The reader may wonder why algae, including both macroalgae (‘seaweeds’) and microalgae (e.g. phytoplankton), and which are popularly thought of as ‘plants’, would be good candidates to serve as alternatives to fishmeal in fish feeds. One fundamental consideration is that algae are the base of the aquatic food chains that produce the food resources that fish are adapted to consume. But often it is not appreciated that the biochemical diversity among different algae can be vastly greater than among land plants, even when ‘Blue-Green Algae’ (e.g. Spirulina), more properly called Cyanobacteria, are excluded from consideration. This reflects the very early evolutionary divergence of different algal groups in the history of life on earth. Only one of the many algal groups, the Green Algae, produced a line of descent that eventually gave rise to all the land plants.Therefore it can be difficult to make meaningful generalisations about the nutritional value of this extremely diverse group of organisms; rather it is necessary to consider the particular qualities of specific algae.
Protein and amino acids
Fishmeal is so widely used in feeds largely thanks to its substantial content of high-quality proteins, containing all the essential amino acids. A critical shortcoming of the crop plant proteins commonly used in fish feeds is that they are deficient in certain amino acids such as lysine, methionine, threonine, and tryptophan (Li et al. 2009), whereas analyses of the amino acid content of numerous algae have found that although there is significant variation, they generally contain all the essential amino acids. For example, surveys of 19 tropical seaweeds (Lourenço et al. 2002) and 34 edible seaweed products (Dawczynski et al. 2007) found that all species analysed containedall the essential amino acids, and these findings are consistent with other seaweed analyses (Rosell andSrivastava 1985, Wong and Peter 2000, Ortiz et al. 2006).
Analyses of microalgae have found similar high contents of essential amino acids, as exemplified by a comprehensive study of 40 species of microalgae from seven algal classes that found that, “All species had similar amino acid composition, and were rich in the essential amino acids” (Brown et al. 1997).
One often-overlooked nutrient is the non-protein sulphonic acid taurine, which is sometimes lumped with amino acids in discussions of nutrition.Taurine is usually an essential nutrient for carnivorous animals, including some fish, but it is not found in any land plants. However, although taurine has been much less often investigated than amino acids, it has been reported in significant quantities in macroalgae such as Laminaria, Undaria, and Porphyra (Dawczynski et al. 2007, Murata and Nakazoe 2001) as well as certain microalgae, for example the green flagellate Tetraselmis (Al-Amoudia and Flynn 1989), the red unicellular alga Porphyridium (Flynn and Flynn 1992), the dinoflagellate Oxyrrhis (Flynn and Fielder 1989), and the diatom Nitzschia (Jackson et al. 1992).
A few algae are used as sources of pigments in fish feeds. Haematococcus is used to produce astaxanthin, which is responsible for the pink colour of the flesh of salmon. Spirulina is used as a source of other carotenoids that fishes such as ornamental koi can convert to astaxanthin and other brightly coloured pigments. Dunaliella produces large amounts of beta-carotene.
In addition to its high content of high-quality protein, fishmeal provides lipids rich in ‘PUFAs’, or polyunsaturated omega-3 and omega-6 fatty acids. These are the ‘fish oil’ lipids that have become highly prized for their contribution to good cardiovascular health in humans. But it is not always appreciated that algae at the base of the aquatic food chain in fact originate these ‘fish oil’ fatty acids.These desirable algal fatty acids are passed up the food chain to fish, and they are indeed essential nutrients for many fish.
Algae have been recognised as an obvious alternative source of these ‘fish oil’ fatty acids for use in fish feeds (Miller et al. 2008), especially eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid (ARA). There is a substantial literature devoted to analysis of the PUFA content of microalgae, particularly those used in aquaculture, because they have long been recognised as the best source of these essential nutrients for production of zooplankton necessary for the first feeding of larval fish, as well as filter-feeding shellfish.
Many shellfish producers are aware the sterol profile of feed lipids is of critical importance, but much less attention has been paid to the importance of the sterol profile of fish feeds. Aside from alterations in the normal sterol profile of the fish, the possible endocrine effects of plant phytosterols in fish feeds (e.g. soy phytohormones) have yet to be thoroughly investigated (Pickova and Mørkøre 2007).
[Read more on Use of algae in formulated fish feeds & Choosing the right algae at thefishsite]