studies appear to have been selected largely for convenience, because they are low-cost and commercially available. For example, microalgae such as Spirulina, Chlorella and Dunaliella can be produced by low-cost open-pond technologies and are marketed as dry powders, and their nutritional profiles are well documented. Macroalgae such as the ‘kelps’ Laminaria, Undaria, and Durvillea, and the brown rockweed Ascophyllum, occur in dense stands that can be harvested economically, and they have a long history of use as sources of iodine, as soil amendments, and animal feed additives to supply trace elements.
In recent years there has been great interest in the potential of algae as a biofuel feedstock, and it has often been proposed that the protein portion remaining after lipid extraction might be a useful input for animal feeds. However, the algae chosen for biofuel production may not be optimal for use as a feed input, and the economic pressure for the lowest-cost methods of fuel production is likely to result in protein residues with contamination that makes them unfit for use as feed.
By contrast, the high-value microalgae that are used in shellfish and finfish hatcheries are generally produced in closed culture systems to exclude contaminating organisms, and they cannot be dried before use without adversely affecting their nutritional and physical properties, greatly reducing their value as feeds. Inevitably their production costs are higher, but their exceptional nutritional value justifies the extra expense.
Just as it would be senseless to arbitrarily substitute one conventional crop plant for another (e.g. potatoes for soybeans) when formulating a feed, the particular attributes of each alga must be carefully considered. In addition to the protein/amino acid profile, lipid/PUFA/sterol profile, and pigment content, there are important additional considerations.
The type and quantity of extracellular polysaccharides, which are very abundant in certain algae, can interfere with nutrient absorption, or conversely be useful binding agents in forming feed pellets. The thick cell walls of microalgae such as Chlorella can prevent absorption of the nutritional value of the cell contents. Inhibitory compounds such as the phenolics produced by some kelps, and brominated compounds produced by red algae such as Laurencia, can render an alga with an excellent nutritional analysis unsuitable for use in a feed. Depending on growth and processing conditions, algae can contain high concentrations of trace elements that may be detrimental.
Further careful study of the properties of numerous algae will be necessary in order to optimally exploit the great potential offered by this diverse group of organisms. But it is already apparent that algae will play an important part in the effort to move the formulation of fish feed “down the food chain” to a more sustainable future.